[time-nuts] DMTD: Mixer DC offset will result in time offset at zero-crossing detector out?

[Stephan Sandenbergh]

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?
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