Evaluating a DMTD - Page 1
Forum section: electronics / general

author: erikka
author_rank: Regular Contributor
timestamp: October 12, 2022, 08:02:29 am
content: To measure accuracy and stability of various 10MHz frequency sources a small DMTD was build and the performance is being checked.For performance evaluation a 1Gsample/second 2 channel 16bit DDS directly connected to the two inputs of the DMTD is used.This allows the generation of two signals with well defined phase and frequency differences.To test phase stability and phase noise, the phase/frequency difference between two identical input signals at different tau was measured and plot as power spectrum and ADEVTo test if there is any pulling, two input signals 0.01Hz apart where measured and the linear residue of the unwrapped phase difference versus time was plotSome plots are added to this post, each indicate the used tau and the frequency difference between the two input signals.The power spectrum did not show anything interesting and was not added.The phase plot with 0.01Hz difference shows some +/- 3ps phase pulling.The long term phase plot with 0Hz difference shows the phase is not very stable (12 ps variation), this is possibly due to temperature variationsThe frequency data and stability plots suggest it should be possible to observe 0.1 micro Hz difference with a tau of 1 second.Overall the performance seems to be good enough.Any suggestions for other tests that should be performed to gain confidence?

author: erikka
author_rank: Regular Contributor
timestamp: October 12, 2022, 04:15:37 pm
content: A practical measurement for illustrationOCXO (not completely warmed up) versus a small Rb

author: mendip_discovery
author_rank: Super Contributor
timestamp: October 12, 2022, 08:03:38 pm
content: So which Dual Mixer Time Difference (DMTD) setup did you build?

author: erikka
author_rank: Regular Contributor
timestamp: October 13, 2022, 05:10:54 am
content: I created my ownIt does not use zero crossing detection (a TIC) but uses a DFT to calculate the phase.To reduce spectral leakage a PLL keeps the mixer frequency plus IF frequency "close" to the reference input.See the block diagram

author: citizenrich
author_rank: Newbie
timestamp: October 14, 2022, 06:04:26 am
content: I hope folks here provide feedback, but if you want some more then I suggest joining the time-nuts mailing list (if you haven't already) where DMTD designs are often discussed. The time-nuts may have lots of questions for you, thoughhttp://www.leapsecond.com/time-nuts.htm

author: erikka
author_rank: Regular Contributor
timestamp: October 14, 2022, 06:20:44 am
content: Thanks

author: erikka
author_rank: Regular Contributor
timestamp: October 16, 2022, 08:39:05 am
content: A next evaluation action was to compare the DMTD with a Picotest U6200A timer/counter.The U6200A claims a time resolution of 100ps and up to 12 digits/second frequency accuracy.As can be seen in the attached plot the ADEV of the DMTD is roughly an order of magnitude better than the ADEV of the U6200A.Not bad for a small standalone device as shown in the last attachment.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 18, 2022, 09:55:18 am
content: Do you plan to open schematics and sources?

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 10:29:31 am
content: Not sure yetIt may be sold as a ready product.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 18, 2022, 10:47:21 am
content: Quote from: erikka on October 18, 2022, 10:29:31 amWhat is the expected price?

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 10:48:17 am
content: Also not decided but could be around $100

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 18, 2022, 10:54:23 am
content: Quote from: erikka on October 18, 2022, 10:48:17 amWhen will it be possible to order?

author: erikka
author_rank: Regular Contributor
timestamp: October 21, 2022, 08:15:39 am
content: There are currently no solid plans, and there will neven be the option for advance ordering.

author: erikka
author_rank: Regular Contributor
timestamp: October 21, 2022, 12:00:48 pm
content: After further analysis it appeared to be possible to model the phase pulling as a function of the phase difference enabling the measurement, modeling and removal of most of the phase pulling.This did great for the ADEV when measuring not equal frequency input signalsBelow ADEV was measured with 0.1Hz difference between the two input signals to ensure any phase pulling, when still present, would be visible