[time-nuts] AD9912 DDS frequency resolution measurement?

[Anders Wallin]

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!?

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...

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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