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

[WarrenS]

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

************
----- 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?
>>>
>>>
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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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>
>