[time-nuts] Thunderbolt? (re simple gpsdo.)

EWKehren at aol.com EWKehren at aol.com
Sat Dec 31 12:21:09 UTC 2011


In my opinion you have to look at it from the point of application.  
Hopefully I will be able to share the test results soon. For us DNL is key, INL  
is specked over the full range and since we use it in a filter application,  
based on the data that I have, can be ignored.
In a Rb application I use 1.5 E-14 steps with a total range of 1 E-9, with  
OCXO's the steps are 1.5 E-13 and rage 1 E-8.  In some applications I use  
smaller step sizes on the OCXO at the expense of range. Some OCXO's aging 
allow  using 1.5 E -14. 
Do not forget that step sizes are 61 uV and take that into consideration  
when you look at the temperature specs. I also use it in an environment where 
 temperature is better than + - .2 C. The other nice thing about the 1655 
is that  you have a reference output that is perfect for setting output range 
and even  changing the output to + -. 
 
 
In a message dated 12/30/2011 9:48:38 P.M. Eastern Standard Time,  
davidwhess at gmail.com writes:

Did you  test the LTC1655 INL?  The data sheet says plus or minus 20
counts  maximum.

I suspect Linear Technology designed those low DNL high INL  parts for
just this sort of application where only monotonic behavior  really
matters.  Their equivalent current output DAC costs about twice  as
much not including a precision transimpedance amplifier but has an  INL
specification of plus or minus 1 count.

Every couple years I  consider the design of a digitally adjusted
oscillator and do a search for  likely parts.  I wonder if it would be
more cost effective to use an  instrumentation ADC to correct a less
expensive DAC design like one based  on a PWM.

On Fri, 30 Dec 2011 17:48:24 -0500 (EST), EWKehren at aol.com  wrote:

>Over the last two years along with two list members that may  want to pipe 
 
>in, I have spend a large amount of time on D/A's  and we went as far as  
>developing a test board using the LTC 2440  and testing numerous D/A's 
taking in  
>to consideration  performance, solderability, cost, availability and the 
>winner  is  LTC 1655 by a long shot, is even available in a DIP with 16 
bits 
>more  than  you need for any Rb and if you want 20 bits, dithering is an  
option. 
>My testing  consistently shows with OCXO's aging that  will in most cases 
>allow operation of  an OCXO for 3 years with  out intervention. To top it 
off 
>the LTC1655 cost less  than $ 10.  Testing the old AD 1861 was an eye 
opener 
>but considering what  its  purpose was and its time the best choice.
>Bert
>  
>In a message dated 12/30/2011 4:24:37 P.M. Eastern Standard  Time,  
>timenuts at n4iqt.com writes:
>
>The DAC   and it's voltage reference looks to be the weak link in the  
>digital  
>control and the "simple" goal. The CPU I  mentioned before on closer look  
>doesn't have a good DAC. The 20  bit TI DAC1220 looks better but not sure  
>you 
>can find it  in the same package as the CPU. The cheap Rb standards  with  
>digital control would not need a DAC and maybe this points to a   simpler 
>GPSDO that doesn't control the XO with analog but corrects it  with  a DDS 
>but 
>again finding them both in one chip is  the problem. I have seen  OCXO and 
>DAC 
>in the same  package and even the DDS and OCXO combined but  they didn't 
fit  
>the simple goal. Not even sure how good they were. I know  they  are hard 
to 
>find.
>
>Stanley   
>
>
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