[time-nuts] AD9912 DDS frequency resolution measurement?

[Bill Slade]

Hi,
Are you losing the least significant digit in your frequency measurement?  A double precision word has 52 bits of significance+sign, roughly corresponding to 15 decimal places.   Adding 3uHz to 1Ghz (or hundreds of MHz) is just about on the limit of generating significance errors due to word length. Just a thought.
Cheers,
Bill

________________________________
From: time-nuts <time-nuts-bounces at lists.febo.com> on behalf of Anders Wallin <anders.e.e.wallin at gmail.com>
Sent: Monday, February 18, 2019 2:47 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] AD9912 DDS frequency resolution measurement?

Hi all,
We've tried to measure the 1e9/2**48 = 3.55uHz frequency resolution of an
AD9912 DDS (clocked at 1GHz SYSCLK), but found that the output corresponds
to an FTW with the LSB set to zero.
Results around 10MHz output, where we expect a step of 3.55 uHz for each
step of the FTW, but instead see a step of 7.1 uHz every second step.
https://drive.google.com/file/d/1U5nbg2RSOpVRa8VLOJ6bCWsyihcSBhBZ/view
https://drive.google.com/file/d/1c6CoQIWzRoM4y8CLoRLhi7sh2pMCywwo/view

has anyone else worked a lot with the AD9912 and verified the frequency
resolution?
(I can describe the setup&measurement more in detail, but we believe we
know roughly what we are doing and tried this with two different AD9912
boards and two different measurement systems)

regards,
Anders
(this question is cross-posted to the AD forums also)
_______________________________________________
time-nuts mailing list -- time-nuts at lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.