[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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