[time-nuts] Another "atomic" clock question

Lars Walenius lars.walenius at hotmail.com
Fri Mar 7 13:28:40 EST 2014

Chris, about using one Arduino for two GPSDO controllers:

Even if a microcontroller has lots of capacity I would recommend to use separate controllers for each oscillator. One of the reasons is what Tom van Baak said about using only one interrupt to avoid jitter and even if you trigger both channels from the same PPS and have just one interrupt you will have a problem that you can´t read two ADC´s at the same time.

Even the HC390 I wouldn´t use for two different oscillators to prevent crosstalk. Both the processor and HC390 is so cheap it isn´t worth the risk IMO.

Actually I would also recommend to put them in separate boxes even if it is more work  (and I´m lazy ) to get best performance.

Having two GPSDO´s that you can compare is very nice as long as you understand how they correlate , if that is not what you want to test. Of course you can also set one or both in hold mode to test them freerunning.

I have thought of connecting the M12 to the Arduino and if someone can help with code to get the sawtooth correction value into the Arduino and decoded I would be glad to have it.


From: Chris Albertson

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


Chris Albertson
Redondo Beach, California
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