[time-nuts] yet another GPSDO design, or so

WarrenS warrensjmail-one at yahoo.com
Tue Jun 29 12:02:41 EDT 2010


Attila

The fact that is possible to build a 24 bit D/A dos not make it a practical 
solution for any that CAN not build it or that do not want to put in ALL the 
engineering time that would be needed to perfect such a design or the cost 
to buy one.

There is a simple and low cost compromise solution available for those,
If they have or can make the course range a low noise Manual adjustment 
thing.
The fine Dac then ONLY needs to cover how much range the Osc will drift 
between manual adjustments time periods,
and even if the manual adjustments are limited to say monthly or even 
yearly,
don't need more than about 12 bits to do a good job for many typical 
oscillators, after they are past their initial fast aging rate.
This is not the universal solution, but for many it is 'good enough' and 
KISS simple, and the more important thing for some,
it can use standard off the shelf parts, and yet still provide lower noise 
than the 24 bit DAC universal solution whose main advantages is that it 
needs no manual adjustment.

example:
the HP 10811 with manual adjustment.
It has a + - 5 volt EFC range.
but a 10 to 100 mv EFC range can go months between needing to reset the 
manual adjustment,
because the aging rate of the old well run in units can be orders of 
magnitude better than their worse case spec after the first month or so of 
continuous operation.

ws

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[time-nuts] yet another GPSDO design, or so
Attila Kinali attila at kinali.ch
Tue Jun 29 13:31:34 UTC 2010

On Tue, 29 Jun 2010 21:32:10 +1200
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> Its possible to build a 24 bit resolution D/A using a synchronously
> filtered PWM circuit.
> A pair of PWM outputs and a few relatively low precision resistors and
> capacitors together with a low noise low drift reference are required.
> The technique takes advantage of the fact that the required EFC voltage
> changes slowly and isnt updated at a highg rate.
> The synchronous filter technique eliminates the very long time constant
> RC filters required with an asynchronously filtered PWM waveform.

I've thought about that, but i'm afraid that this will add too
much phase noise trough EFC noise. Though, i have not calculated
how much noise this would generate.

   Attila Kinali 





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