QRP Labs QCX-mini CW, Руководство по сборке, Страница: 76 / 121

Adjustment of I-Q balance
Now turn the rotary encoder “one click” clockwise to measure I-Q balance.
Note that the previous alignment used the audio signal BEFORE the final amplification
stage, so the gain control had no effect on the signal level. In contrast, the I-Q balance and
audio phase shift adjustments use the audio signal AFTER the audio amplification stage.
This is necessary because these alignment adjustments inject a signal into the opposite
(unwanted) sideband, and the signal level is much lower, therefore it needs to be amplified
for the microcontroller to be able to measure it accurately. In this case therefore, the gain
control DOES now have an effect. I suggest adjusting the gain control approximately to
mid-way to start with. This will provide enough gain, yet not so much gain that the amplifiers
are driven into overload, which distorts the signal and measurements.
You should try to carry out the adjustments with the measured audio values in the range 5
to 10 (as indicated by the division ratio in the top right of the display). If it reaches 12, the
operational amplifiers are limiting, clipping the signal which will make it difficult or
impossible to make the adjustment accurately. If the displayed value (top right of the
display) is only 2 or 3, that indicates the gain is too low. Therefore, adjust the volume
control so that the display is something like 9.
The I-Q trimmer potentiometer is R27. It is a multi-turn trimmer potentiometer so it may
need to be turned quite a few times to get to the optimum value! For this adjustment, you
are looking for the MINIMUM amplitude, not the maximum we adjusted the BPF trimmer to.
We adjust for minimum because now the injected signal is measuring the unwanted
sideband. We want to MINIMIZE the unwanted sideband level.
Adjustment of 90-degrees audio phase shift
Similarly turn the rotary encoder one more “click” clockwise, which automatically sets the
unwanted sideband audio signal to appear at 600Hz, and adjust the “low audio phase shift”
trimmer potentiometer, R24. Again, adjust it for MINIMUM signal.
Turn the rotary encoder clockwise one more “click” and adjust the “high audio phase shift”
trimmer potentiometer R17, again for minimum signal.
Now it is necessary to go back and forth between these three menu items for minimum
unwanted sideband:

8.8 I-Q Bal (adjust R27)

8.9 Phase Lo (adjust R24)

8.10 Phase Hi (adjust R17)
This is because to some extent, these adjustments influence each other. Obtaining the
optimum set of adjustments is an iterative process. So, turn the rotary encoder a click at a
time anti-clockwise or clockwise, back and forth through these three menu items. Each time
make small further adjustments to the appropriate trimmer potentiometer and observe the
lower amplitude. Keep doing this until you see that you cannot really manage to get the
unwanted sideband any lower in any of those adjustments. Pressing the “Exit” button twice
leaves the menu system and returns the radio to normal operation.
Other items in the alignment menu relate to the calibration of the 27MHz reference
oscillator of the synthesizer, and the 20MHz system clock oscillator of the microcontroller.
These adjustments can be made manually, or by connection of a GPS module such as the
QRP Labs QLG1 GPS receiver kit. However, since this calibration is a lot less urgent than
QCX-mini assembly Rev 1.05
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