Monday, 30 September 2013

Balancing Act

Several tricks have been tried with the 10m Wispy to try and get the carrier balance right, so far none have been entirely successful. A balance control potentiometer had no effect at all, and changing the diodes seemed to have no effect either. So, I reverted the circuit back to its original form, put a pot in place of R11 to make it adjustable again, and totally rewound T2, physically symmetrically. An RF bypass capacitor has also been put on the AF modulation input

Things do seem a little improved. The carrier is still present, but I can get it at least to look more like DSB should do. Adjusting R11 for best 2nd harmonic resulted in what looks to me to be a sensible spectra for a doubler. The 28MHz 2nd harmonic is dead center, the first two peaks from the left are an image of the 14MHz fundamental, and the machines zero marker

The harmonics are in the form of a 'staircase' going down in relative strength. Adjusting now for best carrier balance at the mixer output, I managed to get the signal looking like this

Yes, its much weaker than the doubler, but I suppose taking into account the differences from changing the analyzer settings, and the insertion loss of the mixer, and the likely mismatch between the analyzer and the input to the driver where this signal was taken, it at least shows that the carrier is down on the sidebands. This is with a 10kHz 50mV AF input. 10kHz is needed to be able to see the sidebands apart from the carrier on this machine. Looking again at the doubler, shows that when set for best carrier null, it isnt quite as good an output on the 2nd harmonic, and the 3rd isnt down as far as it was

Putting the probe now to the output of the PA, just after the Low Pass Filter, we see an amplified DSB signal, still with significant carrier though. Whether this is good enough for on-air use I dont yet know. Im not certain by how much I should expect the carrier to be suppressed, Roger claimed 30dBm, mine I think is about 14dB down, assuming im reading the scale properly!

I shall have to make a point of listening to the output on a general coverage receiver in SSB mode to see how noticeable the carrier is. The 3rd harmonic isnt very well suppressed either, being barely below the wanted second, but interestingly its carrier is almost none existent!  There are also more noticeable extra mixer products. The picture isnt so good as the camera captured the interval of the sweep time! but the upper sideband can be made out

The 3rd harmonic then needs looking at, perhaps making the driver and PA inductors as tuned circuits will help here. I could not observe any greater harmonics though.

Next step then I think is to sort out the biasing on the driver and PA, so they dont get so hot, and to measure the output power, see if it really is producing what it should be! But these are probably all tasks for another day.

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