[time-nuts] 4046 experiment for gpsdo

[Magnus Danielson]

Hi,

On 09/27/2015 02:04 AM, Bob Camp wrote:
> Hi
>
> If you digitize the beat note it’s fairly simple:
>
> The beat note is not really a sine wave. It’s periodic, but not a pure sine. The
> reason is fairly simple. The frequency changes as the beat note changes the EFC.
> You have a lower frequency as it gets closer to the “zero frequency”.

In closed-loop yes. I actually proposed to start in open-loop mode, 
estimate the frequency and then close the loop after a coarse frequency 
adjustment have been done.

In closed loop, the beat-note shape, which comes from non-linear 
modulation, clearly indicates the direction. You can use this knowledge 
to learn the polarity, as one of many methods.

A semi-closed loop will get you the shape, i.e. a loop where you only 
close the proportional part, but not the integration part. Looking at 
the shape (PWM) of the beat-note you can learn the polarity and the rate 
the amplitude of the frequency error. Once you know the polarity and 
error, just initiate the integrating part accordingly and close the 
integration part of the loop (by setting a non-zero integrating constant).

Another method is naturally to do a quadrature detection. As I said, 
there is so many ways to do it, and I wanted to illustrate a different 
approach. If you can get a 360 degree phase measure, frequency measure 
is almost trivial to derive from that.

> The net result is a signal that is a bit flat on the side that is towards the zero. It’s a
> bit sharper away from the zero. Fit it with a sine wave and you will get a residual. From that
> you can work out which side is which.

You can also measure the PWM if you like.
PWM < 50% => positive frequency
PWM > 50% => negative frequency

Cheers,
Magnus

> Bob
>
>> On Sep 26, 2015, at 5:47 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>
>> magnus at rubidium.dyndns.org said:
>>> Another method would be to measure the phase-detector beat-note  frequency
>>> (most have mixer-like behavior), which you should be able to  measure with
>>> quite good precision, then set the EFC accordingly and then  close the loop.
>>
>> How do you get the sign out of a beat note?
>>
>>
>> --
>> These are my opinions.  I hate spam.
>>
>>
>>
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