[time-nuts] GPS lock of the FE5680. Current experiment and question

paul swed paulswedb at gmail.com
Fri Feb 10 22:53:51 UTC 2012

Just for fun have built up a simple multi-chip project to try my hand at
locking the FE5680 using the external efc connection mod. mentioned in
time-nuts. Typical counter micro rs 232 port stuff.

Essentially the system is 2 X 24 bit counters that toggle back and fourth
every interval selectable from 1 to 65000 seconds accumulating counts. I am
measuring the 10 Mhz of the RB and using a novatel superstar II GPS rcvr. I
do have a better timing rcvr but this was handy and at this point most
likely good enough.

My question is this. It seems that the system must use pretty large
accumulation times like 1000 or 10000 seconds and higher to see the drift
in 1 sample.

I think this actually all makes sense. Since the frequency accumulating is
1 x 10^ 7 and the gate so far is 1 X 10^3 or the total resolution is 1 X 10
10th. Essentially accumulating 3 or so counts and averaging most likely
only gets the lock down to 1 X10^10 or a bit more perhaps. Using a
accumulate of 10,000 would be the 11th.
Is my logic on target here.

I only see a few counts one way or the other when I purposely increase or
decrease the RB offset. But it does indeed go in the correct directions. I
can adjust the dac by the drift and time method to get down in the -12th
region. So essentially the eyeball method is pretty good.

If this is all correct then for a given accumulated time there is an exact
24 bit number to always drive the RB to. I have an excel that figures out
that number for any accumulation time is.

By the way took Berts suggestion some weeks back of using the LTC1655 16
bit Vdac. Talk about easy to work with and in a 8 pin DIP.

Appreciate any guidance. Thanks.

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