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

EWKehren at aol.com EWKehren at aol.com
Tue Jun 29 17:45:09 UTC 2010


Hi
 just bought four AD 1861 on ebay with shipping was $11 each. Will see  
what I get, but they are out there and 18 bit will cover in my opinion most  
applications
Bert
 
 
In a message dated 6/29/2010 12:21:31 P.M. Eastern Daylight Time,  
lists at rtty.us writes:

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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