[time-nuts] GPS DO Alternatives

[Bob Camp]

Hi

In order to get a useful average, you need to have the pps arrive just a bit ahead and a bit behind the 10 MHz out of the OCXO. If the GPS PPS moves more than 100 ns between samples you get data from different cycles on the OCXO. That does not give you useful information to average. If the pps out of the GPS and the OCXO are both very accurate, you only get a one or a zero out. Again you can't average the result very well.

Assuming everything is arranged "just right", you need to average a *lot* of samples to get the job done. The gain of this phase detector is highly dependent on the jitter of both the GPS and the OCXO. Since gain is critical to the setting of all the loop coefficients, that can be an issue. It also means that precise measurement of both these jitter terms is something that must be done before you are likely to come up with  loop that will run without oscillation. 

Bob

On Dec 9, 2012, at 12:03 PM, Chris Albertson <albertson.chris at gmail.com> wrote:

> On Sat, Dec 8, 2012 at 11:14 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> 
>> albertson.chris at gmail.com said:
>>> What if I use a flip flop.  The PPS from the GPS connects to the "set"
>> input
>>> of the FF and also to the Arduino interrupt.   The output of the OCXO
>> goes
>>> to the reset pin of the same FF.
>> 
>> I don't think that is going to do what you want.  The problem is what
>> happens
>> if set and reset are on at the same time?
>> 
> 
> This would be the best case.  The result will be random and we'd read an
> average value of 0.5 from the flip flop.
> 
>> 
>> If I was doing that, I'm make the PPS from the OCXO 50% duty cycle and use
>> the PPS from the GPS as a clock with the PPS from the OCXO as the data
>> input.
>> 
> 
> I think you want everything to be edge triggered.  The flip flop should
> look only at the leading edge of each pulse.
> 
>> 
>> 
>> Which brings up another worm for the can.  How are you going to get a PPS
>> from the OCXO?
> 
> 
> You don't get a PPS from the OXCO.  It runs at 10MHz (Or whatever you like)
> so it resets the flip flop every 100 nanoseconds.
> 
> The way this works is the PPS from the GPS sets the FF to "1" once per
> second and then some time later the OXCO resets it back to zero.   If we
> sample the FF at some fixed time after the PPS and that fixed time is less
> then the OXCO period then the FF will be either 1 or 0.   THe computer
> tries to drive the OXCO so that the sampled value is 0.5 on average.   So
> it looks at the last 1000 samples and tries to hit "500".
> 
> Basically what we have is a one bit counter but if we average enough
> samples we get pretty good resolution.
> 
> 
> 
>> That's going to be hard without some soldering.  My straw man
>> would be to use the OCXO to clock whatever uP you end up using and generate
>> the PPS with a counter/timer.
>> 
>> An alternative would be 7 TTL chips or a CPLD/FLGA.
>> 
> 
> If you used the CPLD then you have to agree to supply programmed chips for
> life and to make any change any experimenter asks for with fast turn
> around.
> 
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
>> 
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> 
> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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