HP 33120A, frequency instability approx 100 µHz - Page 1
Forum section: electronics / testgear

author: binary01
author_rank: Contributor
timestamp: March 10, 2022, 01:45:44 am
content: Hi all,Hoping you might be able to offer some advice.I recently obtained a low-cost HP 33120A Opt 001 from the used-equipment-gambling-centre, also known as ebay. My intention was to use it as a backup signal generator for basic frequency calibrations while my main calibrator is away for external calibration. I routinely need to perform frequency calibration checks in the range 1 Hz to 50 kHz with an uncertainty better than 2 ppm. The unit is in visually good condition and operates without any errors reported, however it appears to have some short term instability in the output frequency that manifests as frequency noise of approx 50 to 100 µHz when measured by a frequency counter (I have checked with multiple counters). The degree of noise is unchanged when the 33120A is using its internal clock or locked to GPSDO (although the mean value is corrected by using the GPSDO). The output frequency can be quite stable for some seconds, but will then bounce fast and then slow by approx 50 - 100 µHz, sometimes more. At higher frequencies (>100 Hz), instability at this level is insignificant for my purpose, but when testing at 1 - 10 Hz, it is problematic. Can any others with 33120A indicate if I am simply expecting too much from this instrument, or if this performance is indeed a fault? If it's important, I am usually using square wave 5 Vpp.Thanks in advance for any feedback.

author: jwet
author_rank: Frequent Contributor
timestamp: March 10, 2022, 05:29:40 am
content: I have a 33120A and a Racal 1992 counter with a TCXO that pretty much dead on and I can verify similar things. The reading bounce around a fair amount. I ran the counter in frequency and period mode and its not super stable. I haven't researched the spec's but I will look around. It would seem like an error .0001 Hz on a 10 Hz signal wouldn't be out of line for this gadget but I have to look at the spec's. If you'd like me to do any special tests, let me know, these two instruments are on my main bench and I use them daily.Below is a pic with a 5 Hz input and the 1992 measuring period- it bounces around quite a bit.

author: J-R
author_rank: Super Contributor
timestamp: March 11, 2022, 07:36:28 am
content: Ran some tests on my 33120A without option 001. 5Hz, 5Vpp, square wave. Roughly over a 5-10 minute period.K2010:Low - 4.999824High - 5.000114Avg. - 4.9999698Std. Dev. - 28.367uHz34401A:Low - 4.999874High - 5.000105Avg. - 4.999986

author: binary01
author_rank: Contributor
timestamp: March 12, 2022, 02:40:56 am
content: Big thanks jwet and J-R.It looks/sounds like you are seeing similar behaviour to me, so I guess it is just a limitation of the 33120A.So far I have just been watching the numbers bouncing around live on the frequency counters, but when I get back in the lab on Tuesday I'll do some logging and see if a similar std.dev to J-R, which might confirm it is just the nature of design of the 33120A.By comparison, when I use my Fluke 5522A to generate 1 Hz square wave 5 Vpp, the measured frequency shows no change at all when measurement with 1 µHz resolution. Just for interest I'll log the 33120A and 5522A outputs at 5 Hz with 0.1 uHz resolution and compare std. dev.Does anyone have a suggestion for cost effect waveform generator that would be good for generating stable frequencies in the range 1 Hz to 2 MHz with low jitter/wander?I guess I didn't research the performance of the 33120A well enough, but at the cursory glance of spec sheet, I think only really lists a frequency accuracy in ppm without a specific lower/floor term.

author: S57UUU
author_rank: Regular Contributor
timestamp: March 12, 2022, 08:03:26 am
content: The 33120 is a DDS type generator, no VCOs that could dance around, any such behavior should, with high probabilty, be in the reference oscilator. The relative frequency error should be independent of output frequency, and absolute error in Hz should be proportional to frequency, so 10000X lower at 10Hz than 100kHz.The 33120 has a 12 bit output DAC, so maybe it is the quantization error. With a low output frequency, the steps will be long in time, and your counter's threshold might alternate between two adjacent steps. For example, an 10Hz triangular waveform will go up and down 4096 steps twice in a 100ms period, so one step will be 12.2 microseconds, and a little less in the steep part of a sinewave.The 33120 uses a 40MHz DDS clock. Try to measure a frequency that is 40MHz/2^N, where the step pattern repeats every cycle.Of course, it could also just be a case of "dirty" signal, grounds or bad caps in the supply.Marko Cebokli

author: nfmax
author_rank: Super Contributor
timestamp: March 12, 2022, 09:27:11 am
content: I made a quick test on a 33120A with option 001 fitted, using my latest eBay score Yokogawa 520 Time Interval Analyser! I'm still learning how to use this isntrument, and I'm not 100% sure it is fully functional, but I did manage to persuade it to measure the periods of 600 successive output cycles of the function generator at a range of frequencies: 100Hz, 1kHz, 10kHz, 100kHz and 1MHz. At frequencies 100kHz and above, the output looks very clean, but at lower frequencies you increasingly see 'spikes' of a short cycle followed by a long one. The magnitude of these disturbances, as well as their frequency of occurrence, increases as the frequency is reduced, although they seem to happen fairly randomly.Although this might br caused by the limited DAC resolution, I suspect that the DDS ASIC is not always rounding correctly when generating the phase output. If so, it is a design fault and they will all do it. I have two more 33120A's here I can try out if/when I get the time.Since it doesn't seem to happen at 100kHz and above, you might possibly be able to use a digital frequency divider between the generator and your test circuit, depending on what you need to do.An interesting problem!

author: nfmax
author_rank: Super Contributor
timestamp: March 12, 2022, 09:27:48 am
content: More pictures...

author: nfmax
author_rank: Super Contributor
timestamp: March 12, 2022, 09:29:13 am
content: Last one!

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2022, 11:51:20 am
content: There are 2 more problems that can come up with the very low frequency: LF noise of the DAC output / output amplifier can become an issue and also the trigger circuit in the counter may not be as accurate in triggering on a rather slow slope.At least the last part could be checked by reducing the amplidude: this part should be worse with a smaller amplidude.I don't think there is problem with rounding in the DDS algorithm, as this is usually just using only the upper bits to use for the lookup table. So not much to go wrong there, and if so it would be more like a desaster that would also be obvious with other frequencies.There could be a problem with the DAC DNL, that gets more relevant with lower frequency and the rather small steps used.

author: jwet
author_rank: Frequent Contributor
timestamp: March 12, 2022, 08:29:24 pm
content: I'll went to do a long term test and noticed that my Racal Freq Counter has a bad HPIB interface. It was disconnected internally and when I reconnect it the box won't boot. I have another good counter that I use for higher Freq's with a good GPIB (HP5386a). This is all a good excuse to dust off my lab automation stuff that I seldom use anymore.

author: nctnico
author_rank: Super Contributor
timestamp: March 12, 2022, 09:23:56 pm
content: Quote from: nfmax on March 12, 2022, 09:27:11 amIs this happening when using square waves or sine waves? A potential problem could be in the way the square wave is created from a sine or triangle waveform. If that is done using a comparator to turn such a waveform into a square wave, you can get these effects. If there are schematics available (or a block diagram) for the 33120A then it can help to study those in order to understand how the square wave is created.

author: J-R
author_rank: Super Contributor
timestamp: March 13, 2022, 04:44:16 am
content: Repeated test using some other gear:Owon AG1022:Low - 4.999999High - 5.000002Avg. - 5.0000015Std. Dev. - 1.21uHzAnalog Discovery 2:Low - 4.999831High - 5.000071Avg. - 4.9999489Std. Dev. - 118.76uHz

author: Someone
author_rank: Super Contributor
timestamp: March 13, 2022, 05:39:22 am
content: Quote from: jwet on March 10, 2022, 05:29:40 amWhat is the important metric for you? cycle to cycle jitter, 1s aperture?Remember these sorts of signal generators are going to be using DDS with jitter approximately equal to their DAC sample frequency (40MSa/s for the 33120A) so depending on exactly how the "frequency" is measured you can easily get a misleadingly high value from short term error. Newer models up the sampling frequency, or you can use exact divisors if you need better short term accuracy.

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 07:21:18 am
content: As in the OP, I used a 5V square wave output. The service manual explains this is generated from an internal sine wave using a comparator. This approach minimises jitter.

author: nctnico
author_rank: Super Contributor
timestamp: March 13, 2022, 08:59:15 am
content: Quote from: nfmax on March 13, 2022, 07:21:18 amThen there is the source of the problem! Long story short: using a comparator minimises jitter but the hysteresis of the comparator and noise dominate the jitter specification at low frequencies. This is a limitation of the instrument.

author: jonpaul
author_rank: Super Contributor
timestamp: March 13, 2022, 09:11:52 am
content: Hello no experience and that generator butlow jitter generators are available, mostly RF sine like classic HP 8640B, 500 khaki..1024 MHzCavity analog oscillator feeds frequency dividers and band passed filters.Results is a super low jitter stable sine.We use Yokogawa TA720 time interval Analyzer for 25 ps intrinsic système jitter, the time histogramme function is unique and very usefulBonsoirJon

author: binary01
author_rank: Contributor
timestamp: March 13, 2022, 09:26:21 am
content: Very interesting data and discussion here, thank you all. It looks like a similar frequency blip is being detected by a few of us. The time interval plots are very interesting.J-R, is there a chance your Owon AG1022 is integrating over a longer time to achieve such a lower stdev than your other counters? I think I would need to set a very long counter gate to get similar performance from my 33120.I have tried with sine wave too, and lower amplitude but performance seems worse in both situations.Thank you jonpaul for the suggestions, I'll take a look at those models. I was considering even trying a Siglent 2042 synced to GPSDO.

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:19:20 am
content: I had a short time available this morning to pursue this a bit further, in particular to investigate whether the problem lies in the DDS implementation itself, or in the analogue side - DAC, comparator, output amplifier.The 33120A clocks its DAC & DDS engine at a frequency of 40MHz (how quaint!) and presuming binary arithmetic is used, there should be no issues with output frequencies that are an integer binary fraction of 40MHz. The generated pattern of phase values should repeat exactly, every output cycle. The advertised frequency resolution of the 33120A is 10μHz, so the lowest such frequency that can be programmed is 2441.40625Hz, i.e. 40MHz/16384. This is well within the 'problem' range of frequencies that I found last time.For comparison, I used a frequency of 2441.40000Hz. Very close, but not an exact binary fraction. Again, I set the generator output to 5V pk-pk square wave, with a 2.5V offset. I have extended the measurement to 14600 cycles, just under 6 seconds of data.The first screenshot IMG_2202.jpg shows the histogram plot when the generator is set to 2441.40000Hz. The distribution of periods is bimodal! This looks very much like something DDS-related. But there is quite a spread on each peak, and there are outliers on both sides. IMG_2203.jpg shows the same data in the time domain. A big, lopsided 'fuzz' in the middle, with the large spikes we saw last time.

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:22:55 am
content: Zooming in to a 300ms window of the same data in IMG_2204.jpg shows that the 'fuzz' is actually a series of +ve going spikes at approximately 10ms intervals, qualitatively different from the previously observed low-then-high bipulses we saw last time. This looks like a DDS remainder issue, but obviously it's not the whole problem

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:27:13 am
content: Now changing the programmed frequency to 2441.40635Hz gives us a unimodal distribution in the histogram (IMG_2205.jpg) but still with the outliers. The time waveform no longer shows the 'fuzz' (IMG_2208.jpg - note the change in Y axis scale factor)

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:29:33 am
content: Zooming in on the same data shows the periodic spikes have gone away, but there are still bipulses of much larger than normal magnitude (IMG_2209.jpg)

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:33:01 am
content: Next, I compared the main and sync outputs from the generator, again at the 2441.40625Hz frequency setting. IMG_2212.jpg is the main output (5V pk-pk squarewave) and IMG_2213.jpg is the sync output (TTL level), both at the same axis scales. To my eye, the sync output looks cleaner

author: nfmax
author_rank: Super Contributor
timestamp: March 13, 2022, 11:40:15 am
content: Finally, for a bit of a laugh, I measured my Agilent 33522B set to generate a square wave at the same output level and 2441.40625Hz frequency. This is no longer a 'magic' frequency, as the 33522B has a DAC clock frequency of 250MHz. It also has a 16-bit DAC instead of of 12-bit, and uses the Agilent/Keysight 'Trueform' asynchronous interpolation technique to generate the amplitude values at each sample point.The results, I think, speak for themselves. IMG_2210.jpg is the histogram, IMG_2211.jpg is the time plot (note the axis scales!)I hope you have enjoyed this little exercise, and found it interesting - I know I did. You can learn something new every day, if you aren't careful

author: Performa01
author_rank: Super Contributor
timestamp: March 13, 2022, 07:11:23 pm
content: Here is a quick test with the cheapest Siglent AWG, the one that is built into the SDS2000X Plus and this should be equivalent to an SAG1021.For obvious reasons, I've used a different DSO for the measurement - otherwise the SDS2000X Plus could have performed it just as well.The attached screenshot shows the situation, using a 10 Hz square signal:By using a 100 ms/div timebase I'm resembling the 1 s gate time of a regular frequency counter.The Measurement statistics shows all the relevant numbers:Amplitude was 3 Vpp;Period was 100 ms, with a peak to peak deviation of 18 ns and a standard deviation of 4.52 ns;Frequency was 10 Hz, with a peak to peak deviation of 1.834 µHz and a standard deviation of 452.11 nHz;Rise and Fall times measurements read slow because of the low sample rate, but this is not an issue here.Cycle to Cycle Jitter is 16 ns peak to peak and has a standard deviation of 1.95 ns.As you can see, the Siglent DSOs of this class generally look at each individual signal period within one record, instead of just the first one, when measuring timing related parameters.The CCJ (Cycle to Cycle Jitter) measurement has the advantage, that we get an information about the worst frequency jump within a supposedly uniform signal.We can calculate the cycle to cycle frequency deviation from the CCJ measurement:16 ns is 0.16 ppm of the 100 ms signal period.Consequently, the peak to peak deviation of the cycle to cycle frequency changes is 0.16 ppm as well.Likewise, the standard deviation of the cycle to cycle frequency changes is 19.5 ppb.

author: nctnico
author_rank: Super Contributor
timestamp: March 13, 2022, 08:23:09 pm
content: @Performa1: did you connect both the oscilloscope and generator to the same reference clock for this measurement to rule out any clock drift?

author: 2N3055
author_rank: Super Contributor
timestamp: March 13, 2022, 08:27:37 pm
content: Quote from: nctnico on March 13, 2022, 08:23:09 pmNo, he said that he uses SDS2000X+ internal AWG, and other scope looks like SDS6000A.Neither have external clock option.All measurements include all clock errors of both uncorrelated clocks..

author: Performa01
author_rank: Super Contributor
timestamp: March 14, 2022, 10:27:09 am
content: Quote from: nctnico on March 13, 2022, 08:23:09 pmNo, these are totally independent systems.That's why I said:QuoteOf course, the result could be much better if I had used the SDS2354X Plus to analyze its own integrated AWG, as they're both using a common clock reference. But this might fail to detect any unexpected frequency jumps or other glitches.As it is now, my measurement shows the combined errors of both the signal generator and the DSO (SDS6000). The quality of the clock in the SDS6000 is significantly higher than in the SDS2000X Plus, so its contribution shouldn't be too big a deal.EDIT: oh, I did not look any further, so I missed this famous old power transistor has been much faster again!

author: nfmax
author_rank: Super Contributor
timestamp: March 14, 2022, 10:32:30 am
content: Some more observations:Generating a non-square waveform from the 33120A, and relying on the TA520's input triggering circuit gives a much larger random cycle-to-cycle period variability, enough to swamp any spikes that may possibly be presentWith the square wave duty cycle set to other than 50%, the spikes are significantly larger in magnitudeThe presence of a DC offset makes no discernible difference (with the TA520 trigger threshold adjusted appropriately)

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 14, 2022, 05:56:43 pm
content: Hello,the 3325A/B might be the predecessor of your generator, so the following analysis is probably equivalent.I also noticed such an instability at lower frequencies, and measured the Allan variance for different frequencies over tau.This can easily be done by a T.I. / universal counter and TimeLab by John Miles, KE5FX:http://www.miles.io/timelab/beta.htmFrank