On 4/19/2016 4:10 AM, Rob wrote:
rickman wrote:
On 4/18/2016 11:29 AM, Rob wrote:
rickman wrote:
How important are time and frequency references to amateur radio
operators? I've been working on a radio controlled clock design that
would be capable of generating a 32.768 kHz, 60 kHz, 240 kHz, 1 MHz and
10 MHz frequency references in addition to providing the time and date.
Initially it would be capable of receiving the 60 kHz transmissions of
WWVB and MSF. With minor tweaks other stations could be received.

Would this be useful to others?

We use GPSDOs in our co-channel diversity repeater.
The signals we require a 10 MHz and 1PPS.
It would be convenient when modules for arbitrary frequencies could
be inserted (like 10.240 MHz). Our GPSDO allows only evenly divisible
fractions of 10 MHz (like 5, 2, 1 MHz) as auxilliary outputs.

Another thing is that the short-term accuracy is not optimal.
These references "wander around" the correct frequency.

Short term accuracy is what costs real bucks. You need a very stable
VCTCXO or ovenized VCXO. How much accuracy are you seeing and how much
would you like?

We have ridiculous requirements :-)
We already have such things but it is not enough.
Probably we should try a rubidium GPSDO.

The 10 MHz is used to lock a 430 MHz transmitter and it should remain
in phase with another one. It is preferable that the frequency remains
stable if a bit off.

What happens now is that the DAC in the GPSDO steps up and down every
couple of seconds, and the oscillators wobble around the correct frequency
in the 1E-9 area (with of course an average that is very close, more like
1E-11), and this results in funny interference patterns. What is
happening is clear when you put the 10 MHz outputs of two independent
boxes on the scope in X-Y mode.

I don't know what they are using for an oscillator, but if you have
control over it, can you reduce the corner frequency of the LPF on the
control loop? It sounds like the control loop is hunting to me. But at
10-9 I suppose it could be ambient thermal drift too. Yes, I think a
rubidium GPSDO would do the job.

I remember a couple/three years ago Symmetricom came out with a chip
scale atomic clock that can sync to a 1 pps. "Two orders of magnitude
better accuracy than oven-controlled crystal oscillators". Only $1,000.
Might do the job. They likely package this in a box level product
that will do what you want. Check out this one...

http://www.microsemi.com/products/ti...-2750#overview

They were bought by Microsemi it seems.

--

Rick