[time-nuts] AD9912 DDS frequency resolution measurement?

[Magnus Danielson]

Hi Anders,

On 2019-02-21 14:47, Anders Wallin wrote:
> AD replied to my message on the AD forums: (upon inspection they will
> revise the datasheet to say the resolution is 47 bits now... ha!)
> https://ez.analog.com/dds/f/q-a/107510/ad9912-ftw-lsb-always-zero
> the datasheet has a copyright "2007-2010" so maybe the chip has been out
> for 11-12 years already (?) but nobody bothered to measure the frequency
> resolution!?

First of all, good catch!

I bet it took some internal process only to conclude that they knew of 
it but thought nobody would be affected. It could be that they didn't 
know, set up a simulation only to conclude that they did not nail all bits.

> We might give the visual oscilloscope-method a go also at some point - just
> for kicks.
> For example one could set the DDS frequency so that there's a "beat" of
> 3600 seconds (1 h) - i.e. the 10MHz reference and the DDS output drift one
> period (100ns) during an hour.
> If I calculated correctly when the frequency is changed by one LSB that
> 3600s beat should change by around +/-45 seconds. If the change is 2LSB the
> beat-period changes by around 90seconds.
> To see the 'coincidence', maybe we need to collect traces from the
> oscilloscope at 5s or 10s intervals, and do cross-correlation...

If you use Linear Regretion / Least Square frequency estimation methods, 
it will be easy to be conclusive.

The counter/oscilloscope setup would work. You could even record the 
Lissajou pattern shift ever so slowly.

Cheers,
Magnus

>
> Anders
>
>
> On Wed, Feb 20, 2019 at 2:00 AM John Miles <john at miles.io> wrote:
>
>>> A friend of mine has a x96 multiplier from 108 MHz to 10 GHz but we don't
>>> have access to any counters capable if measuring the difference.
>> For sanity-checking an extremely small frequency discrepancy, perhaps you
>> could set up the DDS to output a signal very close to a plesiochronous
>> reference of some kind.  E.g., a binary fraction of its own clock frequency
>> plus or minus 1 LSB.  Then trigger a scope from the DDS output while
>> watching the clock slide past it on the other channel.
>>
>> This turns the question into one that can be answered definitively with a
>> stopwatch, rather than ambiguously with a counter. :)
>>
>> -- john, KE5FX
>> Miles Design LLC
>>
>>
>>
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