DX Engineering DXE-Z10000K2-KIT Instructions manual

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.

Version 2.0 Buffer Amplifier with the Elecraft K2

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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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© 2017 - DX Engineering - 1200 Southeast Ave. - Tallmadge, OH 44278

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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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© 2017 - DX Engineering - 1200 Southeast Ave. - Tallmadge, OH 44278

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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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© 2017 - DX Engineering - 1200 Southeast Ave. - Tallmadge, OH 44278