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"
<ssandenbe...@gmail.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@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.griffi...@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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