[time-nuts] Thunderbolt? (re simple gpsdo.)

[Tom Harris]

I too have been mulling over a minimal GPSDO.

Has anyone mentioned a PWM DAC? Even a "toy" microcontroller like the
Atmel AVRs can generate 16 bit resolution PWM, which sounds like it
would be heaps. As long as the voltage reference is low noise, it does
not matter if it drifts slowly, since the control loop adjusting the
OCXO frequency will adjust it. Of course you will need a long time
constant filter to smooth the PWM, but a 2 pole filter constructed
from an opamp will do the job.

The micro can also capture the 1PPS from the GPS if it is clocked by
the OCXO, and use the value to close the control loop.

So a minimal GPSDO looks like a simple microcontroller (I would use
Arduino since we use these at work whenever we need to build something
in an afternoon) with a filter for a PWM DAC, so one opamp and a few
discretes. Of course as I write software for such beasts as a trade I
am giving the software a zero cost.

On 31 December 2011 06:46, Stanley <timenuts at n4iqt.com> wrote:
>
>
>>
>>> "What is the simplest design for a GPSDO that uses only the PPS signal
>>> from
>>> a modern GPS?"
>>
>>
>> Some sort of oscillator with a voltage control.
>> CPU with a timer/counter that can capture the PPS.
>> DAC.
>> Software.
>>
>
> How about MSC1200 : http://www.ti.com/lit/ds/symlink/msc1200y3.pdf
>
> Stanley
>
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-- 

Tom Harris <celephicus at gmail.com>