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

ewkehren at aol.com ewkehren at aol.com
Tue Jun 29 17:57:21 UTC 2010


Hi again.
 Any one interested should hold off for a day buying. I have contacted the seller asked how many he has and what a quantity price would be. To much interest and the price will go up. I think the market for audio equipment replacement is shrinking and there will be more reasonable offers out there for this device, because repair of the audio equipment is prohibitive
Bert






-----Original Message-----
From: EWKehren at aol.com
To: time-nuts at febo.com
Sent: Tue, Jun 29, 2010 1:45 pm
Subject: Re: [time-nuts] yet another GPSDO design, or so


Hi
just bought four AD 1861 on ebay with shipping was $11 each. Will see  
hat I get, but they are out there and 18 bit will cover in my opinion most  
pplications
ert


n a message dated 6/29/2010 12:21:31 P.M. Eastern Daylight Time,  
ists at rtty.us writes:
Hi
Summing a pair of DAC's and checking them with an ADC is  one way to get the
ob done. It's been used quite a bit. 
16 bit  DAC's are sub $3 items these days with pretty good specs on the
arts. A  multi channel <1 ppm accurate 24 bit "DC" ADC is a fairly common
art as  well. Raw parts cost from Digikey for PIC and the rest of it  
except
eference) likely would be sub $20. If you have all the parts  already I
uppose it could be free. Even with a $50 charge for a quick turn  PCB
here'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
erm  stable stand alone EFC drive setup. My guess is that reference noise  
n
ffordable parts may drive you to the big R/C's. Putting at least  one
hannel of the ADC after the big R/C lets you get a handle on  leakage
ssues. 
Bob
-----Original Message-----
rom:  time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
ehalf Of  Bruce Griffiths
ent: Tuesday, June 29, 2010 5:32 AM
o: Discussion of  precise time and frequency measurement
ubject: 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 
or
> 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
o
> find  and now expensive and  the single step of the D/A is intended for a
.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
    
ts possible  to build a 24 bit resolution D/A using a synchronously 
iltered PWM  circuit.
 pair of PWM outputs and a few relatively low precision resistors  and 
apacitors together with a low noise low drift reference are  required.
he technique takes advantage of the fact that the required EFC  voltage 
hanges slowly and isnt updated at a highg rate.
he  synchronous filter technique eliminates the very long time constant 
C  filters required with an asynchronously filtered PWM  waveform.
Bruce

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