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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jun 29 09:32:10 UTC 2010

Attila Kinali wrote:
> Moin,
> On Sat, 26 Jun 2010 21:14:02 EDT
> EWKehren at aol.com wrote:
>> What you want is basically a Shera Board. That design has been around for
>> quite some time and has served me very well.
> Yes. The Shera Board and similar designs serve as an example for me.
>> I have a total of six running
>> including two controlling Rubidium. There are in my opinion a couple of
>> problems: not every 4066 works on the design the 18 bit D/A is very hard to
>> find  and now expensive and the single step of the D/A is intended for a 1.7
>> E-13  frequency step.
> Yes. My goal is to update the venerable 4066 with something more
> modern and have components that are easy to get trough farnell, digikey,
> mouser, and all the other distributors. Yes, 16bit D/A seems to
> be the maximum that is currently available. It crossed my mind
> to build a 24bit R-2R D/A using discrete components, but this might
> have actually a worse performance than a off the shelf 16bit D/A.
> (temperature drifft, resistor values missmatch, EMI, etc)
> 			Attila Kinali
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.


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