vhf baixa com tda7000

18

VHF-LowExplorer

elektor electronics - 4/2004

Gert Baars

This article, we hope, will have serious consequences.Not negative, mind you, because apart from enablingbeginners to experience the thrill of picking up radiotraffic in a generally neglected band, the arrival of thisinexpensive 4-m receiver should help to get the IARUsection of the 4-m band released to radio amateurs incountries all over the world in due course.

A low-cost NBFM receiver for 68-88 MHz

For many decades the UK has beenhome to the 4-m amateur radio band,also known as 70 MHz, and theIARU-approved band plan shown inFigure 1 is on the wall in many a radioshack up and down the country. So far,only a few other countries includingIreland, Denmark and quite recently Croatia have followed suit byreleasing the frequency rangebetween 70.000 and 70.5000 MHz foruse by licensed radio amateurs. Unfor-tunately, in many other countries therelevant frequency range is in use bygovernment or military services whichneed to have a few arms twisted (punintended) before they move out.Although it is too early to say whetheror not the arrival of new communica-tion systems like Tetra, C2000 andAstrid on the European Continent andelsewhere will free up the 70 MHzband to amateurs, there can be noharm in increasing the pressure onvarious national radio regulatoryauthorities to do the necessary paper-work. At least in Holland and Poland,the word is out that amateurs areinterested in the 70 MHz band. Letshope the pressure rises as the 4-mband is fantastic for VHF DX-ing. Elek-tor Electronics being an internationalpublication, we will gladly assist inspreading the word in as many coun-tries as possible.

Whats in it for meWhile the radio amateur fraternity ispoised to grab their share of thespectrum around 70 MHz, it shouldbe noted that the 68-88 MHz bandhas other, equally interesting usersand applications including Govern-ment, MoD and PMR (private mobileradio) communications (notencrypted in many cases), securityservices, telemetry and the odd TVstation. Unless you live in a reallyremote place, even a simple antenna

in your loft will bring in a surprisingnumber of stations using the 4-mband. Tune and Explore!

Designconsiderations

From the very start of this project, thedesign was to remain as simple aspossible. This decision has importantconsequences as well as a back-ground wed like to share with you.Sure, a receiver for the 68-88 MHzband could be a double-conversionsuperheterodyne design employing a10.7-MHz filter, a 10-turn pot for thetuning and a final intermediate fre-quency (IF) bandwidth of 15 kHz tosuit NBFM (narrow-band frequencymodulated) signals picked up at a sen-

sitivity of 1 V or so, not forgetting asquelch to make sure the receiver isquiet when nothing is received. Greatshopping list, but such a receiver willbe expensive as well as difficult toadjust by beginners. Next, please!The good news is that an attractivealternative is available in the form ofthe TDA7000 chip from Philips thatsbeen around for more than 10 yearsnow, which is quite remarkable for aconsumer-market chip. This ever-green, then, contains a completeradio receiver with a very low IF ofjust 70 kHz. Okay, so image frequen-cies occur just 2 x 70 kHz = 140 kHzaway from the desired signals, butthat need not be a problem becauseon the positive side we do not have toworry too much about the input filter-ing. Also, the IF filter responsible for

4/2004 - elektor electronics 19

Figure 1. If and when radio regulatory authorities eventually decide to allocate the4-m to radio amateurs then the IARU recommendations will be followed, with strongguidance available from the UK example.

the selectivity may be realised as asimple R-C network, obviating theneed for expensive and esotericquartz or ceramic filters.

Block diagramEven if you are not a radio boffin, theblock diagram of the proposed receiverin Figure 2 should be largely self-explanatory. The TDA7000 contains amuting circuit which is activated at alevel of about 6 V. As we will want touse a whip antenna as the bare mini-mum, an RF preamplifier will have tobe inserted between the antenna andthe input of the TDA7000. Everythingfrom the output of the RF preamp rightup to the input of the audio amplifier iscontained in the TDA7000. If you wantto know everything about the chip, geta copy of the datasheet (see Webpointers).

Inside the receiverFigure 3 pictures the circuit diagramof our little receiver. MOSFET T1 at theantenna input provides a gain of about18 dB across the band, driving theTDA7000 RF input via coupling capac-itor C5. The receivers input imped-

ance is 50 to match most types ofcoax cable available these days. Anumber of capacitors strewn aroundthe TDA7000 ensure an IF bandwidthof about 70 kHz. The VFO (variable fre-quency oscillator) inside the chip istuned by a varicap (D1) which gets itsbias voltage from tuning pot P2. IC2, a78L05, supplies the regulated 5 voltsfor the receiver chip, the preamp and,importantly, the tuning pot.

The circuit configuration around theTDA7000 follows information fromPhilips on making the chip bettercompatible with NBFM signals. Afterall, the TDA7000 was originallydesigned for reception of VHF FMbroadcast stations, which at100+ kHz deviation are much widerthan the thin PMR signals (3 kHz)were interested in. None the less, asthe IC will produce a rather low nettoutput signal, some extra amplifica-tion is furnished in the audio sectionby adding an electrolytic capacitorbetween pins 1 and 8 of the LM386AF power amp (another evergreen).Hang on, where are the adjustmentsand the dreaded home-made coils inthis receiver? Well theres only trimmerC20 to adjust the tuning to 68-88 MHz.

The receiver employs off the shelfminiature chokes only, so there are 0(say, zero) coils to wind.

Build it!At this point, you should have enoughconfidence and inside knowledgeabout the receiver to start building it,if necessary with the help of a friendwith RF experience. If you do not havethe means to make your own board,you can easily order a ready-made onethrough our Readers Services. Theboard, pictured in Figure 4 togetherwith its external elements, is single-sided with a large copper plane at thesolder side to assist in RF stability,screening and decoupling. There aremany small ceramic capacitors on theboard which need to be positivelyidentified before they are soldered inplace. The same goes for the threeminiature chokes, the coloured bandson them indicating the value in micro-henries. The TDA7000 should be sol-dered directly on to the board.The 4-legged MOSFET T1 is mountedat the solder side of the board, percheddirectly onto four solder pads. Theclose-up photograph in Figure 5should help to get our message across.

We would suggest using a smalldiecast case from Hammond to housethe receiver and the battery. The case isthen drilled to secure the volume pot,tuning pot and the loudspeaker. Bat-tery powering is not a must however,and you may decide to power thereceiver from an existing DC sourcelike a cheap mains adapter. This willrequire one additional hole to be drilledand filed for the mains adaptor socket.

Caveats and limitationsDue to the simplicity of the design,some inherent limitations should be


DET

020416 - 12

68 - 88 MHz

68 - 88 MHz

70 kHz

6

13 2

TUNE

TDA7000

Figure 2. Block diagram of the single-conversion receiver. Note the low intermediatefrequency of just 70 kHz which in our case has a number of advantages!

Resistors:R1=100kR2=150kR3=100R4=22kR5=330kP1=50k logarithmic potentiometer

P2=50k linear potentiometer

Capacitors:C1=39pFC2=27pFC3,C6,C14=10nFC4,C11,C13,C19,C23,C24,C25,C26,

C29=100nFC5,C12=1nF, lead pitch 5mmC7=100nF, lead pitch 5mmC8=220pFC9,C18=330pFC10=10pFC15,C17=3nF3, lead pitch 5mmC16=180pFC20=22pF PTFE trimmerC21=150pF

COMPONENTS LIST

taken into account. First, the receiverwill be found rather susceptible tocross modulation, breakthrough andgeneral interference from nearby FMbroadcast transmitters. This not at allsurprising in view of the nearby fre-quencies (89-107 MHz) and power lev-els in the kilowatts range. Good shield-ing, coax cable and a tuned antennafor 4 m (see Antenna inset) shouldremove most of the interference. Sec-ond, a small problem with spuriousoscillation was discovered when thereceivers RF input is not terminated


TDA7000IC1

14

18

16 11

13

17

12

10

15

1

2

4 8

7

6

5 39

C1

39p

C4

100n

C3

10n

C6

10n

C13

100n

C12

1n

C14

10n

C16

180p

C17

3n3

C7

100n

C8

220p

C9

330p

C21

150p

C11

100n

C19

100n

C2

27p

C5

1n

C10

10p

C15

3n3

C18

330pR3

100

R2

150k

R1

100k

R4

22k

L1

C27

2n2

C25

100n

C23

100n

C24

100n

C29

100n

C22

100p

L2

330nH

C26

100n

BF981

G2

G1

D

S

T1

G 1BF981

G 2

S

D

L3

180nH

C20

22p

P150klog.

ANT1

D1

BB911

R5330k P2

50k

IC278L05

LM386N-4

IC32

35

6

4

1

7

8

C28

10

C30

100

LS1

TUNE

VOLUME

+9V...+15V

020416 - 11

100nH

16V

16V

C31

10016V

Figure 3. The circuit diagram of the VHF-Low Explorer has few surprises and proves the simplicity of the design.

(C) ELEKTOR EPS020416-1

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11C12

C13

C14

C15C16

C17

C18C19

C20C21

C22

C23C24

C25

C26

C27

C28

C29

C30

C31

D1

IC1

IC2

IC3

L1

L2

L3

P1P2

R1 R2R3

R4

R5

T1

EPS020416-1

+

0

T

LS1

SG1

G2D

9V TUNE

VOLUME

ANT1

LS1

S1

Figure 4. A drawing to show how the board is connected to its external elements.The copper track layout may be found elsewhere in this issue.

C22=100pFC27=2nF2C28=10F 16V radialC30,C31=100F 16V radial

Inductors:L1=100nH (brown, black, silver)L2=330nH (orange, orange, silver)L3=180nH (brown, grey, silver)

Semiconductors:D1=BB911T1=BF981IC1=TDA7000IC2=78L05IC3=LM386 N4

Miscellaneous:8 ohm, 1 watt miniature loudspeakerDiecast case: e.g., Hammond type 1590B

9V battery (PP3 / 6F22) with clip-on leadsPCB, order code 020416-1 (see Readers

Services page or www.elektor-electronics.co.uk)

with 50 . For the rest, nothing to stopyou from exploring the 4-metres band.

(020416-1)

Web pointers

TDA7000 datasheet:www.semiconductors.philips.com/pip/TDA7000.html#datasheet

70 MHz info page and news reflector:www.70mhz.org

International Amateur Radio Union (IARU):www.iaru.org

Radio Society of Great Britain (RSGB):www.rsgb.org

4-m band in Ireland:www.qsl.net/ei7gl/vhfpage.htm#70mhz

70 MHz Yagi antennas (DK7ZB):www.qsl.net/dk7zb/start1.htm

7-element Yagi for 70 MHz (M1CCZ):www.qsl.net/zr6dxb/PROJECTS/4mBeam/4MBeam.htm

Figure 5. This is as close as we couldget a lens to the MOSFET at the solderside of the board. Use the MOSFETpinout drawing in the circuit diagramto get the device positioned the rightway around.

For some strange reason varicaps (orvariable capacitance diodes) havealways been rather elusive compo-nents. Try this: design, engineer andpublish a design at instant T and youllfind that the varicap youve specifiedafter hours of careful researching hasdisappeared from the market at [T+1day]. Here at Elektor were optimistsby nature but because we anticipatesupply problems with the BB911 vari-cap specified for this present receiver,we thought wed give you a few cluesto help you find equivalent types.

The component values in the circuitdiagram guarantee a tuning range of68-88 MHz, with trimmer C20 defin-ing the edges of the tuning range andthe capacitance ratio of the BB911defining the width of about 20 MHz.In other words, trimmer C20 shifts thetuning range and D1 determines thewidth of the tuning range. The twoparameters have some interaction, ofcourse.

If you are only interested in, say, the4-m amateur band (70.0-70.5 MHz)then a narrow tuning range is suffi-cient and you will have no trouble get-ting just about any old VHF varicap towork in the receiver, simply by adjust-ing C20 to a known-good signal inthe band (ask for assistance from alocal licensed radio amateur).

If, on the other hand, you want every-thing from low-band TV (68 MHz) topolice, MoD and government PMR (insome cases just below 87 MHz) thensome thought should be given to theselection of the varicap.

The Philips BB911 was chosenbecause of its relatively large capaci-tance range of 25 pF to 65 pF for acorresponding tuning voltage of 0.6 Vto 5 V see Figure A (courtesyPhilips Semiconductors). Thats right,the capacitance presented by a vari-cap is inversely proportional to thevoltage applied across the device!Mathematically, though, [delta-C /delta-V] is a dimensionless device con-stant which, in the case of the BB911,works out at about 9 for the linearpart of its capacitance range.

If you cant get the BB911locallytheres no reason to abandon the proj-ect or send Blue Murder emails to theEditor because there are lots of alter-natives. Do not be afraid to experi-ment. In many cases, unlabelled vari-caps picked up at radio rallies or sal-vaged from an FM radio may beused, provided you know they are forVHF. Connecting a few varicaps inparallel (stacking) is perfectly legiti-mate in order to arrive at the desiredV/C value and hence the receiverstuning span.


About the authorGert Baars (42) has been active in electronics from a young age. In 1988 he graduated in electron-ics at the Polytechnic of Alkmaar in the Netherlands. Gerts main interest is RF electronics, but also soft-ware and digital hardware. He has had over 20 projects published in this magazine since 1997,including the successful Airband Receiver, the 20-m Band Receiver and the DDS RF Signal Genera-tor. In the future, Gert hopes to write about Atmel micros in control of RF equipment, and possiblythe design of a UHF sweep generator. He invites comments and suggestions by email,[email protected].

10210-1 10.6 5

0

75

50

2525

65

10VR [V]

Cd[pF]

f = 1 MHz; Tj = 25 C

020416 - 20

Varicap selection or the delta-C / delta-V issue

A

The propagation of radio waves is afascinating phenomenon because mostof it is guesswork and sheer surprise.That is not to say the subject has notbeen studied extensively byresearchers and radio amateurs farfrom it, a number of underlying princi-ples have been described in scientificterms as early as the 1920s by NobelLaureate Sir Edward V. Appleton(1892-1965). Appleton discoveredthat radio waves, depending in theirfrequency, were subject to refraction,reflection and (partial) absorption bycertain regions of the earths atmos-phere. These regions are marked bydifferent electron densities and occurat heights of 60-400 km above theearth. The basic distribution is shownin Figure A. You will search in vainfor the A, B and C regions. This isbecause Appleton first discovered theregion around 100 km height andcalled it electron region. The D and F(actually F1 & F2) regions were dis-covered later when the name E regionwas already established. Today,researchers employ extremely sophisti-cated radio equipment as well asobservations from radio amateurs inan attempt to prove the existence ofmore layers in the atmosphere.

Because it is easily ionised, the Eregion is favourable for reflection andrefraction of signals in the 70 MHz

and VHF bands in general. Apartfrom rather unexpected behaviour,usually during periods of high airpressure, the E region is also pre-dictable in that the electron densitydrops considerably at sunset due to alesser degree of ionisation. As anaside, the E region reflects medium-wave band signals at night time whenthe absorption by the D region largelydisappears.

Sporadic E (Es) is what we are afterfor our 70-MHz receiver, and thereason should be obvious if you lookat Figure B. Normally, the range ofa transmitter T using the VHF-Lowband is governed by line of sight, so

it will reach receiver R1 as the far-thest location. However, with a bit ofhelp from Es the signal may bereflected and reach receiver R2which, seen from T, is way below thehorizon. In extreme case the signalmay even bounce within the E layerand reach receiver R3.

Es is due to the formation of cloudsof densely ionised regions in theatmosphere at a height of 100-125km. Es typically occurs during sum-mer months, but exceptions have beennoted. Given a sufficient degree ofionisation (sometimes helped bysunspot outburst), radio contacts viasporadic E have been made over dis-tances of 2000 miles and more. Agood way to check for Es activity is touse your receiver to monitor the signalstrength of one of the many beaconsin the 70-MHz radio amateur band,or TV stations near the low end of theband. Many years ago, thanks to apeak in sunspot activity coupled withmassive Es cloud activity across theAtlantic Ocean, police cars from

Boston and New York could be heardloud and clear in Europe, some sig-nals even making it across policerepeaters on this side of the ocean.Starski & Hutch Ten-four!


Features at a glance Single conversion receiver

Frequency range 68-88 MHz (VHF-Low band)

Free-running VFO

TDA7000 FM Radio Circuit modified for NBFM

MOSFET preamplifier

Single-board construction

On-board audio amplifier

1.7 V sensitivity for 12 dB SINAD (3 kHz deviation)

Power supply 9-18 VDC, 20 mA (muted)

1000

800Min.Max.

Night-time

Daytime

600

400

200

150

Electron density [cm-3]

Altit

ude

[km]

100

80

6010 102 103 104 105 106

F2

F1

F

E

020416 - 18

E

D

D

T

R1 R2

R3 (60...100)

(100)(>100)

020416 - 17

D

EF1

F2

Propagation the total surprise factor

A

B

No receiver is complete without amatching antenna. Commercial offer-ings for the 4-m band being few andfar between (or scrapped by PMR fleetowners), we decided to present adesign for a low-budget get-u-goingdipole. Not sophisticated, want adirectional antenna? Then try the linksat the end of this article. Too difficult?It doesnt get much simpler than this,so give this antenna design a try andyoull be pleasantly surprised. Theantenna is great for initial experimentseven when installed on your attic.

Our ingredients and tools are:

a length of 50 RG213 or RG8coax cable (10.3 mm outside diame-ter)

a piece of copper pipe, 15 mm out-side diameter, length 965 mm

two aluminium rods, 6 mm diameter,length 1 m

two cable eyelets

a round T-junction box for electricalconduit, 20 mm openings

some not too thin wire

a powerful soldering iron (>50 watts)

nylon or plastic bushes, 20 mmdiameter

permission from the missus

The drawing in Figure A is intendedas a guide to constructing the anten-na. The copper pipe acts as a balun(balanced to unbalanced transformer),not only matching the asymmetricalcoax cable to the symmetrical dipole,but also stepping down the dipoleimpedance of about 72 to the cableimpedance of 50 . RF buffs will liketo refer to it as a bazooka or sleevebalun. Unless you have electrical con-nection materials to fit to the rod ends

(like a 60-A electrical chocolateblock terminal strip), simply flattenand drill the ends of the aluminiumrods to allow screws to be used for theconnection with the cable eyelets. Forextra rigidity, the rods are fed throughbushes (drilled to accept them) wherethey enter the junction box, and thebox itself may be filled with pottingcompound or hard setting siliconesealant. The other ends of the alu rodsshould be deburred, rounded off andsealed to prevent moisture ingress.

If the antenna is to be used out ofdoors, the junction of the copper pipeand the coax braid should be protect-ed as well. This may be achieved byinserting the balun assembly in alength of 20-mm dia conduit and fill-ing the lot with silicone sealant. Allsoldering to the coax cable should be

done as quickly as possible to preventdeformation of the PTFE (Teflon) coreand consequently creating impedancehumps.

Variations on the theme are possible,but be careful if you lack experience.You may, for instance, decide to usemuch thinner material for the dipolearms (for instance, lengths of weldingrod), thinking it will make no differ-ence as it is only the length thatcounts. Wrong, because your antennawill lose much of its broadbandresponse and will present an accept-able VSWR at about 75 MHz 2 MHz only. A larger rod diameterincreases the bandwidth so the anten-na can be used for the entire VHF-Lowband (68-88 MHz). That is why baseantennas use for PMR services in theVHF bands are so thick!


A dipole antenna for 4 metres

965 mm

1 m x 6 mm alu tube (2x)(not to scale)

RG213coax

15 mm copper pipe

junction box for20 mm conduit

remove coax sleeve andconnect braid to copper pipe braid not connected

On the rack!

While tidying up the design of theVHF-Low Explorer for publication inthis issue, an opportunity arose tohave our little receiver tested on ahigh-end instrument called the Rohde& Schwarz CMS 54 RadioCommunications ServiceMonitor. This instrument (picturedhere) can perform automated meas-urements across the frequency range0.4 MHz to 1 GHz which is not nor-mally within the capacity of the RF testequipment available in the ElektorElectronics design laboratory. Theoffer came from Mr. Ed WarnierPA1EW who is totally conversant withthis piece of kit, handling and operat-ing it as if he were driving his car tothe supermarket. Not only the test

results obtained from the receiver areworth telling you about, but also thebasics of some of the specific tests theinstrument can perform, as they maybe unknown to many readers enteringthe radio hobby.

A few facts had to be established first.Our receiver is designed for narrow-band FM (NBFM) reception between68 MHz and 88 MHz, these valuesmarking what is generally referred toas the VHF-Low communications band.As the receiver is of specific interest toradio amateurs, it was decided to tuneit to 70.250 MHz being the centreof the 4-metre band as defined by theIARU. The receiver being VFO tuned

A

plicity of the design. The test was car-ried out using a test tone of 1 kHz anda deviation of 3 kHz. As you can seein the screendump, the CMS 54 alsodisplays a real-time image of thereceivers output signal.

Absolute sensitivity andsquelch action

Since the CMS 54 is capable of inter-preting audio signals with such amaz-ing precision it has no problems at alldetecting when such a signal is passedor muted by the receiver. The latteraction is taken care of by the squelch(mute) function built into theTDA7000. An automated, steppedmeasurement was launched again, thistime to establish the RF signal level atwhich the squelch closes. The result,about 1.6 microvolts, can be seen inScreendump B. Ed kindly informedus that a squelch hysteresis of just

0.2 dB is not favourable for NBFM lis-tening, a value of 2-3 dB being thestandard. A bit more hysteresisensures that stations dropping into thenoise do not cause the squelch to closeabruptly. Rather, the receiver will fol-low such flutter-infested signals which,although barely intelligible in the noisewill not cause the squelch to chatter.

The CMS 54 has a plethora of otherfunctions for some really gruelling testson radio communications equipmentand in particular PMRs. We hope tobe able to use it again some time inthe future.

and lacking a frequency readout,reverse thinking quickly lead to theCMS 54 being set to 70.250 MHzand cheerfully tuning the receiver untilthe test signal was audible.

Hooray, it works!

Receiver sensitivity measurement

With the generator tuned to thereceiver (or was it the other wayaround?) a great moment arrives were ready to decrease the RF out-put level on the generator until thereceiver under investigation loses thesignal. The VHF-Low Explorer nothaving an adjustable squelch orsquelch defeat switch, the transitionfrom very noisy signal to mutedwas found to be fairly abrupt (moreabout this further on).

Receiver sensitivity is defined as the RFsignal level at which the receiversaudio output signal achieves a certainsignal-to-noise ratio. For NBFMreceivers, SINAD = 12 dB is consid-ered the standard meaning that thesignal we would like to hear is 12 dBabove the sum of noise and distortion(hence the acronym SINAD; an oldermeasurement standard, S/N, employsnoise only in the quotient). The RF sig-nal level is usually given in microvoltspd (potential difference) although emf(electromotive force) is preferred bypurists. The CMS 54, then, had its RFsignal output connected to the receiverinput (by a length of RG58 coaxcable) and the audio input, to thereceivers audio output. After a manu-al adjustment of the volume control onthe receiver, an automated measure-ment is started on the CMS 54 whichsteps down its RF signal level until itmeasures an audio signal of 12 dBSINAD. The RF signal level at whichthat happens is frozen and displayed see screendump A. In our case,a sensitivity of about 1.7 microvoltsfor 12 dB SINAD was obtainedwhich is not bad at all given the sim-


A B

vhf baixa com tda7000

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