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

[Chris Albertson]

On Fri, Feb 10, 2012 at 2:53 PM, paul swed <paulswedb at gmail.com> wrote:
> 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.

Do you ever need such a large counter?  As you say the counts are only
small integers.   If the two are close they would not overflow even a
4-bit counter.   One way to go is to have only a two bit counter (just
a pair of flop flops) and end the integration as soon as the second
bit flips.  If you are building a FLL you only need to know which is
faster.

If you want faster convergence I think you have to measure relative
phase and use a PLL.   Then you don't need to wait for a full cycle.
Maybe only a few degrees of phase difference is enough

My plan is to use the 74HC4096 is the detector.  It has the required
flip flops inside.  Use that to dive the RB5680 and then when they are
reasonably close the 74HC4096 also contains an XOR type phase
detector, so switch to that.

Chris Albertson
Redondo Beach, California