homebrew rf transceiver design

Winter 2004 3 HOMEBREWER

A Nuts and BoltsApproach to RF DesignLet’s say you are like many other homebrewing hams ... you mighthave a limited understanding of how radio circuits work and don’thave a shack full of test equipment. You probably have an HF ama-teur band receiver, an oscilloscope, a digital multimeter with an RFprobe, an SWR meter or bridge, a dummy load and an antenna. Youhave a basic set of tools and you have at least begun to get comfort-able with the Manhattan, Island, or Ugly style building methods. Andmost important, you have the desire to design and build some gear ofyour own. If you fit this profile, this project is for you!

Wayne McFee, NB6M

“Nuts & Bolts 101”The principal idea behind the Nuts & Bolts approach is to make use

of what others before you have done. There is no need to re-invent thewheel. Certainly there are new ideas all the time but all of us in ham radiowill continue to use both ideas and proven circuits that others have previ-ously developed. All of the best engineers have files of proven “buildingblock” type circuits that they fit into an overall circuit in order to achievethe desired result.

As you know, every electronic circuit, no matter how complex, ismade up of individual stages. These are shown in simplified drawings asblocks with either their individual stage names or a symbol of their pur-pose in the blocks.

An entire circuit is depicted in simplified form as a “block” diagram.Even if we do not have the knowledge needed to be able to design theindividual stages, we can still design and build a complete transmitter,receiver, or transceiver. Just select proven circuits for the stages or blocksand carefully connect them together.

Suppose we want to design a transceiver, and because of the chang-ing availability of electronics parts, we want to use generic, readily avail-able discrete parts throughout. Using this Nuts & Bolts approach not onlymakes it easier to troubleshoot and fix the rig if anything should go wrong,but it also helps us learn more about how the individual stages work andhow they work together to produce the desired result.

A 40 Meter CW Transceiver designed and built using the Nuts & Bolts approach.

HOMEBREWER 4 Winter 2004

Philosophy• Use discrete, readily available parts throughout.• Design for reasonably low current drain on receive.• Use a VFO as the LO, in order to realize full band coverage.• Provide appropriate audio output for modern, low impedance head-phones.• Provide for transmitter power output up to 1.5 Watts.• Use electronic T/R circuitry

Building BlocksKeeping our design philosophy in mind, and perusing available proven

designs, one relatively simple superhet receiver circuit described in Chap-ter 12 of “Experimental Methods in RF Design” is called the S7C. Itseems to fit our needs, so for this exercise we will use it as the basis for our40 Meter transceiver design. The S7C circuit is shown below, minus theVXO LO which we will not use.

This receiver uses all discrete parts, has low current drain and pro-vides a reasonable amount of selectivity in a superhet configuration. Formore detail on the design, see Chapter 12 of Experimental Methods in RFDesign. As published, the S7C receiver has a 10 MHz IF frequency anduses a VXO LO with limited frequency range. Crystals for 4.000 MHzwere already on hand so the IF frequency was changed. We would like fullcoverage of the CW portions of 40 Meters, so a VFO will be used as theLO.

If we use this receiver circuit as a base, what do we need to add inorder to design a complete transceiver? To make it easier, let us look at theS7C as a block diagram and consider a block diagram of the remainder

needed to complete a transceiver circuit. Both block diagrams are shownon the next page.Design Discussion

In order to form a complete transceiver, we will add all of the build-ing blocks shown. Using the Nuts and Bolts design philosophy, we willperuse available published circuits and use parts of several in order toprovide the needed circuitry.

Let us start with the Local Oscillator (“Permeabililty Tuned VFO” onnext page). We changed to a VFO configuration to realize a much widerfrequency range. The choice of oscillator is up to the builder and anythingfrom crystal control to a digital VFO could be used.

For simplicity I have used an analog VFO that I have had very goodluck with. It is stable and is relatively easy to build. It uses permeabilitytuning and can provide us with entire band coverage if we so desire. Thebasic circuit was published on the WA6OTP website and I have modifiedit and added it to this rig. It is a nice little VFO.

Next in our chain of blocks is the LO Buffer Amplifier. The circuitchosen is the very familiar one from the “Ugly Weekender” by KA7EXMand W7ZOI, and is a design by W7EL.

The next block we need to consider is the Transmitter Mixer. Wecould simply duplicate the dual JFET mixer used in the S7C, but since it isa single ended mixer with no balance, it might either allow or introducespurs into our transmitter signal. In order to help prevent this, we will usea diode ring, doubly balanced mixer. We can either use a commercial +7dBm mixer, such as the TUF-1 or SBL-1 from Mini-Circuits, or build themixer from discrete parts.

S7C Transceiver, Minus LO


LO Buffer Amplifier

Because of our desire to use all discrete parts, and our choice of thediode ring, doubly balanced mixer, we will discuss the mixer and its re-quirements in terms of input signals.

Some things to consider in our design process are the characteristicsand input signal level requirements of the mixer we have chosen. The di-ode mixer has no gain. In fact it has about a 6 dB loss. It needs an input


from the LO of at least +6 dBm. At the same time, we want the signal fromthe Transmitter Oscillator to be down around the –10 dBm level. This isdone to help reduce spurious output.

We used the BFO circuit from the S7C receiver, changing the trim-mer cap to an inductor to create the needed amount of frequency offset.We will also add a resistive attenuator pad to adjust the signal level. Be-cause we don’t know yet exactly how much output we will get from theoscillator, we will have to build it, measure the output, and choose anappropriate amount of attenuation in order to set the injection level to themixer at the desired –10 dBm.

Let us discuss the next stage: the Transmitter Bandpass Filter.

In order to get the most out of our bandpass filter and to define theimpedance felt by the output of the mixer, we will use a 50 Ohm, 6 dBresistive attenuator pad betweenthe mixer and bandpass filter. Thiswill ensure that the mixer will beproperly terminated for all fre-quencies present.

From there, we can usereadily available computer soft-ware to design a 50 Ohm bandpassfilter. This one is designed for acenter frequency of 7.1 MHz anda 300 kHz bandwidth.

We purposely kept a low RFlevel from the transmitter oscilla-tor. This fact, in combination withthe 6 dB loss in the mixer, the 6dB loss introduced by the 50 Ohmpad used to provide a good termi-nation for the mixer, and the lossthrough the bandpass filter, meanswe need to add a 20 dB Post MixerAmplifier. This brings the trans-mitter signal back up to a usablelevel for the following stages.

At the coupling capacitorfrom the output of the Post MixerAmplifier, we add a PI networkthat is designed to provide an im-pedance transformation from the50-Ohm level that we have beenworking in to the 2.2 K Ohm levelthat is defined by the 2.2 K resis-tor to ground at its termination.

At that point, we add the 0.1

uF DC blocking cap and go right into the input of the emitter follower TXBuffer Amp, which is the next link in our transmitter chain. To simplifythe remainder of our design, we will use a large portion of the transmittercircuit used in the SW40+, from the end of the bandpass filter to the out-put, including the buffer amplifier, driver, PA, output filter, keying circuit,and T/R circuit.

In the SW40+ transmitter, either a 2SC2078 or 2SC1969 is used toproduce 1.5 Watts of output. A 2SC799 was used here because it was onhand. The output filter is the familiar half wave design. The T/R circuit isa version of the circuit introduced by W7EL in the Optimized QRP Rig.

With the transmitter, keying and T/R circuits all now complete, theonly things left are the muting circuit for the receiver and the peripherals -- the FreqMite CW frequency enunciator and the Tick Keyer.


In order to provide receiver muting, we inserted the very familiarJFET switch that was introduced by W7EL, but we integrate it into thediscrete component chain as shown below. This allows us to hear the trans-mitter signal, so no sidetone oscillator is needed.

The value of the resistor bridging the JFET will have to be deter-mined by experimentation in order to set the tone to the desired level. Inthe Small Wonder Labs “SW” series rigs, a value of 4.7 MegOhms is usedand that should be a good starting point for us.

The addition of the FreqMite and TICK keyer will make the rig com-plete. The audio from each is fed into the audio output amplifier at thepoint specified in the complete circuit diagram. Next let us take a look atthe block diagram for the entire transceiver.

The complete circuit diagram is appended on the last page of this

article, also showing how the FreqMite and TICK Keyer are tied into thecircuit.

ConstructionIn order to demonstrate the viability of this Nuts & Bolts design ap-

proach, a 40 Meter CW transceiver was built, using the “Ugly” method,completely without “Manhattan” or “Island” type pads. This method isstraightforward and easy. No supporting structure such as Manhattan padsor a printed circuit board have to be made, so circuitry goes together veryfast and lends itself to easy modification or change. To make changes, allyou need do is remove and replace the actual parts themselves, with noother changes needed to the basic circuit structure.

As shown in the overall photo at the start of the article, the circuitry islaid out in a “U” shape, beginning with the receiver audio amps in upper

Post Mixer Amplifier and Tx Buffer Amp


left, the product detector and BFO next, then the IF amps and dual crystalfilter, the cascode FET mixer, and the double-tuned Receiver Input Filterin upper right. Centered behind the front panel is the permeability tunedVFO, and to its left are the Keying Circuit and Receiver Muting Circuit.

Just below the VFO Tuning Coil is the VFO Buffer Amplifier, and toits left are the SBL-1 Mixer and the Transmitter Oscillator. The RCA jackon the front panel is where an outboard FreqMite Audio Frequency An-nunciator plugs in. Continuing to the left of the SBL-1 Mixer are theTransmitter’s Double Tuned Filter, RF Amplifier, LC Impedance-Match-ing Network, Buffer Amplifier (with small trimpot), Driver Amplifier(2N2222A), Power Amplifier (2SC799) and Output Filter. The diode T/Rcircuit can be seen just above the PA heatsink.

At first this method might seem daunting. In actuality however, it isreally quite easy if one lays out the circuit logically, in almost the samefashion as the circuit diagram itself is arranged.

Construction started with the Audio Output Amplifier at one cornerof an eight inch by eight inch sheet of single-sided pc board material, andfollowed the signal chain as it was described above. Let us take a closerlook at the layout and construction of individual stages and sections of theentire circuit. Stages were built in the following order:

• Audio Output Amplifier• Audio Preamplifier• Product Detector• BFO• 2nd IF Amplifier• Dual Crystal IF Filter• 1st IF Amplifier• Cascode FET Mixer• Double-Tuned Receiver Input Filter• VFO• VFO Buffer• Transmitter Mixer• Transmitter Oscillator• Transmitter Double-Tuned Bandpass Filter• Transmitter RF Amplifier

• LC Impedance Matching Network• Transmitter Buffer Amplifier• Driver Amplifier• Power Amplifier and Output Filter• T/R Circuit• Keying Circuit• Receiver Muting Circuit

Each stage was tested as it was built by injecting test signals into thecircuitry for the Audio Amplifiers, IF Amplifiers and Receiver Mixer. Anoscilloscope and a multimeter with RF probe were used to test the BFO.Signal level from the BFO injected into the Product Detector is set by thevalue of the coupling capacitor between the two, at +7 dBm. Transmitterstages were tested with a combination of frequency counter, 50-Ohmdummy load, multimeter with RF probe, and Oscilloscope.

As already stated, construction began with the receiver’s Audio Am-plifiers. The picture below shows the layout of the Audio Amplifier, BFO,and Product Detector stages. The top of the large inductor to the left of thetri-filar-wound toroid transformer is the signal injection point into the Prod-uct Detector from the 2nd IF Amplifier. The shielded cables running off tolower left go to the Audio Volume Control on the front panel.

Product Detector, BFO, and Audio Amplifiers


Next in the receiver chain is the 2nd IF Amplifier, Dual Crystal IFFilter, and 1st IF Amplifier. Although some might want a narrower IFFilter, this one provides good opposite-sideband rejection, and has a nice,clean and crisp sound.

The inductor on the rightmost side is the input to the Product Detec-tor. The 0.1 uF capacitor at left is the signal injection point from the Re-ceiver Mixer to the 1st IF Amplifier.

The Cascode FET Mixer and Double-Tuned Bandpass Filter com-plete the Receiver Chain, as shown here.

The small, shielded cable running underneath the upper edges of thetwo larger toroids is the signal input to the receiver from the T/R Circuit.The shielded cables above that are the audio lines going to the volumecontrol pot on the front panel.

Now, the project really started to get exciting. With the addition ofthe LO, the receiver section could be brought to life and its very first re-ceived signals could be heard.

The VFO used as the Local Oscillator in this superhet-based trans-ceiver is shown below. It is a permeability-tuned oscillator using a tuningcoil wound with #30 wire on a 5/16th-inch diameter plastic drinking straw.The actual tuning element is a 6-32 brass screw. All NP0 capacitors wereused in frequency determining locations and very short leads were sol-dered, where appropriate, directly to the copper substrate.

The 9-turn link taking RF to the VFO buffer is wound with #22 mag-net wire around the outside of the main coil. The entire coil is covered withfive-minute epoxy so as to prevent any shifting of the coils which wouldcause drift. Tuning is smooth throughout the approximate 165 kHz range.The tuning rate is such that there are about 4 kHz per full turn of the tuning

knob at the low end of the band, and about 8 kHz per turn at the upper endof the tuning range. This tuning rate is more than slow enough to ensureease of tuning individual signals.

The coil mount was made from a small piece of double-sided copper-clad board material, with a 6-32 brass nut soldered to either side of thebracket. One nut was rounded to provide a slip fit for the drinking strawcoil form, and a clearance hole for the tuning screw was drilled throughthe coil mount.

The coil mount was soldered to the copper substrate of the main pieceof single-sided PC board material, and a clearance hole for the tuning screwwas drilled in the double-sided PC board front panel. Using the tuningscrew as a guide, a brass nut was soldered to the outside of the front panel.Having this nut, and the pair of nuts on the coil mount itself, separated bya little over a quarter of an inch provides good physical stability for thetuning screw.

Another small piece of PC board material is soldered into place as astrut between the upper edge of the tuning coil mount and the inside of thefront panel, rigidly holding the assembly in place.

The lower end of the tuning range of the LO was set by adjusting thevalue of C36, in the circuit diagram shown below, in addition to that ofC22. The final tuning range is from just below 3.000 MHz, to about 3.165

First IF Amp, IF Filter, and 2nd IF Amp

Double-Tuned Receiver Bandpass Filter, Cascode FET Mixer, and 1st

IF Am

MHz, which, with the 4 MHz IF frequency, provides for coverage of thelower 165 kHz of the 40 Meter band.

C37 was placed in parallel with C23 in the VFO circuit, so as to setthe signal injection level to the Receiver Mixer at just over 5 Volts, peak topeak. That injection level is recommended for this mixer, as detailed inthe S7C Receiver article in Experimental Methods in RF Design.

It is always a thrill to bring a receiver to life, even more than a trans-mitter, and listen to those first signals it receives. This receiver did notdisappoint. One very nice advantage of the all-discrete construction is thevery low noise level. When no antenna is connected no noise is heard atall. However, upon plugging in the antenna, it immediately comes to lifewith whatever signals are out there. This receiver has plenty of gain for 40Meters. Although it would be nice to have speaker-level audio output,there is more than sufficient audio when good quality stereo headphonesare used .

The VFO Buffer Amplifier was built next, and is shown below. Thelead taking RF to a jack for the outboard FreqMite Audio Frequency An-nunciator can also be seen. This Buffer Amplifier provides +7 dBm inputto the Transmitter Mixer.

Next to be added were an SBL-1 Mixer, the Transmitter Oscillator, aresistive, 50 Ohm, 6 dB attenuator pad, and the transmitter’s Double-TunedBandpass Filter.

The LO tuning coil can be seen at lower left in this photo. The crystaloscillator is the Transmitter Oscillator, with its output running through afilter to a resistive attenuator pad that sets the injection level at –10 dBm.


An SBL-1 diode ring mixer was used in this transceiver. Its case issoldered directly to the copper substrate, locking down its location. Thetwo transistors below the crystal oscillator are part of the Keying circuit.The toroid transformer at the right edge of the picture is part of the RFamplifier that follows the bandpass filter.

Two 12 uH surface mount inductors were placed in series with thecrystal in the Transmitter Oscillator in order to provide the correct amountof transmitter frequency offset to match that of the receiver.

A 50-Ohm Input, 50-Ohm output RF Amplifier, an LC ImpedanceMatching Network, and the Transmitter Buffer Amplifier were added next,and are shown next.

The trimcap at far left is part of the Transmitter’s Bandpass Filter.The trimcap at lower right of center is part of the LC impedance matchingnetwork that transforms the 50-Ohm output of the RF Amplifier to the 2.2K Ohm impedance set by a resistor at the input to the Buffer Amplifier.The trimpot is in the emitter lead of the Buffer Amplifier and is where thesignal is tapped off going to the Driver Amplifier.

Completing the transmitter, the Driver, Power Amplifier, and output

be needed in order to achieve the desired result.In a Superhet design we depend on signals from the BFO and the

Transmitter Oscillator to mix with signals from the LO or the IF to trans-mit and receive on another frequency. Those oscillator outputs need to beclean and free from spurious output or the result will be either a spuriousoutput from the transmitter or a very rough and distorted audio output

filter were added next. The Diode T/R circuit can be seen just below thePA heat sink. The toroid to the lower right of the PA transistor is the RFChoke in its collector circuit.Construction Summary

As usual with any electronics project, changes have been made in thistransceiver since it was first completed in order to improve its operation,ensure that it functions completely as desired and ensure that its transmit-ter output more than meets FCC specifications for a rig of this power level.

All changes were very easily made since there was no pc board to re-design and order and no Manhattan type pads to break away from thecopper substrate. Parts were simply unsoldered and removed, new partswere installed in whatever new location was needed, and the transceiverput in operation once again.

Those changes are discussed in detail in a follow-up article to thispresentation, coming in the next issue of HOMEBREWER.Closing Thoughts

Quite a bit of circuitry goes into the making of a superhet-based trans-ceiver. However, taken one stage at a time, it is a relatively simple matterto pick and choose proven circuits and use them to complete your owndesign.

A few key considerations come quickly to mind. Impedances shouldbe properly matched between stages. Signal levels between stages shouldbe adjusted so that subsequent stages can function as intended. Overdrivingeither amplifier or mixer stages very definitely causes spurious output.When the rig is built one stage at a time, so that the performance of thestages can be evaluated, some adjustment or even stage substitution may

VFO Buffer Amplifier

TX Osc, Mixer, 6 DB Attenuator, and Double-Tuned Bandpass Filter

RF Amplifier, LC Impedance Matching Network, and Buffer Amplifier

Buffer Amp, Driver, Power Amp, Output Filter, and Diode T/R Circuit


The addition of the Keying Circuit, and a Receiver Muting Circuit com-pleted the transceiver.

The author may be reached by mail at 2379 Saint George Drive,Concord, California 94520, or by email at [email protected].

NOTE: The full schematic of the 40-Meter transceiver appears on the next page.

from the receiver.In this transceiver, the outputs from the LO and the BFO were fine,

but the output from the Transmitter Oscillator was not. The fix was simplyto place a filter between the Transmitter Oscillator’s output and the resis-tive attenuator pad used to adjust the signal injection level to the Transmit-ter Mixer. This is detailed in Part II of this article.

If you get stuck there is nothing wrong in asking for help, either onlineat such places as QRP-L, or individually to any of several more experi-enced designers. Wes Hayward, W7ZOI, provided me with very valuableguidance on this project, particularly with avoiding the pitfalls in design-ing the transmitter chain from the Transmitter Mixer through to the emitterfollower Buffer Amplifier. Many thanks, Wes.

Designing a rig can be a lot of fun. Building, testing, adjusting, modi-fying, and, finally, operating it on the air is even better.

Give the Nuts and Bolts approach a try!

Another view of the completed transceiver. Note that the circuitry is laid out in a “U” shape, beginning with thereceiver audio amps in upper left, the product detector and BFO next, then the IF amps and dual crystal filter, thecascode FET mixer, and the double-tuned Receiver Input Filter in upper right. Centered behind the front panel is thepermeability tuned VFO, and to its left are the Keying Circuit and Receiver Muting Circuit.

Be sure the check out the online HB Extra! web pages on the AmQRPwebsite containing a complete set of full-color and full-resolutionimages presented here in this NB6M article. Point your browser to towww.amqrp.org/hbextra.

AmQRP Homebrewer, Issue #4 1 Copyright 2005, AmQRP, All rights reserved

Wayne McFee, NB6M

A Nuts and Bolts Approach to RF Design ... Part 2 This homebrew transceiver was the subject of a presentation made during the QRP Symposium at Pacificon 2003. I focused on the process of designing and building this transceiver from scratch by selecting portions of proven circuits and stringing them together in order to achieve the desired result. In the previous issue, I presented a step-by-step approach to designing and building this rig using some simple "nuts and bolts" techniques. This time I’ll take you through the detailed circuit analysis, measurement and tuning procedures.

DESIGN OVERVIEW

When attention is paid to appropriate detail, building this or a similar transceiver is well within the ability of many homebrewers. Please refer to the schematic diagrams presented in Part I, as the discussion continues.

As can be seen in the photos, the entire transceiver was built "Ugly style" with discrete parts, using no Manhattan style pads or other devices.

Laying out the circuit in a logical and straightforward fashion, almost the same as the circuit diagram itself, kept interstage interference to a minimum, and allowed space for future modifications and improvements.

It has been shown that building RF circuitry onto a solid copper substrate helps to ensure that circuits perform more cleanly, spectrum wise. Evidence also shows that circuits constructed this way simply work better, and more as designed. An additional benefit of building this way is that the circuit can be built very rapidly, and then can be changed easily, as needed.

In this particular case, a simple superhet design was used as the base, and proven circuitry from many sources was added in order to complete the transceiver design.


Key ingredients for ensuring the success of the design were appropriate impedance matching and signal level adjustment between key stages. Taking appropriate steps to ensure the spectral purity of the transmitter output also provided for good results in the end.

An analog VFO using a permeability tuned variable inductor was chosen as the local oscillator (LO). This oscillator is quite stable, inexpensive, relatively easy to build, and allows for wide frequency coverage. In this case, the rig covers the bottom 165 kHz of the 40 Meter band.

VFO output was routed to the receiver mixer through an NP0 capacitor, the value selected so as to provide just over 5 Volts peak-to-peak to the mixer. Because the VFO Buffer Amplifier chosen for the transmitter chain required a signal taken from a source at DC ground potential, a 9 turn link was wound around the tuning coil, and the VFO signal was taken from there to the buffer amp.

Output of the buffer amplifier provides a little better than +7 dBm for the transmitter mixer chosen, a diode ring mixer, and also provides more than ample output for the FreqMite CW Frequency Annunciator. Although different mixers could be employed, the desire to use discrete devices and the spectral purity benefits of using a balanced mixer as a transmitter mixer, dictated the choice here.

The output from the Transmitter Oscillator was routed through a resistive attenuator pad so as to set the TXO injection level to the Transmitter Mixer to approximately -10 dBm.

One of the inputs to the diode ring mixer needs to be of sufficient amplitude to ensure that the diodes conduct fully on both halves of the input cycle. Hence, the +7 dBm level was chosen, as set by the VFO Buffer Amplifier. However, the second input needs to be kept quite low, about -10 dBm, so as to minimize spurious output from the mixer.

In addition to this, the mixer output was terminated in a 50-Ohm resistive 6 dB attenuator pad so as to provide a true 50-Ohm termination for all RF from the mixer. This was followed with a double tuned, 50-Ohm input to 50-Ohm output bandpass filter so as to select the proper signal from the mixer output for amplification by the transmitter amplifier chain.

A 50-Ohm input, to 50-Ohm output RF amplifier was added next in order to compensate for losses introduced by the 6 dB attenuator pad and the double-tuned bandpass filter, and to boost the signal to a usable level for the transmitter’s driver amp. An LC impedance matching network follows this RF amplifier, in order to transform the 50-Ohm output of the RF amplifier to the 2.2 k Ohm input at the base of the transmitter driver amplifier.

Driver amplifier, power amplifier, and T/R circuitry, all borrowed from popular QRP transceiver designs, completed the transmitter. The initial transceiver design was completed with the addition of the muting circuitry.

Preliminary Results

Even in its first form, after initial construction, this transceiver showed very good promise. The receiver has a very low noise level and has plenty of sensitivity for the 40 Meter band.


Although the dual crystal IF filter might not be narrow enough for some, it does provide good opposite sideband rejection and has a nice, crisp, clean sound.

While the audio amplifier chain in this receiver does not supply a ton of audio, the audio output is quite sufficient and is very clean sounding as long as good quality stereo earphones are used.

The transmitter puts out 1.5 to 2.0 Watts, which is more than enough for many contacts. This lower power level was chosen because of readily available outboard power amplifiers for those times when band conditions warrant.

As with all construction projects, changes are made when the actual building starts. Some changes are made simply because of having substitute parts already on hand. Others are made because new ideas prompt improvements or additions to the original circuit. Further, some are made necessary because the final results may not be quite up to expectations.

In this project, once the injection levels to the receiver and transmitter mixers were properly set, and the receiver and transmitter frequency offsets were adjusted, the basic receiver and transmitter sections worked very well. The analog VFO is quite stable and it is really nice to be able to tune all of the CW portions of the band. Although it would certainly be nice to have a digital VFO, this one is so easy to build and gives such good results with so few parts that it is well worth using in more than one project. The addition of a FreqMite CW frequency annunciator, or another frequency reporting device, gives us the complete confidence of knowing exactly where we are in the band.

However, even without a digital readout or a FreqMite-type annunciator, it would be difficult to stray outside the band. The IF is exactly on 4.000 MHz, and when the LO is tuned to 3.000 MHz, effectively putting the rig on 7.000 MHz, a birdie is heard in the receiver, providing a very effective band edge marker. This birdie, plus the fact that the upper end of the tuning range is 7.165 MHz, helps to ensure that all transmissions stay inside the band.

One area of the transceiver that needed improvement was in the receiver muting circuit. A definite goal of this project was to be able to hear the transmitter’s signal in the receiver. There are several advantages this, not the least of which is that you have a ready indicator that the transmitter is actually working. In addition, once the receiver and transmitter offset frequencies are adjusted so as to coincide, all one has to do in order to zero beat another station’s frequency is to tune the rig so that the received tone matches the tone heard on transmit.

In this particular case, the JFET mute switch inserted between the audio preamp and the audio output amp did not quite produce the desired effect. Although it did provide some muting, the audio tone was too loud and had a strident quality, which indicated that muting was needed in previous stages of the receiver.

The first step in evaluating the problem and determining a correction was to move the muting circuit from between the two audio amplifiers, and place it between the product detector and the audio preamp. This did not correct the problem, so an experiment was conducted to see what audio level and quality would be had if Vcc was removed from the two IF amplifiers on transmit.


In order to answer this question, the 12-Volt leads to the two IF amps were unsoldered and disconnected. The results were that, on transmit, the audio tone quality was good and the audio level was just about right. So, the following circuit was inserted between the 12-Volt bus and the Vcc leads to the two IF amps.

The audio tone heard on transmit was louder than it was with the two DC leads simply disconnected. This was probably due to capacitive coupling, although this simple switching circuit did shut off the Vcc to the IF amps completely,

In an experimental effort to see if using another type of switching circuit would provide better results, the switching circuit shown below was installed. The results were the same. Although the audio tone heard was of good quality, it was still too loud at the highest settings of the receiver’s volume control.


On-the-air experiments were conducted to determine if this was truly going to be a problem, or if it was a situation where the audio level on transmit was at an acceptable level at the normal settings of the volume control.

In practice, it was found that almost all of the time the volume control was at less than maximum, and the audio tone on transmit was at a comfortable level. However, in those situations where it had been necessary to have the volume control at maximum, one had to turn the volume down before transmitting, in order to keep the audio tone at a comfortable level.

Further experimentation showed that a keyed, bipolar transistor switch, connected in such a way as to short the audio from the wiper of the volume control to ground on transmit, gave a very satisfactory reduction in sidetone volume, and a much nicer sounding tone as well. Here is the circuit used.

Two other areas of concern cropped up, and showed the advantage of having a spectrum snalyzer on hand while the circuit was being built. Using the SA, I could actively check and modify stage performance as each stage was added.

First of all, the output of the transmitter did not initially meet FCC specs. There were two spurs – one at 6.88 MHz, and one at 7.20 MHz – that were -28 and -30 dBc (dB below the carrier) respectively, as shown in the spectrum analyzer photo below.


Spectrum analyzer shows spurs. (Test and photo courtesy of W7ZOI)

FCC specs for a transmitter with a power output less than 5 Watts require all spurious output to be more than 30 DB below the carrier, or main signal. The fact that the spurs were relatively close to the carrier frequency, definitely not harmonics of the carrier, and the fact that the output from the VFO Buffer had previously been checked, led to investigation of the output of the Transmitter Oscillator, which turned out to be the culprit.


Differences Between Receiver BFO and Transmitter Oscillator

The receiver’s BFO frequency offset was accomplished with the addition of a 56 pF capacitor in series with the BFO crystal. This was done to utilize the lower sideband as the mode of reception in this 40 Meter rig.

BFO

Adding capacitance in series with the crystal raised the oscillator frequency. The amount of capacitance was arrived at experimentally by adjusting the value so as to provide enough offset to give good rejection of the opposite sideband, while allowing for reception of the desired lower sideband.

The BFO output waveshape is a pretty nice looking sine wave, indicating little harmonic energy. This is borne out by the fact that the receiver’s audio output is very clean sounding.

The Transmitter Oscillator’s frequency was adjusted downward by the addition of two 12 uH inductors in series with the TX Oscillator crystal. This arrangement closely matched the transmitter’s frequency offset to that of the receiver, but in the opposite direction frequency-wise. This places the transmitted signal very close to zero beat of a received signal when the rig is tuned so that the receiver’s audio output tone matches the frequency of the transmitter’s sidetone. This addition is shown below.


Transmitter Oscillator Mods

Again, the amount of inductance needed was determined experimentally.

In this case, on investigation, the waveshape of the Transmitter Oscillator’s output was not a clean sine wave, indicating the presence of harmonic energy. As shown in the preceding photo of the transmitter’s output on the Spectrum Analyzer, this harmonic energy caused strong spurs to appear relatively close to the main carrier frequency. The answer, and correction of the problem, was to filter the output of the Transmitter Oscillator, before its output was applied to the Transmitter Mixer.

A five-element Chebyshev filter, with a cutoff frequency of 4.5 MHz, was added to the output of the Transmitter Oscillator, as shown above.


The addition of this filter completely eliminated the close-in spurs, as shown in the Spectrum Analyzer picture below.

(Test courtesy of WA6AYQ, photo by NB6M.)

The second area of concern was that the wideband spectrum analysis of the transmitter’s output showed that the second harmonic was at –33 DBC, in spite of the fact that all harmonics of the carrier were more than –30 dBc. Refer to the photo below.



While this was certainly within FCC specs for transmitters at this power level, further harmonic attenuation was desired.

A five-element, halfwave output filter had been used in the transmitter’s final amplifier stage. In order to further attenuate the second harmonic, and to see just how much difference it would make in the purity of the output, a seven-element Chebyshev filter was installed in the transmitter. This filter has a cutoff frequency of 8.5 MHz.

7-Element Filter

Although a seven element-filter is a bit of overkill in this case, it was interesting to see the improvement made in the spectral purity of the transmitter output.


As the Spectrum Analyzer photo shows below, the addition of this filter, all spurious output is now reduced to approximately –60 dBc.


Suggestions to Builders

Obviously, we all would love to have a complete set of laboratory test equipment such as spectrum analyzers, tracking Generators, and other high priced gear. Not all of us can afford to purchase such items, however, and we may not yet have the skills to build our own. What can one do with limited equipment in order to help ensure that a homebrew rig such as this operates as intended, and has a transmitter output that should meet current FCC specifications?

First of all, proper signal injection levels can be set for key stages such as the transmitter mixer. To do this the output of stages such as the VFO Buffer and the Transmitter Oscillator should initially be terminated with a 51-Ohm resistor, without running the signal further. Then the output can be measured with either a high impedance voltmeter and RF probe or with an oscilloscope, and the signal level adjusted as necessary.

The appropriate signal levels are often stated, such as the minimum of +7 dBm needed for one of the inputs to a diode ring mixer. These levels relate to the amount of power provided to a true 50-Ohm load, such as a 51-Ohm resistor used to provide termination for measurement purposes, and are not what would be read at the input of the mixer itself once the signal is properly routed.


In this case, the output of the VFO Buffer was adjusted by simply adjusting the number of turns of the link providing VFO energy to the amplifer, thus setting the output at just approximately +8 dBm.

In the case of a homebrew diode ring mixer using 1N914 devices, it takes a minimum of 0.7 Volts to make one conduct, which means that one of the two signals fed to the mixer should be at least a little over 1.4 Volts p-p. 5 milliwatts, or 0.5 Volts rms, is 1.414 Volts p-p, which is 6.9897 dBm. Therefore, it would seem reasonable that in this case, the VFO Buffer output should be set just little above that level. 7.2 milliwatts, or 0.6 Volts rms, is 8.5733 dBm, and should be adequate for the 1N914 diodes used in a homebrew mixer. Remember, this measurement is made when the output is connected across a 51-Ohm resistor to ground, not when it is connected to the mixer.

This is also true of setting the injection level from the Transmitter Oscillator. A 51-Ohm resistor to ground is connected to the output and the measurement is taken. The dBm level is calculated, and a 50-Ohm resistive attenuator pad is selected to provide the needed reduction. The pad is inserted between the Oscillator output and the mixer. Resistor values for pi-type 50-Ohm attenuators are easily found in such references as the ARRL Electronics Data Book.

As an example, let us say that we measured 0.45 Volts rms across the 51-Ohm resistor terminating the oscillator output. The 0.45 Volts rms signal is 4.05 milliwatts (rms, squared, divided by 50), which is 6.07 dBm (4.05, divided by 1, LOG, times 10). Let’s round that off to +6 dBm.

In order to set the injection level to –10 dBm, we need to reduce the signal level by 16 dB. According to the table on page 5-2 of the ARRL Electronics Data Book, we need resistances of 153.8, 68.8 and 153.8 Ohms, respectively, for a 16 dB, 50-Ohm impedance, pi-type attenuator. We use the closest values, 68 Ohms and 150 Ohms, and there we are.

Now, if one doesn’t have a Spectrum Analyzer on hand, in order to evaluate the outputs of stages such as the VFO Buffer and Transmitter Oscillator, how can we tell if the signal coming from these stages is relatively clean?

An Oscilloscope is needed in order to check the wave shape of the signals when they are terminated in the purely resistive 50-Ohm load. If the wave shapes appear to be clean sine waves, then there should be little harmonic energy. If the wave shape is distorted, then harmonic energy is definitely present, and a filter such as what was used in this rig between the Transmitter Oscillator and the resistive attenuator, should be installed. A 51-Ohm resistor should be connected from the output of the filter to ground and the waveshape should be checked again before routing the signal to the mixer.

The mixer itself will distort the wave shape of the inputs once they are connected, and for that reason, checking input wave shapes at the mixer will be misleading.

The use of the resistive 50-Ohm impedance, 6 dB attenuator at the output of the transmitter mixer helped ensure that harmonics of the two signal inputs were properly terminated. The double-tuned bandpass filter following the attenuator also selected the proper signal for amplification to the transmitter output.


The transmitter output filters with higher harmonic rejection can certainly help ensure that the signal radiated out the antenna is as clean as possible. Thus using the 7-element Chebyshev filter instead of a 5-element half wave filter improved the signal quality.

Thoughts for the Future

While this homebrew transceiver is fun and easy to operate, I am quite sure that it has not reached its final state of development.

Different audio amplifier circuits could be added to give a speaker output with plenty of volume. RIT could also be added to the receiver. The keyer should be built in. The FreqMite or another type of frequency annunciator would also be easy to build-in. Different types of mixer and amplifier circuits can be tried. Of course, there are other bands to build for, and on which to enjoy the operation of a totally homebrew rig.

This little rig has provided not only a wonderfully informative learning experience, but now provides the satisfaction of being able to say "rig HB, from scratch" to each person in QSO. There is tremendous pride in knowing that the rig meets and surpasses expectations on almost all levels. Even more satisfying is the joy of sharing information and ideas with others who may be interested in trying scratch building as well.

Wayne McFee, NB6M may be reached by mail at 2379 Saint George Drive, Concord, California 94520, or by email at [email protected] All material in HOMEBREWER is copyright 2005 and may not be reprinted in any form without express written permission from the American QRP Club and the individual author. Articles have not been tested and no guarantee of success is implied. Safe construction practices should always be followed and the builder assumes all risks. HOMEBREWER Magazine is a quarterly journal of the American QRP Club, published on CD-ROM. Each issue typically contains over 200 pages of QRP-related homebrewing construction and technical articles intended for builders, experimenters, ham radio operators and low power enthusiasts all around the world. HOMEBREWER features include construction projects for beginners all the way up to the advanced digital and RF experimenters. Annual subscriptions are $15 (for US & Canada) and $20 (for foreign addresses). For information, contact editor/publisher George Heron, N2APB at [email protected] or visit HOMEBREWER Magazine home page at www.amqrp.org/homebrewer.


Wayne McFee, NB6M

A Nuts and Bolts Approach to RF Design ... Part 3 The Nuts and Bolts 20 If the rig in this picture looks familiar to you, it should! It looks a lot like the 40m Nuts and Bolts design that was detailed in the previous two installments. There is there is a major difference – this one is a 20m rig that was the product of taking that original design and bringing it a step forward. The result is a useful rig for one of the most popular HF DX bands.

As can be seen in the photo above, the construction layout and building method were the same as those used in the Nuts and Bolts 40. However, in order to put the rig on 20 Meters, several parts of the circuit required changes, and there were some additions, as well. The 40 Meter transceiver described in the previous two "Nuts and Bolts Approach" articles is fun to operate, has a very clean transmitter signal and a nice sounding superhet receiver, and has a stable VFO, which provides coverage of the entire CW portion of the band. It was even more fun designing it, building it, testing it, modifying and tweaking it so that it actually performed as desired. In this article, we will look at the changes necessary to make this basic design work, and work well, on a higher band. Because 40 and 20 Meters have always been the two most popular HF bands, it seemed only natural to design and build a transceiver for 20 Meters, since the first Nuts and Bolts transceiver was designed for 40. DESIGN REVIEW


What are the changes needed, in order to make the switch from 7 MHz to 14 MHz operation? In order to clarify our needs, let’s go back and take a quick look at the basic structure of the Nuts and Bolts 40. Here is the Block Diagram.

Remember that we used a simple superhet design, the S7C, borrowed from "Experimental Methods In RF Design" i as the basis for designing this transceiver. An IF frequency of 4.000 MHz was chosen, and a 3 MHz VFO was used so that the transmitter mixer output and receiver mixer inputs would be on 7 MHz. Looking at the block diagram above, the key things that make this transceiver operate well on 40 Meters are: • T/R circuit tuned to 7 MHz • Receiver Preselector Filter tuned for the 40 Meter band • VFO frequency coverage of 3.000 to 3.165 MHz • IF Filter frequency of 4.000 MHz • BFO crystal frequency 4.000 MHz, with the oscillator frequency tuned so that a received

LSB signal centered in the IF filter passband provides a 600 Hz tone in the headsets • Transmitter Oscillator crystal frequency 4.000 MHz, with the oscillator tuned so as match

the transmitter signal frequency to a received signal frequency that is centered in the IF filter passband.

• Transmitter Oscillator output filter, with a 4.5 MHz cutoff frequency (Chebyshev 5 element)

• Transmitter Bandpass Filter tuned for the 40 Meter band • Impedance matching circuit (Pi Network) between the Transmitter Post-Filter Amplifier

and the Transmitter Buffer Amplifier tuned to 40 Meters


• Transmitter output filter (Low-Pass, 7 Element Chebyshev) designed with an 8.5 MHz cutoff frequency, so as to provide great attenuation of any harmonics of the transmitter signal.

CHANGES NEEDED FOR 20 METER OPERATION Beginning with the receiver, the basic changes we need to make are: • Select appropriate IF and VFO frequencies • Change the BFO crystal frequency to match the IF frequency • Change the T/R circuit so that it is tuned to 14 MHz • Change the Receiver Preselector Filter so that its passband centers on the lower half of

the 20 Meter band IF and VFO FREQUENCY CONSIDERATIONS First let’s address the IF and VFO frequency selection. In the Nuts and Bolts 40, several different IF/VFO frequency pairs could have provided for 7 MHz operation. For those of us building crystal filters for ourselves, relatively cheap microprocessor crystals are available in a wide range of frequencies. Using these for the IF, the following are some of the mixing schemes giving coverage of the lower 150 KHz of the 40 Meter band: • 4.915 MHz IF and 2.085 to 2.235 MHz VFO, mixing IF + VFO • 10.000 MHz IF and 2.850 to 3.000 MHz VFO, mixing IF – VFO • 4.000 MHz IF and 11.000 to 11.150 MHz VFO, mixing VFO – IF • 9.000 MHz IF and 1.850 to 2.000 MHz VFO, mixing IF – VFO • 12.000 MHz IF and 4.850 to 5.000 MHz VFO, mixing IF – VFO • 4.000 MHz IF and 3.000 to 3.150 MHz VFO, mixing IF + VFO Here are some of the factors that should be considered when choosing IF and VFO frequencies: First, the IF and VFO frequencies should be selected so that they are not harmonically related to each other. This is done so that no strong birdies occur in the receiver, but, more importantly, so that bandpass filters can effectively screen out any harmonic output of the basic IF and VFO frequencies, to help ensure the spectral purity of the transmitter. A second consideration is the type of VFO we are going to use. In this case, we are going to use an analog VFO, and it is easier to build a stable VFO for relatively low frequencies than it is for higher frequencies. A third consideration is whether or not we would like to modify the basic design so as to provide dual, or multiple band operation in the future. And, as always, we want to use parts that we have on hand, rather than buying more.


At the time the Nuts and Bolts 40 was being designed, the crystals on hand were 4.000 MHz and 9.000 MHz. Using a 4.000 MHz IF and 11 MHz VFO was ruled out, due to the probable instability of the VFO. The 4.000 MHz IF and 3 MHz VFO were chosen over the 9 MHz IF and 2 MHz VFO scheme because of the fact that a 3.0 MHz VFO could be used with the 4.000 MHz IF, by the addition of a frequency doubler, to add 30 Meter operation at a future time. Now that we want to change the design so as to put the rig on 20 Meters, and given that 9.000 MHz crystals were already on hand, using them seemed the right thing to do since a 9.000 MHz IF and a 5 MHz VFO would allow for the addition of modifications in the future which would give the rig the capability of operating on both 20 and 80 Meters. IF FILTER DESIGN The G3UUR method for measuring quartz crystal parameters, described in "Experimental Methods In RF Design" ii, chapter three, under "Crystal Measurement and Characterization", were used in order to determine the Motional Capacitance and Motional Inductance of the 9 MHz crystals on hand. These figures were plugged into the crystal filter design program, XLAD iii, a program provided with the book, "Experimental Methods In RF Design", in order to design the two-pole, 500 Hz bandwidth IF filter used in the receiver. The filter evaluation program, GPLA iv, also provided with the book, was used to plot filter performance. Here is the filter used.

BFO CHANGES With the IF frequency chosen, and the IF Filter designed, BFO and TX Oscillator crystal frequencies fall into place, using crystals from the same 9.0 MHz batch on hand.


Trimmer capacitors were used in each oscillator, so that each could be finely tuned. The BFO circuit is below.

T/R CIRCUIT CHANGES In the Nuts and Bolts 40, a 12 µH inductor was used in series with two capacitors paralleled to give a total of 43 pF. This combination provided a series resonance of 7.006 MHz, which, in this low-Q application, gives good coverage of the low end of the band. In order to give us a series-resonant circuit for 20 Meters, a 27 pF capacitor and 4.7 µH inductor are used. This works out to resonance at 14.128 MHz. Again, because of the low-Q application here, that is close enough. The circuit below shows these parts as C69 and L18.


You will notice that the usual back-to-back arrangement of diodes, used to short the bottom end of C69 to ground on transmit, have been replaced here by a switched NPN transistor. This change was an experiment, which resulted in the same amount of muting as was achieved using the diodes mentioned before. This setup seems a touch fussier, needing a 12V+Tx line run to it, so the back-to-back diodes seem simpler. RECEIVER PRESELECTOR FILTER CHANGES Because of the fact that more RF gain is needed in a 20 Meter receiver than one to be used on 40, the changes in the Preselector Filter were incorporated into an RF Preamp. The RF Preamp selected is a 20 Meter version of the Low Noise Amplifier shown as part of the General Purpose Monoband Receiverv Front End, from "Experimental Methods In RF Design". Because of the addition of an RF Preamp to the receiver, the 10K pot used previously as an Audio Gain control was removed from the audio circuit and moved between the T/R circuit and the input to the RF Preamp, to act as an RF Gain control. See the circuit above, as well as the RF Preamp Circuit below.

Appropriate changes were made to the second half of the double-tuned filter on the output of this preamp, because of the fact that the input of the Cascode coupled Mixer used in our receiver is a high impedance. The second half of the double-tuned filter shown as part of the general purpose receiver front end is designed to work into a 50 Ohm impedance. Software is readily available to assist in designing double or triple-tuned bandpass filters, such as W7ZOI’s DOS based Dttcvi and newer, Windows based DTCvii programs. However, in this case, the changes were simplified by the fact that, since the FET is working into the high impedance load presented by the first half of the existing double-tuned filter, it was only necessary to duplicate those values in the second half. Note L4, C7 and TC3 in the circuit above.


Q1 is kept turned on by the switched 12V+Rx line, providing a DC ground path for the for the FET source. When that voltage is switched off, on transmit, Q1 is shut off, effectively providing about 40 DB of signal attenuation, helping to mute the receiver. VFO CHANGES The same basic, Permeability-Tuned VFO circuit from the Nuts and Bolts 40 was used in this rig. A J310 JFET was used as the oscillator. Changes were made to the frequency determining components, in that a single NP0 capacitor of 150 pF was used from the Drain of the oscillator to ground, C12, and the number of turns on the tuning coil was reduced to 35 turns, as shown below. These changes resulted in a VFO tuning range of from just below 5.000 MHz to over 5.150 MHz, when an 8-32 brass screw was used as the tuning element. A #10 brass screw gave complete coverage of over 350 KHz.

As was done in the Nuts and Bolts 40, the value of C15 above was selected so as to present a 5 Volt, peak to peak signal to the Receiver Mixer. The number of turns of the link taking VFO energy to the buffer amplifier can be adjusted as necessary to provide appropriate output from the buffer. With the above noted changes, the receiver section has now been re-designed for the 20 Meter band. Now, let’s take a look at the changes made to the transmitter.


TRANSMITTER CHANGES The first change made in the transmitter was to change the TX Oscillator crystal frequency to 9.000 MHz. A 22 µH inductor was used in series with the crystal, along with a 5-50 pF trimmer capacitor, so that the exact frequency could be adjusted to match the frequency of a received signal, as heard in the audio output in the form of sidetone. Remember, one of the design goals with the Nuts and Bolts 40 was to be able to hear the transmitter’s signal in the receiver. This provides both an indication that the transmitter is working, and, when the BFO and Tx Oscillator frequencies are properly adjusted, a ready reference as to what audio tone on receive indicates that the signal is zero beat with the transmitter. Also changed were the component values in the five-element Chebyshev filter used to prevent spurious output from the TX Oscillator from reaching the TX Mixer. This filter was designed using the computer program, LowHiviii, from "Experimental Methods In RF Design". The new component values give the filter a 10 MHz cutoff frequency. See the TX Oscillator circuit below.

Note also that, as in the Nuts and Bolts 40, a 50 Ohm resistive attenuator was used in order to adjust the output of the TX Oscillator to the appropriate level. PI and T network resistive attenuator values are charted in the ARRL Electronics Data Book. The next change in the transmitter is to the component values in the transmitter’s double-tuned bandpass filter. Again, software such as the above-mentioned Dttc program is readily available to assist the designer. One can plug design parameters such as desired bandwidth, center frequency, and termination impedance into Dttc, in order to receive both a circuit diagram and component


values. Another program that can be used along with the before-mentioned DTC or LowHi programs is GPLAx. This program takes files outputted from the design programs and allows one to plot the results. It goes without saying that for many of us who don’t have an engineering background, these programs are invaluable. Here is the new Transmitter Bandpass Filter circuit. It has a center frequency of 14.100 MHz, and a 250 KHz bandwidth.

As in the Nuts and Bolts 40, the Tx Mixer output is terminated in a 6 DB 50 Ohm resistive pad. Since the Transmitter Post-Filter Amplifier is broadband, no changes will be needed there. But we will need to change the impedance-matching Pi network between that amplifier and the Transmitter Buffer. It is important to note that the impedance levels throughout the transmitter thus far, from the VFO Buffer and Tx Oscillator, through the Tx Mixer and the double-tuned filter and Post Filter Amplifier, have been kept at 50 Ohms up to this point. The Pi network on the output of the Post Filter Amplifier transforms that 50 Ohm impedance to the 2.2 K Ohm impedance set by resistor R65, at the input to the Transmitter Buffer Amplifier. Using the software program, Zmatxi, we find that the values needed to transform 50 Ohms to 2.2 K Ohms at 14 MHz are approximately 215 pF, 2.99 µH, and 46 pF. We will use 220 pF, 32 turns on a T37-6 Toroid core, and a 15-65 pF trimmer capacitor, as shown in the circuit below.


Because of the fact that the Transmitter Buffer, Driver Amplifier, and PA circuit are broadband, there is one last change to make in the transmitter circuit, and that is the final Output Network. In the Nuts and Bolts 40, we found that a seven element Chebyshev filter did a really great job of suppressing harmonics of the carrier frequency. So, as they say, "if it ain’t broke, don’t fix it". Using the program LowHi, once again, a seven element, Chebyshev filter with a 15.5 MHz cutoff frequency was designed. The component values are shown in the circuit below.

Filter response plotting in GPLA showed that setting this filter’s design cutoff frequency at 15.5 MHz provided the desired harmonic rejection, and at the same time, did not attenuate the 14 MHz signal. While, again, this seven element filter is a bit of overkill, the results are very comforting, in terms of spectral purity. The transmitter’s two fundamental signal frequencies, the 5 MHz VFO signal, and the 9 MHz Tx Oscillator signal, are at –50 DBC. The second harmonic of the Tx output signal is at –60 DBC, with subsequent harmonics even lower in strength, as shown in the Spectrum Analyzer photo below.

Photo by NB6M, test courtesy of WA6AYQ.


FINAL TWEAKING The last item that needed to be addressed was the receiver’s muting scheme. In the Nuts and Bolts 40, the two IF amplifiers were switch on for receive, and off for transmit, by the following circuit.

And, a secondary muting circuit, consisting of a 1 µF capacitor, shorting the audio signal going from the wiper of the audio gain pot to ground on transmit, was also used. The same setup was tried in this rig. The audio signal going from the Product Detector to the Audio Preamp was shorted to ground through a 1 µF capacitor, on transmit, by a switched 2N3904 transistor, as shown in the circuit below.


It was soon apparent that in this receiver, with 40 DB of muting being achieved on transmit by turning the Receiver RF Preamp off, turning both IF amps off as well as using this audio mute circuit resulted in almost no sidetone being heard at all. Next, the receiver circuit was modified so that only one of the IF amps was turned off on transmit. With that done, the sidetone was now a bit too loud. So, the value of the capacitor used to short audio to ground in the audio muting circuit was increased from 1 µF to 10 µF. This reduced the sidetone level considerably, but still not enough, as the sidetone was still too loud at high settings of the RF Gain control, and the sidetone did not have a good, clean sound. The third modification to the muting scheme involved removing the above Rx Mute circuitry, and installing a J176, P-channel JFET switch between the audio preamp and the final audio output amp, as shown in the circuit below.

The J176 is switched off on transmit by the application of 12V+Tx. And, R38, 2.2 M Ohms in this case, can be adjusted in value to set the sidetone level. It was found that with this muting circuit in place, neither of the two IF amps needed to be turned off on transmit. Having the receiver’s RF Preamp switched off, and having the J176 circuit above switched off, provided for a very comfortably low level of clean-sounding sidetone. Therefore, the receiver IF circuitry was modified so that both IF amps receive 12V+ continuously. The final, very satisfactory muting scheme involves just three key items. First, either the familiar back-to-back diodes or the switched transistor in the T/R circuit, shorting the Tx RF to ground on transmit. Second, turning the Receiver RF Preamp off on transmit, through the means of the transistor switch placed in the Source lead of the JFET. Third, using the P-channel JFET switch between the Audio Preamp and the Audio Output Amp, with its bridging resistor value adjusted so as to provide the desired amount of sidetone.


CONSTRUCTION As can be seen in the photo below, this rig was laid out exactly as its predecessor.

NUTS AND BOLTS 20 CONSTRUCTION OVERVIEW See the construction photos and accompanying text from Nuts and Bolts Approach to RF Design, Part I xii for details of the construction of each stage. The dual trimmer capacitors associated with the Receiver RF Preamp can be seen in the lower left corner, just to the right of the RF Gain pot. Visually, the addition of this amplifier is about the only striking difference between the NB40 and the NB20, other than the relative shortness of the tuning coil in this rig as compared to the 40 Meter version.


A close-up shot of the RF Preamp is below.

Nuts and Bolts 20 Receiver RF Preamp

Its muting transistor is at upper center, and the two transistors at lower right are part of the Cascode Mixer. The NP0 capacitor in upper right is the 4.7 pF cap bringing VFO energy to the mixer. The photo below shows the BFO, and, just above the crystal, peeking out from under the Tx Final Amp transistor is the transistor switch used in the T/R Mute circuit.

Nuts and Bolts 20 BFO and T/R Mute

In both the BFO and Tx Oscillator, 22 µH inductors and 5-50 pF trimmer capacitors were placed in series with the crystal in order to accurately tune their respective offset frequencies.


A shot of the Tx Oscillator is below. The two toroid coils are part of the five element Chebyshev filter used at its output to help ensure spectral purity of the transmitter.

Nuts and Bolts 20 Tx Oscillator and Chebyshev filter

As can be seen from the pictures, Ugly Style construction is Three-Dimensional. With leaded parts, the resulting structure is quite mechanically stable. Both the NB40 and NB20 have survived long distance trips at the hands of the U. S. Mail Service, which, of itself, is testimony to their durability. This type of construction could be made even more durable by the use of compounds such as RTV to fix toroid coils and other less rigid parts in place. If you haven’t yet tried this construction method, you will be pleasantly surprised at how quickly circuitry goes together. The only real trick to it is in soldering parts together in logical order so that one part isn’t unsoldered when the next one is being installed. This technique will be quickly learned, with just a little practice. CLOSING THOUGHTS Re-designing the Nuts and Bolts 40 so as to put it on 20 Meters was a really fun project, which resulted in a rig design that performs well on all levels. The transmitter’s output was left at 1.5 Watts because of the ready availability of an outboard Miniboots amp to boost the signal when needed. The spectral purity of the transmitter is excellent, much better than that required by current regulation. The receiver is sensitive enough to pull QRP signals through on 20 Meters, and the VFO provides stable coverage of over 150 KHz at the low end of the band. And, after some experimentation, the receiver muting is clean and efficient, providing a nice sounding sidetone from the transmitter’s signal.


The complete circuit for the transceiver follows, at the end of this article. Today, more than at any other time in Amateur Radio history, we have both information and tools at our instant beck and call, through the medium of the internet. Currently available computer programs allow those of us who need a little help with the math to design segments of our projects that were beyond our reach before. As you will be able to tell from the footnote references below, I have been very pleased with the arrival on the scene of the book, "Experimental Methods In RF Design". While it does cost a few dollars more than one might, at first, think of spending, I can only say that it is worth every penny, in terms of ideas and methods, as well as the included software programs. For the homebrewer, EMRFD has taken "Solid State Design For The Radio Amateur" a huge quantum leap forward. I would like to express my thanks to Wes Hayward, W7ZOI, Rick Campbell, KK7B, and Bob Larkin, W7PUA, for giving us this wonderful addition to our reference library. Using the methodology of the Nuts and Bolts approach, and these tools, many of us can either design and build rigs of our own, or modify existing designs so as to put them on the band of our choice. If you are not yet comfortable with building in the Ugly Style, as these transceivers were built, use whatever method works for you. But, at the very least, give it a try. Enjoy, Wayne NB6M * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

i. Hayward, Campbell, and Larkin. "Experimental Methods In RF Design", ARRL, 2003, pages 12.16 through 12.19

ii. Hayward, Campbell, and Larkin, "Experimental Methods In RF Design", ARRL, 2003, pages 3.18 and 3.19

iii. Wes Hayward, W7ZOI, © 2002, "Experimental Methods In RF Design", ARRL, 2003 iv. Wes Hayward, W7ZOI, © 2002, "Experimental Methods In RF Design", ARRL, 2003 v. Hayward, Campbell, and Larkin, "Experimental Methods in RF Design", ARRL, 2003,

page 6.32 vi. Wes Hayward, W7ZOI, ARRL, 1994 vii. Hayward, Campbell, and Larkin, "Experimental Methods In RF Design", ARRL 2003 viii. Hayward, Campbell, and Larkin, "Experimental Methods In RF Design", ARRL 2003 ix. Doug DeMaw, "The ARRL Electronics Data Book", ARRL, 1988-96, page 5-2


x. Hayward, Campbell, and Larkin, "Experimental Methods In RF Design", ARRL 2003 xi. Wes Hayward, W7ZOI, ARRL, 1994 xii. Wayne McFee, NB6M, "A Nuts and Bolts Approach To RF Design", AmQRP

Homebrewer Issue #3, Winter 2004 xiii. Wes Hayward, W7ZOI, and Doug DeMaw, W1FB, "Solid State Design For The Radio

Amateur", ARRL, 1977 Wayne McFee, NB6M may be reached by mail at 2379 Saint George Drive, Concord, California 94520, or by email at [email protected] All material in HOMEBREWER is copyright 2005 and may not be reprinted in any form without express written permission from the American QRP Club and the individual author. Articles have not been tested and no guarantee of success is implied. Safe constructions practices should always be followed and the builder assumes all risks. HOMEBREWER Magazine is a quarterly journal of the American QRP Club, published on CD-ROM. Each issue typically contains over 200 pages of QRP-related homebrewing construction and technical articles intended for builders, experimenters, ham radio operators and low power enthusiasts all around the world. HOMEBREWER features include construction projects for beginners all the way up to the advanced digital and RF experimenters. Annual subscriptions are $15 (for US & Canada) and $20 (for foreign addresses). For information, contact editor/publisher George Heron, N2APB at [email protected] or visit HOMEBREWER Magazine home page at www.amqrp.org/homebrewer.

homebrew rf transceiver design

Documents

design philosophy

our40 meter transceiver

s7c circuit

s7c receiver

nuts bolts approach

electronic circuit

proven circuits

able discrete parts