Charles wrote: "I like Didier's suggestion of the FM tuner, but the phase modulation will need to be substantial to detect it that way."
The deviation (modulation index) of a FM broadcast signal is very large, on the order of +/-75KHz. The phase modulation will need to produce large frequency deviations to pick up 137Hz phase modulation. A VHF radio or scanner is a narrow band device, you might have better luck if the deviations are small. The modulation index is a measure of the deviation vs the deviating frequency, but it's the AMPLITUDE of the 137Hz that produces the FREQUENCY deviation, whether phase or frequency modulation. Certainly a good FM receiver will pass the low 137Hz, but a VHF receiver or scanner may have a high pass filter, yet a scope might still detect it. Do you have a HP8901 on hand? How about a spectrum analyzer? Bob ----- Original Message ----- From: "Charles P. Steinmetz" <charles_steinm...@lavabit.com> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Tuesday, June 15, 2010 11:05 PM Subject: Re: [time-nuts] 5061A Problem Child > Joe wrote: > >>Short of fabricating a 'phase detector', how can I prove that the A3 unit >>is >>functioning in my 'problem child' unit. If I have to fabricate a 'phase >>detector', how easy (or difficult) is that? > > I have no idea how much phase modulation there is supposed to be. If > it is substantial, you might get a crude qualitative idea by looking > with a stably-triggered 'scope at a scale where there are, say, 10 > cycles per cm (~100 cycles across the screen). If the signal at the > right of the screen is fuzzy (cycles run together) while at the left > it is sharp (cycles clear and distinct), you have phase > modulation. Turning the mod off should make the signal look sharp at > both ends. > > If you are lucky enough that the modulation is sufficient to show up > this way, you can compare the two units to see if the progression > from sharp to fuzzy looks about the same from one to the other. > > I like Didier's suggestion of the FM tuner, but the phase modulation > will need to be substantial to detect it that way. > > Best regards, > > Charles > > > > _______________________________________________ > 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.