introduction [01 september 2006 - i've extensively revised ... · within a fully-loaded k2....

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Version 2.0 Buffer Amplifier with the Elecraft K2

Introduction

[01 September 2006 - I've extensively revised this page to reflect the version 2.0 buffer amplifier and to add a discussion on Elecraft K2 BFO leakage. I've deleted older material no longer relevant.]

I've found a lot of interest in the SpectraScan panadapter from Elecraft K2 owners. The K2 has a nominal IF frequency of 4914 KHz, and the SpectraScan works well at that frequency. (My early Z90 design used an internal IF of 9830 KHz, and I built several experimental filters with 9830 KHz crystals. However, you may notice a relationship here -- 9830 KHz is almost exactly double the K2's 4914 KHz IF. Internal birdies generated by this combination made me rethink the SpectraScan's IF frequency and I changed to 8.000 MHz for K2 compatibility.)

Unlike some receivers and transceivers, the K2 does not have a IF output jack. Hence, it's necessary to add an IF connection. I've designed a general purpose IF buffer amplifier that fits within a fully-loaded K2.

The buffer amplifier can be built in two versions, by choice of components:

A version optimized for the K2, with a bandpass response, centered around 5 MHz for the K2's 4915 KHz IF; or

A broadband version that will work with any IF from near DC to 100 MHz and is accordingly not limited to just the K2.

Both versions share a common PCB, approximately 1.375" x 1.25" (35mm x 32mm). I will ship the board with the parts necessary to build either the K2 version or the broadband version.

I also have designed a BNC-connector version of the broadband amplifier, but this seems to be of little interest and it likely not become a finished product. If someone wants one or two, I can make available a version without silk screening or solder mask.

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Connection Point in the K2

The schematic fragment shows the recommended connection point.

The circuit impedance at this point is relatively low, on the order of 150 ohms, so the buffer amplifier's input impedance will result in minimum disturbance of the K2's operation.

Prototype Buffer Amplifier

I've designed and built a generic high impedance buffer amplifier. In this context, high impedance is a relative term; my target was to get the input impedance in the several kohm range over a frequency range of up to 75 MHz, and to have the possibility of gain ranging from -6 dB up to 10 to 15 dB. The target impedance range should permit a non-disturbing connection to most receivers.

Of course, a higher impedance design is possible, such as an emitter follower or source follower, with an additional gain stage. After building up several prototypes, I decided to go with the AD8007 device.

The net gain is adjustable by a single programming resistor, from negative 6 dB to positive 12 dB or more. The gain will be set by the builder based on the receiver or transceiver with which it is used. In testing the amplifier with a K2, the net gain (considering filter loss and other factors)

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should be approximately 0 dB.

Power requirements: 12V @ 20 mA. The board has a 9V regulator and can operate with an input voltage between 12V and 24V.

Here's a top view of version 1.1 of the buffer amplifier PCB. The current version 2.0 is essentially identical in the top view.

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Version 2.0, bottom view. The components at the lower left comprise a five-pole low pass filter to shape the frequency response.

This particular board was assembled with plug-in connectors for easier testing. I do not recommend plug in connectors at the buffer amplifier board for installation in a K2.

If assembled in "wideband" mode, the response is is as shown at the right--flat up to about 30 MHz, where it starts to peak. The 3 dB bandwidth exceeds 300 MHz in this configuration. At 4.914 MHz, the net gain is +6.5 dB.

When measuring gain of a high impedance amplifier with 50 ohm equipment, you must use a 50 ohm through at the input, or else you will see a false 6 dB gain increase, as the source voltage doubles into what is nearly an open circuit with respect to the 50 ohm source. The gain data presented is with a 50 ohm through termination on the input.

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Here's the frequency response of the buffer amplifier when assembled in the recommended K2 bandpass filter configuration.

At the K2's IF frequency, the response is flat over the Z90's 200 KHz range.

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Connecting the buffer to the K2

How the buffer amplifier is connected to the K2 depends on whether the K2 has the optional noise blanker board.

Stan Rife, W5EWA, has been very helpful in working through an elegant mounting and connection arrangement for the K2 buffer amplifier board.

The buffer board's input wiring consists of a short length of Teflon coaxial cable, and a single piece of hookup wire for power. These wires terminate in a standard 3-pin 0.1" spaced male header plug, insulated with heat shrink tubing.

If the Noise Blanker is not Installed:

The buffer board will come with an 8-position 0.1" female header socket, to be installed at J12. This is the same header socket Elecraft provides with the noise blanker kit, so if you decide to add the noise blanker kit later, you will not have to remove any parts associated with the buffer board.

The 3-pin plug from the buffer amplifier plugs into the J12 socket at pins 1, 2 and 3.

If the Optional Noise Blanker is Installed

Remove the noise blanker board and turn it upside down.

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With an X-Acto knife, trim away the plastic from the male header pins 1, 2 and 3, leaving the pins soldered in place.

Solder the provided 3-pin female header socket to the three header pins, with the socket parallel to the noise blanker PCB.

Another view of how the three-pin socket is solder to the modified noise blanker header.

The 3-pin lead from the buffer board plugs into the newly added 3-pin socket.

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Update on Testing with the K2

A prototype version 2.0 PCB installed in my K2. The board requires a small notch to fit against the noise blanker, but I'll move the mounting hole for the next revision to avoid a notched board.

The board is mounted with a 4-40 male/female threaded stand-off in an existing hole.

I've made the input and output coaxial cables longer than desirable so that I might experiment with alternative mounting locations.

Close up of the output cable. The cable terminates in an SMA cable jack bulkhead connector. The cable remains shielded in the connector.

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The output SMA connector installed in a spare hole in my K2. If you have no spare holes, the SMA connector mounts in a 0.250" (6.35 mm) diameter hole.

Here's a view of the 20 meter band, from 14000 - 14200 KHz, as seen via my K2 connected to a prototype Z91 via the prototype version 2.0 buffer amplifier.

BFO Leakage

Stan, W5EWA, reported seeing a steady signal displayed on the Z90, even with no antenna connected to his K2, but it was not clear if this signal was an aberration in his particular K2, or if it will be generally seen in all K2 transceivers.

I'm seeing the same signal, and it appears that it is BFO leakage through the reverse gain of the

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IF system. The BFO leakage level is approximately -80 dBm as seen at the IF pick-off point.

Whether the BFO leakage signal will be a problem depends on your perception. Here's what I've found with my K2, and Stan's results are similar.

The Z90/91 display, connected to a K2 with the buffer amplifier and with no antenna connected to the K2. This is the worst-case for the BFO leakage signal, as there are no other signals to be seen on the panadapter. I've also selected a narrow span which exaggerates the BFO leakage signal.

The BFO signal is about 25 dB above the Z90's noise floor.

To verify that the BFO leakage is not an artifact of the Z90, I observed the same signal with an HP8558B spectrum analyzer.

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In normal operation, the BFO leakage is much less visible. This shot shows the same frequency, but with a wider span and an antenna connected to the K2, with the K2's pre-amplifier turned on.

Narrow span (10 KHz), antenna connected but K2 pre-amplifier off. The BFO leakage is noticeable.

Where does the BFO leakage come from?

It seems that the BFO signal is passed back to the buffer amplifier's input via one of two possible paths, as illustrated in the marked-up K2 block diagram below.

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It's not clear which of the two possible paths is the culprit. Considering that the BFO signal is quite strong (I measured nearly 2 volts peak-to-peak at U11), even if it is attenuated 90 dB, it still shows up as a clearly visible signal at the IF pick-off point.

I've tried alternative buffer board locations, and the BFO leakage signal remains the same, which indicates the problem is unlikely to be stray coupling into the buffer amplifier. Rather, the BFO leakage is directly in the signal chain. (I've had E-mail exchanges with Wayne Burdick at Elecraft confirming this is the likely source of the signal.)

Why does the K2 have BFO leakage but other transceivers do not?

Several possible reasons:

The K2 is a single-conversion design, and hence the BFO is at the same frequency as the point where we sample the IF frequency. A multiple conversion receiver will not have the BFO on the same frequency as the high IF.

Relatively simple IF chain. There are only two active stages between the IF pick-off point

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and the BFO. Thus, the BFO suppression is critically dependent upon (a) U11 (NE602) balance and the reverse gain of U12 (MC1350) IF amplifier. If both U11 and U12 have -40 dB reverse gain (how strong the signal at the amplifier's input is when fed into the output), the BFO will be attenuated only 80 dB at the IF sample pickup point. Other receivers have more IF amplification stages, which improves the overall BFO leakage proportionally.

Single board construction. At one extreme, commercial and military grade receivers have each major module constructed in a separate, shielded compartment. The K2 has all its RF components on a single PCB. This provides a significant cost benefit, but may contribute to the BFO leakage.

If you tap off an IF stage operating at the same frequency the BFO, you may well see BFO leakage.

Why Doesn't the BFO Leakage Bother Normal Receiver Operation?

Because it is at the same frequency as the BFO and because it is much weaker than the direct signal. The leakage that shows up on the Z90 panadapter (and on my HP8558B spectrum analyzer) will have no effect upon normal K2 operation.

How to Remove the BFO Leakage Signal?

The most promising method is to move the buffer amplifier connection point from the recommended output of Q22 to Q22's input, i.e., to the mixer output, at the junction of C159, R80 and R81. This will decrease the BFO leakage signal by Q22's reverse gain.

The problems with connecting at the mixer output are:

Less gain, as Q22 will not be in the circuit. The buffer amplifier gain can be increased to offset this loss to some extent.

Much less mechanically convenient, as the suggested connection point is not brought out to a convenient jack.

Since Q22 has a lot of negative feedback, its reverse gain may not be as high as desired, and the degree of BFO leakage suppression correspondingly modest.

introduction [01 september 2006 - i've extensively revised ... · within a fully-loaded k2....

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