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TRX2A I/Q phase balance adjustment

PostPosted: 09 Oct 2017 22:40
Hello to the group;
I have just finished my TRX2A transceiver kit. I have successfully completed all of the adjustments, except for the I/Q phase balance adjustment.
How do I know when I have made the proper adjustment of trimmer R42 on the DDS board?

In keeping with the manual instructions, I tuned in an S9 cw signal and then cycled the mode button. The cw signal was present on LSB, and then disappeared when I went to USB. This is what the manual said should happen. As I turned R42, I never noticed any change in this pattern; cw station heard on LSB, then not heard on USB. By the way, if it is important, this was on 20 meters.

What should I be listening for while making this adjustment?

Thanks for any help that anyone can offer! 73, Jef Fox, KC9GBX

Re: TRX2A I/Q phase balance adjustment

PostPosted: 10 Oct 2017 05:56
by 5B4AIY
Hi, Jef,

OK, let's see if we can explain this. The output from the DDS on pin 21, Iout, is a current that is converted to a voltage by the input resistor, R15/200 ohms, of the low pass filter. The complementary output from pin 20, Ioutb, is similarly converted to a voltage by R12/100 Ohms. Both the normal and inverted DDS outputs are summed by R13 and R14 and should be exactly zero, decoupled by C9. This is the reference comparison voltage for the comparator and is coupled to pin 15, the negative comparator input. The normal output from the DDS is low-pass filtered and the sinusoidal signal is coupled to the positive input of the comparator on pin 16. The normal and inverted comparator outputs are coupled to the dual-D cross-connected bi-stable to provide the I and Q local oscillator signals at half the frequency of the synthesiser's output.

In a perfect arrangement the polyphase networks would have exactly 90 degrees of phase shift, but in the practical world there are always minor errors. R42 alters the switching threshold of the comparator by a small amount, and this has the effect of slightly advancing or delaying the phase of the I and Q signals, allowing you to obtain a better unwanted sideband null.

To adjust, inject a S9, 50uV CW signal into the transceiver on your favourite band. Tune the signal in using either the LSB or the USB mode. Switch to the opposite mode and the signal should be greatly attenuated. Carefully adjust R42 for the greatest null in the audio output. There should be a distinct reduction in the audio signal. If you do not get any significant change to the audio output when you adjust R42, then carefully examine the connections involving R55, R42, R40, R14, and R13. In my case, a poor soldered joint was causing the problem.

Adrian, 5B4AIY

Re: TRX2A I/Q phase balance adjustment

PostPosted: 10 Oct 2017 21:19
Hello Adrian;
IT WORKED! Thank you very much for the enlightening electronics description of the function of R42, but more importantly in my case, because I am not so electronically-inclined, the clear description of the adjustment procedure. I have an Elecraft 7.040MHz signal generator that I had used to adjust the AGC trimmer, so when you said to inject a cw signal, I used it again. As I turned R42, I wish that you could have seen the smile on my face when the injected signal faded to a null, and then began to reappear. Back to the null, and DONE. I used the signal generator's 2nd harmonic on 14.080MHz and repeated the procedure, and the null was in the same R42 adjustment spot. My wife and I then went out for lunch to celebrate!!!

73, Jef KC9GBX