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
******************
[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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