Stability of AC voltage from function generators - Page 1
Forum section: electronics / general

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 10, 2020, 02:39:11 am
content: I recently had a reason to look at stability of AC voltages in the kHz range. The measurement is very simple, just connect the function generator to a DMM measuring ACV. I was using HP3457 with 10 NPLC and fast ACBAND, giving about 2 readings/sec.Among various function generators I had on hand, Agilent 33220 stands out as having particularly low short-term fluctuations in AC amplitude. Rigol DG811 is marginally worse than others. FeelTech FY-6600 is in the middle. The overall winner is Keysight B2962 SMU which can generate sine waves up to 10 kHz referenced to LM399 (will say more about it later).Generator Standard deviation of AC amplitude over 500 secRigol DG811 34 ppmRigol DG4162 26 ppmAgilent 33220A 10 ppmAgilent 33522A 25 ppmFeelTech FY-6600 29 ppmKeysight B2962A 3 ppm

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2020, 03:35:49 am
content: What output voltage and frequency did you use for this? And how was the absolute accuracy?I'll run my Siglent 2042X as another data point.

author: bob91343
author_rank: Super Contributor
timestamp: August 10, 2020, 05:21:35 am
content: I don't think these generators were intended to supply very stable amplitudes. If that's what you want, you will have to modify them to insure more stable power rails and perhaps temperature control.The readings you get may not be the true values either. Distortion, power line interference, and other factors enter into the equation. You would need calorimetric voltage measurement to get the true rms without regard to frequency and all the rest. Check your 3457A for specification on the ac ranges. Not as accurate as one might guess from all those digits.

author: tautech
author_rank: Super Contributor
timestamp: August 10, 2020, 05:39:15 am
content: Quote from: bdunham7 on August 10, 2020, 03:35:49 am1 KHz 4V p-p so the graph shows.Your SDG2042X has a datasheet spec of: Accuracy ±(1%+1mVpp) 10 kHz sine, 0 V offset and flatness spec of:Amplitude flatness -0.3 +0.3 dB 0~100 MHz (Included), 50Ω load, 2.5Vpp, compare to 10kHz SineThat rises to+0.4 dB from 100 to 120 MHz for the SDG2122X model.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 10, 2020, 08:30:24 am
content: Short term stability is very different from accuracy or long-term stability. All generators are easily within their amplitude accuracy specs (except B2962, but that's a different story). Short term stability is a measure of intrinsic design quality, such as voltage reference stability, DAC stability, gain resistor stability and short-term temperature fluctuations. It's interesting that an older Agilent model performs better. Perhaps other vintage generators are even more stable if they use higher-quality components. The most stable measurements for B2962 maybe are limited by HP3457 itself, but others are clearly not.The absolute values just for reference are below. Some generators take longer to settle, I generally let them run for at least 30 min. But clearly there are too many digits here. PPM is calculated relative to RMS values measured by the DMM.DG811 1.413947 VrmsDG4162 1.411982 VrmsA33220A 1.411788A33522A 1.410849FY6600 1.424248B2962 1.412711Sqrt(2) 1.414214

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2020, 03:22:59 pm
content: Yesterday, before looking at the title of the chart that Tautech pointed out, I tried my SDG2042X-hat 1Vrms 1kHz and 5Vrms and 5kHz and got 26 ppm and 21 ppm. The unit and the DMM had been on for several hours as I'd been using them all day to test oscilloscopes.This morning cold at the agreed-upon 4Vpp 1kHz, the results were much worse (40ppm or so) so I let them warm up an hour.With no load other than the DMM, absolute accuracy was very good at 1.4138xx. SD was about 26ppm with max deviation of ~90ppm.With a 50ohm load, it was about the same on the first run, but there was small drift down. Absolute accuracy was worse, reading about 1.4110xx at the end. A second run was better with about 24ppm and max deviation of ~80ppm, a change which I attribute to something warming up a bit.As was pointed out, these deviations are much less than the specified accuracy of the DMMs, in my case an 8846A with a specified accuracy of 600ppm + a few counts in this range. However, since that accuracy is specified from 10Hz to 20kHz with a max crest factor of 5 and a 99% confidence interval, I'm pretty sure it will greatly exceed that on a clean sine wave at 1kHz.I also tried my FY6900 with a 50R terminator and after some drift down, it settled at 1.4343xx +/- ~70ppm and 25ppm SD. For all the complaints about the crappy PSUs in these, they're doing pretty well in this department.On both units, once warmed up the deviations appear to be short term, as opposed to a steady drift one way or another.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 10, 2020, 06:23:57 pm
content: Here is a Siglent SDG2122X at 4 Vpp, Sine, 1 kHz, HiZ (default at power on).

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 11, 2020, 01:17:29 pm
content: Here is an interesting article investigating AC amplitude stability of a 1 kHz sine wave generator using AD5791 DAC. They get to 0.2 ppm stability. I wonder if there are any commercial implementations of a source using AD5791 DAC for dynamic signals.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 11, 2020, 01:35:00 pm
content: Looking at the HP3457 specs, it appears to be rated for 5 1/2 digits on AC, 0.13% accuracy + 116 counts.There doesn't seem to be a spec for short term drift. How would one go about measuring the short term drift of the AC range of a DMM? It doesn't seem as simple as just using a shorting plug or something like that...

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 11, 2020, 02:51:55 pm
content: Since Keysight B2962 has two independent channels, one can setup a differential measurement. Here is the data where HP3457 measures the difference between 4V pp and 3.6V pp sine waves at 1 kHz. The RMS value should be 0.14142 Vrms. It is important that two AC waves are not exactly the same, otherwise the RMS measurement would be sensitive to high-frequency noise. The standard deviation of this amplitude is 3 muVrms. This corresponds to 3e-6/1.4/sqrt(2)=1.5 ppm noise for each channel. So the 3ppm measurement above is partly limited by HP3457.It would be interesting to compare to the big guns: Fluke 5730 calibrator measured with HP3458.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 11, 2020, 03:30:37 pm
content: Quote from: maxwell3e10 on August 11, 2020, 02:51:55 pmThey are probably not truly independent? - possibly same power supply, frequency reference, ground path, etc., there might be some common mode misbehaviour shared by the two channels?Have you attempted the same measurement between two different (phase locked) generators?

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 11, 2020, 03:40:14 pm
content: Quote from: SilverSolder on August 11, 2020, 03:30:37 pmThey are largely independent, except for the power supply. See Dave's teardown of a similar unit B2912:In fact, without some additional effort the two channels are not even frequency synced together. But that will be subject of a separate B2962 review.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: August 12, 2020, 12:37:52 pm
content: Here are the raw data for B2962. The rms is indeed 4.3 uV. Comparing with measurements by TiN herehttps://www.eevblog.com/forum/metrology/post-3000/msg1406214/#msg1406214this is actually pretty good compared with the big boys