Adding an I/Q output to the Elecraft KX2
A mini writeup of an ill-conceived little project, adding a homemade option board to the Elecraft KX2 transceiver.
Not all my projects are necessarily based on a good idea.
The below project is not something I would encourage anyone to do, it will certainly void any warranty on your shiny new KX2.
You have been warned :-)
Unlike the KX3, the KX2 does not have an audio I/Q output for use by a Spectrum Scope or by external SDR software. However, this is actually rather easy to add, the signals are readily available, see:
It is not advisable however to connect an external circuit directly to those outputs. We certainly don't want to introduce hum or static to the signal path and any impedance mismatch introduced between the I and Q signals will trash the performance of the receiver, hence we need an isolation circuit:
No magic here, two unity gain amplifiers, in fact a somewhat similar circuit to the one used for the IQ output in the KX3.
C9 and C10 provide a gain rolloff for higher frequencies than we are interested in. Initially I tried without C9 and C10, this resulted in an audible overdrive condition from time to time, presumably caused by a high frequency component present at the I and Q taps. This overdrive condition was especially notable when turning the VFO. The 390pF capacitors however, provide a 6 dB corner frequency at approx. 80 kHz. This is probably a tad bit more aggressive than necessary, but in any case, there is already more rolloff present at the KX2 IQ outputs themselves. More on this later.
Here is the new circuit, plugged into the option slot of the KX2:
Another angle:
And here is the printed circuit board, rather miniscule :)
As I mentioned above, C9 and C10 were not included at first, here they are riding piggyback on top of R4 and R7.
Backside:
As can be seen, the layout certainly has its share of flaws, I will not be making it available – so please don’t ask.
And here we have the new KX2 Audio IQ output wired up to a computer running HDSDR, works like a charm. Notice the audio roll-off slope to the sides, appears to be close to 20 dB per 50 kHz, apparently the main contributor being roll-off circuitry in the KX2, before feeding the AD inputs to the DSP. This is probably the reason Elecraft decided not to provide an IQ output. I would love to see a schematic of the KX2. Anyway, even with this roll-off, it is still perfectly useable.
