[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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