[time-nuts] Rubidium as radio reference

WB6BNQ wb6bnq at cox.net
Mon Nov 5 02:26:40 UTC 2012



   Joseph Gray wrote:

     Internet is flaky lately. I'll be brief, as I'm sending this from my
     phone.

     I don't need timenut accuracy for this. I want to make a setup for
     measuring off the air, so local amateurs can have some way to check
     a radio.

     Using a service monitor, I can get a linear AC voltage from the
     discriminator that corresponds to deviation. I can also measure a DC
     noise squelch voltage that is inversely proportional to RSS, but not
     linear. For carrier error, I get about 3V DC at the discriminator
     for on frequency. Then I get a linear relationship for off
     frequency.

     I still need to figure out how to measure these voltages with an ADC
     and interface to a R-Pi.

     Just a fun project, I thought.

     Joe Gray
     W5JG

   On Nov 4, 2012 4:15 PM, "WB6BNQ" <[1]wb6bnq at cox.net> wrote:

     Hi Joe,

     The reference oscillator is only part of the story.  Being a typical
     "FM" radio
     the normal method is to observe when the carrier is in the center of
     the
     discriminator's bandpass.  The particular center point is not a
     constant, per se,
     because the discriminator is typically adjusted so that the swing of
     the
     modulation is symmetrical and that is not usually at the exact
     center.  The
     discriminator itself may not be symmetrical and often times the
     center is skewed
     because of it.  Therefore you cannot necessarily rely on the center
     point as
     being "on" frequency.  In addition to the so-called reference
     frequency, you
     would also have to stabilize the oscillators that feed the mixers if
     there is
     more than one IF frequency involved, which is usually the case.

     The typical stability of these types of radios are pretty good but
     no where near
     time-nuts type stuff.  When transmitting, the heat build up inside
     the radio will
     alter the transmit frequency by a few hertz to as much as few 100
     hertz depending
     upon the particular radio.  This is normal and usually does not
     affect the
     ability to communicate due to the wide bandwidth of both the IF's
     and the
     discriminator.  At the receiving end you can watch such action in
     real time over
     the length of the transmission.

     If you are really trying to do something more in line with timenuts
     type stuff,
     you will, most likely, need to do a different approach.  Perhaps you
     could give
     more details allowing for a better run of suggestions.

     As for the FE-5680 Rb, it would require some filtering and such.
     Instead of
     trying to modify the radio for a direct feed from the Rb it would
     serve you
     better as a source in a PLL type arrangement.

     Just my two cents.  So give us more details.

     Bill....WB6BNQ

     Joseph Gray wrote:

     > I know that some here are Amateurs and have used external sources
     to
     > provide a more accurate reference for a receiver. So, I have a
     noob
     > question or two.
     >
     > Is the programmable FE-5680A suitable to replace the 12.8 MHz
     > reference in a Midland XTR? If so, where do I feed the rubidium
     into
     > the circuit (see attached screen clip)? X101 is the 12.8 MHz
     crystal
     > and R107 is used for temperature compensation.
     >
     > What I have in mind is to use the XTR radio for measuring
     off-the-air
     > carrier frequency error. It's just part of a project that I've
     been
     > thinking of doing. Things are still in the planning and
     experimenting
     > stage.
     >
     > Joe Gray
     > W5JG
     >
     >
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References

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