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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Nov 22 06:15:15 EST 2013


Stephan Sandenbergh wrote:
> 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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One can always add AC coupling to eliminate this effect as in 
http://www.wriley.com/A%20Small%20DMTD%20System.pdf

Bruce


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