[time-nuts] 5061A Problem Child

Robert Benward rbenward at verizon.net
Wed Jun 16 00:42:07 EDT 2010


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_steinmetz at lavabit.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at 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
>
>
>
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