> > > 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 <[email protected]>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" < >> [email protected]> >> To: "Discussion of precise time and frequency measurement" < >> [email protected]> >> 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 <[email protected]> >>> 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 -- [email protected] >>>>> 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 -- [email protected] >>>> To unsubscribe, go to https://www.febo.com/cgi-bin/ >>>> mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>> >>> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
