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

Stephan Sandenbergh ssandenbergh at gmail.com
Mon Nov 25 11:08:52 UTC 2013


>
>
> 1st section is common mode  low pass filter, 2nd section is differential
> common mode low pass filter.


Ok - makes sense now.

Yes AC coupling after limiting isnt such a good idea.
> AC coupling either between the mixer and the input stage of the limiter
> chain or after the last linear stage would be better.


I assumed so, since limiting an offset sinusoid will produce an
asymmetrical trapezoid which will have a different mean value than the
original sinusoid. But, I wasn't sure so thanks for confirming this.


On 23 November 2013 07:28, Bruce Griffiths <bruce.griffiths at xtra.co.nz>wrote:

> WarrenS wrote:
>
>> Stephan
>>
>> Did you also notice that the AC coupling is done **after** the sine wave
>> has already been clipped by the previous stage (according to the schematic
>> note)?
>> This generally is not a good way to remove DC offset from a low level
>> 'noisy' signal.
>> I doubt that Bruce was recommending doing it that way.
>>
>> ws
>>
> Yes AC coupling after limiting isnt such a good idea.
> AC coupling either between the mixer and the input stage of the limiter
> chain or after the last linear stage would be better.
>
> Bruce
>
>
>> ************
>> ----- Original Message ----- From: "Stephan Sandenbergh" <
>> ssandenbergh at gmail.com>
>> To: "Discussion of precise time and frequency measurement" <
>> time-nuts at febo.com>
>> Sent: Friday, November 22, 2013 4:19 AM
>> Subject: Re: [time-nuts] DMTD: Mixer DC offset will result in time offset
>> at zero-crossing detector out?
>>
>>
>>  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?
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com
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>>>>> and follow the instructions there.
>>>>>
>>>>>  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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>>>>
>>>
>>>
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