You wrote: > Mike,
> He was using an analog mixer, but your comment about XOR mixer does > not apply to analog mixers. Your oversimplification that analog > mixer and XOR gates being the same thing does not apply here, and > thus the assigned missbehavior does not carry over to the analog > mixer case. > Cheers, Magnus Magnus, Thanks for your comment. Here are some attached files: 1. DBMS.PNG This shows the schematic of a double-balanced mixer. Note the mixer output is on pin U24. A low pass filter is at R1C1. 2. DBMWFM.PNG These are the waveforms in quadrature lock. The bottom waveform in red is the signal at pin U24. It is a square wave at twice the signal frequency. This signal is identical to an XOR, such as a 7486 logic ic, except the amplitude is much lower at only 900 mV p-p. The top waveform in green is the signal at the low pass filter. It is a triangle wave, the same as you would get from adding a low pass filter to any square wave. Thus my statement that a double-balanced mixer is an XOR is accurate. 3. DUBLBA01.ASC This is the double-balanced mixer schematic input for the LTspice simulator. 4. DUBLBA01.PLT This is the output waveforms from LTspice. Ordinarily, the triangle ripple output from a double balanced mixer would add considerable jitter to any PLL. Eric's application avoids this problem since his loop bandwidth is so low, at much less than 1 Hz. This makes it extremely difficult for him to obtain lock, which is why I proposed using a phase/frequency detector. The first block diagram I posted earlier, PNA.PNG, contained two errors. I corrected them in PNA2.PNG, which I will post to Eric. At first, I did not realize the significance of Eric's low loop bandwidth, and I erroneously assumed the triangle wave ripple output would cause significant jitter to his loop. It is now obvious the low loop bandwidth will reduce the ripple amplitude to insignificance, and I now retract my claim. Thanks, Mike
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