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

[Bruce Griffiths]

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
>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>>>> mailman/listinfo/time-nuts
>>>> 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
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>>> mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>>
>>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>