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

Stephan Sandenbergh ssandenbergh at gmail.com
Fri Nov 22 12:19:26 UTC 2013


Hi,

Thanks - mystery solved. This is one of the systems that I looked at,
and missed the DC block in the second amplification stage. I guess it is
possibly a large Ceramic 10uF. My bad.

Thank you for putting up those web pages I find them to be very good
references. I spent quite a lot of time reading through them.

Something that puzzles me though is your mixer termination (
http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html). What is the logic in
having the second balun (and connected in that way)?

Regards,

Stephan.


On 22 November 2013 13:15, Bruce Griffiths <bruce.griffiths at xtra.co.nz>wrote:

> 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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