QCX assembly Rev 1.08
66
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 MINIMISE 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 right (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
synthesiser, 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 the Band Pass Filter peaking and
Summary of Contents for QCX 5W CW
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