[time-nuts] Another "atomic" clock question

Chris Albertson albertson.chris at gmail.com
Wed Mar 5 22:21:57 EST 2014


On Wed, Mar 5, 2014 at 5:43 PM, Didier Juges <shalimr9 at gmail.com> wrote:
> Tom and Bob,
> It is not obvious to me that it is "easier" to simply apply a correction in
> nS increments with a range as wide as 100nS. How is this done? Using
> switched delay lines or delay gates?

Here is my plan for processing saw tooth data.  If it's not going to
work I'd rather hear about it now then a month from now after I've put
in some effort.

This is going into Lars' Arduino based GPSDO.  Every second I read the
voltage on a TIC capacitor.  This tells by the phase in nanoseconds
between the PPS and the OCXO.  Then I add whatever the current GPS
sawtooth value is to whatever my TIC said.   I compare this to a set
point.  This is the phase error.  The OCXO is adjusted based on a
filtered version of this error.

So in short, I don't correct even try to delay the pulse.  I don't see
any need to do that.  I measure the pulse and get a number in
nanoseconds.  then I use sawtooth to correct the number.

It seems way-hard and with no purpose to correct the pulse and then
measure it.  Better to correct the measurement.  I think it is more
accurate too a delay could never be perfect.

The controller has LOT of spare capacity so I don't see way I can't
add one of more TIC channels and a few more DACs  I should be able to
discipline an OCXO and my Rb  oscillator from the same GPS PPS input.
 The 74HC360 is only 1/2 used an Arduino has enough spare pins.  Any
one more 74HC4046 and some passive parts would be required to build a
dual channel GPSDO.

It will be interesting to look at andompare the 10MHz outputs of two
oscillators that are being disciplined by the same controller and GPS
receiver.

-- 

Chris Albertson
Redondo Beach, California


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