Thanks for all the comments and advice. I have come to understand the wisdom of divide by 4 IQ circuits. Fortunately it was very easy to convert the divide by two 74AC74 circuit described earlier to a version of the divide by 4 scheme seen above. (From the SDR Ensemble II Receiver: http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm) This change provided a great way to observe 1) the improvement in the output signals from the VFO and 2) the resulting improvement in receiver performance, especially opposite sideband rejection. Here are some numbers. I was very pleased to discover that my Rigol scope will measure duty cycle and phase difference. Thanks Rigol! AD9850 Divide by 4 : 7.212 MHz Duty cycle: 48.3 Phase Difference: 87-90 degrees Si5351 Divide by 2: 7.212 MHz Duty Cycle 49.6 Phase Difference: 83 degrees Si5351 Divide by 4 7.212 MHz Duty cycle 49 Phase Difference: 85-90 degrees Additional improvement came when I switched the power supply to the IQ inverters and Flip Flops. I switched from 3.3 to 5 volts: Si5351 Divide by 4 7.105 MHz Duty Cycle 49.7 Phase Difference: 90 degrees When I took the VFO box and put it back in the receiver with the divide by 4 scheme and the 5 volt supply I immediately noticed a big difference in performance. It was obvious that opposite sideband rejection was back to what I had had with the AD9850, perhaps better. I have a quick and dirty method of measuring opposite sideband rejection: I put an RF signal into the antenna connector. I put the 'scope on the audio output. I tune (on the desired sideband) for 1kHz audio and I measure the output voltage. Then, with the audio gain and RF sig gen output in the same positions, I tune to the opposite sideband, again tuning for 1 kHz, again measuring audio output. With the divide by 4 scheme and the 5 volt supply, the opposite sideband was so weak I had trouble measuring it. I estimate the rejection to be at least 32 db -- this is back in the range of what I had with the AD9850, and significantly better than I had with the divide by 2 scheme.
Now I just need to figure out how to get the Si5351 VFO sketch to tune above 42.94 MHz. For some reason it quits at this point, switching down to 2 kHz output, and keeping me on 30 meters and below. Thanks again to Todd VE7BPO for a lot of help with the hardware and to Tom AK2B for help with the Arduino code.
In response to popular demand, "SolderSmoke -- Global Adventures in Wireless Electronics" is now available as an e-book for Amazon's Kindle.
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The two versions are identical, except for a minor difference in the paper used. That's why the prices are a bit different.
Bill's OTHER Book (Warning: Not About Radio)
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W4HBK's QRSS Grabber: The Amazing Pensacola Snapper (Live!)