[time-nuts] Rubidium as radio reference

WB6BNQ wb6bnq at cox.net
Sun Nov 4 23:15:17 UTC 2012


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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