Hi, I'm playing with dual-mixer time difference stuff again. And, came across this and I find it somewhat puzzling since no one else seems to have encountered it. Possibly because I'm missing something?
The doubly balanced mixers (of the type known to be used in DMTDs and phase noise measurement systems) are known to have DC offsets. So much so that the guys doing phase noise measurements employ elaborate DC removal circuits in their preamps to combat this. Here's my question: why isn't this DC offset removed in any DMTD circuits I've seen? It seems standard practice to attach the filtered mixer output directly to the zero crossing detector. I did a quick simulation (see attached): The mixer beat is a 10Hz sine 0.7Vpp. If you then use a Collins style zero crossing detector the first stage will have a small gain (I chose a gain of 2.83 from Bruce Griffiths pages ( http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html)). I then compare this ideal signal to that of a similar one that is offset by 40mV. Notice the asymmetry in the signal due to offset. 40mV result in 1.8ms offset 4mV result in 180us offset Obviously, once the time offset is there no amount of subsequent slope amplification will remove it. I've tested this in practice and bingo, I now have a very accurate way of plotting relative mixer DC offset over time. Any comments?
<<attachment: Time offset due to mixer DC offset (40mV).png>>
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