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