Why you need a fast and accurate frequency counter - Page 1
Forum section: electronics / projects

author: erikka
author_rank: Regular Contributor
timestamp: October 16, 2022, 04:17:34 pm
content: Some time ago I build two GPSDO. Both claim, based on their internal measurements, that they are stable within 1e-10 of 10MHz. One measures and corrects per second and the other uses up to 400 seconds to sum the pulses to reach sufficient resolution.But its unclear how stable they are on the short term (1 second and below) so a DMTD (Dual Mixer Time Difference) frequency counter was used to measure both with a tau of 0.005 seconds against a Rubidium reference.See attached frequency measurement plotThe red trace is the noise floor of the counter measured by routing the Rb output to both inputs of the DMTD.The green trace is an OCXO measured against the Rb. This was done to get an idea of the frequency noise when using the DMTD with two different inputs. The noise is the same as the green trace.The pink trace shows a frequency up/down jump every 0.5 seconds. This is possibly caused by a supply voltage swing every 0.5 seconds, such as from a blinking led.The blue trace shows a, on the long term, very stable output, but on the short term there is a lot of frequency variation. A possible cause is an interference of the 10MHz output with another signal, close to 10MHz, that is also generated inside the GPSDO.Its clear that, without the ability to measure frequency accurately at short tau, it would be difficult to diagnose these problems.

author: hgl
author_rank: Regular Contributor
timestamp: October 16, 2022, 04:48:16 pm
content: Let's see the ADEV diagram.

author: erikka
author_rank: Regular Contributor
timestamp: October 16, 2022, 05:02:59 pm
content: As requested the ADEV

author: hgl
author_rank: Regular Contributor
timestamp: October 16, 2022, 06:07:45 pm
content: The green line becomes flatter with time, which could mean that the warm up for OCXO and/or Rubidium was too short. GPS is only stable when averaged over minutes. Below that, the PPS signal has too much jitter. There can also be disturbances in the supply and in the control voltage for the OCXO.

author: erikka
author_rank: Regular Contributor
timestamp: October 16, 2022, 07:12:46 pm
content: The idea was indeed to detect small short term disturbances which where invisible with a 1 second gate timeGPSDO referred to the 10 MHz output, not the PPS

author: KE5FX
author_rank: Super Contributor
timestamp: October 16, 2022, 07:38:45 pm
content: The DMTD residual is looking really good. Is that Bill Riley's design?

author: erikka
author_rank: Regular Contributor
timestamp: October 16, 2022, 07:41:34 pm
content: No, my own design.See:https://www.eevblog.com/forum/metrology/evaluating-a-dmtd/

author: hgl
author_rank: Regular Contributor
timestamp: October 16, 2022, 08:40:37 pm
content: See the overnight frequency and stability plot on page 4 from "An Informal Evaluation of a Leo Bodnar GPS Frequency Reference by W.J.Riley"

author: G0HZU
author_rank: Super Contributor
timestamp: October 16, 2022, 10:40:09 pm
content: Many years ago, I bought an old (and faulty) HP 8405A vector voltmeter. This was one of the early versions of the HP 8405A from the mid 1960s and it had lots of age-related faults that affected its performance. I ended up replacing quite a few parts to get it to be stable in operation.One of the confirmation tests I did back then was to test it for phase stability between channel A and B using a common clock at 10MHz. This old VVM based system won't match the performance of your modern DMTD, but I recall getting short term results in the 3E-11 range at sub 0.1 second time intervals. It wasn't a very well controlled experiment at the time and all I did was log the phase port with a fast DVM and this gave me the slope of phase change vs time with about a 50ms time interval.I'd really like to buy a more modern HP 8405A as I probably haven't cleared all the failing parts in mine. It may be possible to get an improvement in the residual noise on the phase recorder output with a newer and healthier HP 8405A than mine. I'll dig it out tomorrow and revisit this stuff. This assumes it is still working! I rarely use it these days, but I'll jump at any excuse to dig it out of storage

author: G0HZU
author_rank: Super Contributor
timestamp: October 17, 2022, 09:21:03 pm
content: I warmed up the HP 8405A this evening and logged the phase output port on the +/- 6 degrees scale whilst feeding the A and B inputs from the output from an Agilent ESGD sig gen at 10 MHz.See below for the first attempt to log the phase stability using a DVM at the phase detector output port. This seems to be about the same result I got many years ago. Zooming in on the data, it looks like the fastest rate of change is a 0.01deg phase change in about 0.07 seconds. The DVM seems to be taking data every 70ms. A newer and healthier HP 8405A VVM will probably be cleaner than this.

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 05:52:09 am
content: For comparison here the phase resolution of my DMTDThis data was gathered with 0.002 seconds time resolution

author: G0HZU
author_rank: Super Contributor
timestamp: October 18, 2022, 08:48:20 am
content: Hi erikka, thanks.Your DMTD does look to be very clean in comparison. I tried swapping the DVM for a low noise scope to look for any detection bandwidth limitation with the HP 8405A and it does look like the phase detector output starts to slowly roll off above a few tens of Hz. This was a bit disappointing. Therefore, there isn't much point trying to log at a faster rate.I don't have a GPSDO here, but I do have my old offair standard. This locks a 10MHz crystal to the local 198 kHz BBC transmitter at Droitwich. I designed this unit many years ago. I'll see if I can capture the short-term drift using the HP 8405A. The 10MHz output tends to jitter +/- 0.02Hz on a fast counter due to propagation effects so it's not going to look very good compared to a GPSDO.

author: MIS42N
author_rank: Frequent Contributor
timestamp: October 18, 2022, 12:34:46 pm
content: I wish there were someone near me to do this. I am into the third version of my GPSDO, but don't have the test equipment to say if it is any good. The method I use is to adjust the control voltage at discrete intervals, and knowing the sensitivity of the OCXO to voltage change, and knowing the change in voltage, I make an estimate of the stability and accuracy of the OCXO. Currently the adjustments are at 1500 second intervals and up to a hundred µV. The control voltage is synthesised 24-bit DAC buffered with a precision voltage source (MAX6350) supposed to be good for 4µV/C. Theoretically the frequency is within 1 part in 10^-10 but there's no direct proof. The OCXO is supplied by its own linear LDO, with the rest of the circuit run from another. Even so, I am getting a daily fluctuation in reported control voltage which, if I could get rid of it, would allow longer periods between adjustments.But to build/buy equipment to prove it is beyond my budget. One day I hope to find a time nut nearby.

author: hgl
author_rank: Regular Contributor
timestamp: October 18, 2022, 02:22:47 pm
content: SRS has a picture on its FS740 page that shows the basic stability of GPSDO with different locked oscillators.https://www.thinksrs.com/images/instr/fs740/FS740_lockedLG.gif

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 03:06:58 pm
content: Quote from: hgl on October 18, 2022, 02:22:47 pmThis post was about short term stability (otherwise you would not need a fast counter), not sure I understand how measuring long term stability relates this, please explain

author: hgl
author_rank: Regular Contributor
timestamp: October 18, 2022, 04:05:50 pm
content: Short term stability depends only on the quality of the OCXO and long term stability can be controlled with GPS. In this figure you can see how much averaging over time is approximately necessary to achieve a desired stability of the GPSDO. For short time stability look at the datasheet of your OCXO.In your case: regulating with 1s average is too short because GPS has only approx. 4E-8 stability in this time. With 400s averaging your OCXO must be stable enough over this time.Just measure your OCXO without GPS in free running mode.

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 04:11:39 pm
content: Sorry, I was not addressing how to discipline a GPSDO (either per second or at much longer interval) but what could be learned if you are able to see 10ms frequency variations. As demonstrated this can show problems that are not visible in the control loop of the GPSDO, even if it runs every second, as both these problems happen much faster then once every second and in the GPSDO loop they are averaged out and become invisible.

author: nctnico
author_rank: Super Contributor
timestamp: October 18, 2022, 06:42:50 pm
content: What you need is a phase noise analyser. Your tiny DMTD design is likely easy to adapt for that purpose. You'd still need a good reference though. I think a good quality OCXO should get you a long way

author: erikka
author_rank: Regular Contributor
timestamp: October 18, 2022, 07:08:15 pm
content: The noise level of the tinyDMTD is insufficient for a true PNAI recently also build a PNA but its lowest frequency is 1Hz and often problems in a build are in the 100Hz to 0.1 Hz range so having a counter with a range that overlaps with a true PNA is nice.