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 <[email protected]> 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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