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

[Bob Camp]

Hi

Summing a pair of DAC's and checking them with an ADC is one way to get the
job done. It's been used quite a bit. 

16 bit DAC's are sub $3 items these days with pretty good specs on the
parts. A multi channel <1 ppm accurate 24 bit "DC" ADC is a fairly common
part as well. Raw parts cost from Digikey for PIC and the rest of it (except
reference) likely would be sub $20. If you have all the parts already I
suppose it could be free. Even with a $50 charge for a quick turn PCB
there's not a lot being spent for the ADC side of things. 

You can spend a *lot* on a reference. That's going to be true for any long
term stable stand alone EFC drive setup. My guess is that reference noise on
affordable parts may drive you to the big R/C's. Putting at least one
channel of the ADC after the big R/C lets you get a handle on leakage
issues. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, June 29, 2010 5:32 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] yet another GPSDO design, or so

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.

Bruce


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