Help: Need low noise stable amplitude audio signal generator or design - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: January 02, 2022, 11:59:40 pm
content: I am working on an AC Voltage Calibrator project. See reference below from reply #82 for the latest version.https://www.eevblog.com/forum/metrology/an-experimental-ac-voltage-calibrator/75/Am now in need of a very stable amplitude low noise signal source, ~1vRMS 20-20KHz, to drive the calibrator.I have tested several signal sources, commercial and home-made, and most have fallen short wrt either amplitude stability or noise.Wavetek Model 80 function generatorLeader LAG126S audio generatorChinese low distortion audio range generator kitASUS Xonar U7 PC USB sound cardObviously there are dozens of good designs out there that can generate stable audio frequencies. What I am looking for is an affordable one to work with.Ideally, for this project, a generator that stabilises in amplitude (<60uV/min) within ~30 min and has less than 30uVpp noise on a 1v/1KHz signal would be OK.Looking for suggestions on equipment or designs. I will post more info and the results of my tests soon.How good is your audio frequency generator for amplitude and noise in a metrology role?

author: enut11
author_rank: Super Contributor
timestamp: January 03, 2022, 04:42:06 am
content: @evb149I have only tested one PC sound card - an ASUS Xonar U7 and it had a very drifty amplitude. See Reply #86 athttps://www.eevblog.com/forum/metrology/an-experimental-ac-voltage-calibrator/msg3911450/#new.Are you saying this can be compensated for in software?

author: enut11
author_rank: Super Contributor
timestamp: January 03, 2022, 06:08:36 am
content: I may look into that later on, ie letting the sound card measure its own output to determine amplitude drift.Meanwhile, I am looking for an AF signal source that stabilises in a reasonable time with minimum interference from me - ie 30- 60min.One thing I did like about the sound card, there was no measurable noise at the output.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 03, 2022, 06:32:54 am
content: Soundcard output is normally pretty stable.For example, for my old and trusted M-Audio Delta 1921+ hour stability in loopback is below 0.01 dB.Long term drift after a month or so when switching on is around -0.04 dB staying there being stable again at this level below 0.01 dB.Maybe a long term stability of -0.05 dBis not good enough for your purpose. You probablycannot expect more from even the best sound cards.

author: enut11
author_rank: Super Contributor
timestamp: January 03, 2022, 08:03:22 am
content: @MesstechnikerThanks for your input. This is useful information and puts a number on what is possible from a sound card. No doubt my inexpereince with these devices does not help.On the negative side, high performance sound cards are very expensive and could not be justified simply to run this AC Cal project.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2022, 09:07:09 am
content: Sondcards have additional adjustements for the ouput amplitude. The stability may depend on the amplitude setting and maybe the output (line out, headphone) used. Another point can be the supply voltage and thus the PC it is used in. Amlitude stability is not one of the main targets, the qualtity in this respect may not correlate with price and there are rarely good data in the describtions. Trying another type may be worth it.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 03, 2022, 02:49:12 pm
content: You might even try on-board audio.Line inputs and outputs can be quitegood on some more modern motherboards.Or absolutely horrible especially on older boards.Note: on-board MIC inputs are always basicallybad to horrible when it comes to moderatelydemanding specs.Lots of penny pinching going on here.

author: moffy
author_rank: Super Contributor
timestamp: January 03, 2022, 02:58:20 pm
content: If you are able to measure the output of the sine wave precisely would that solve your issue? A simple synchronous demodulator(switch based) filtered and fed into a semi decent volt meter will give you a good measure of the average value of the sine wave, from that you can compute the RMS. I don't know if that suits your situation though?

author: 2N3055
author_rank: Super Contributor
timestamp: January 03, 2022, 03:08:45 pm
content: This has been discussed before. Best way is to use DDS chip with a reference. No audio DAC will be PPM stable in amplitude.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2022, 04:07:44 pm
content: The old solution is to make a precision measurements of the AC output, like with a precision rectifier, and use that to control a leveling loop, which is just part of an analog oscillator anyway so that is easy enough. Offset and drift can be removed from the rectifier by AC coupling the gain stage, but that would not be necessary today.

author: djsb
author_rank: Super Contributor
timestamp: January 03, 2022, 04:12:22 pm
content: Have you looked at the QuantAsylum equipmenthttps://quantasylum.com/

author: moffy
author_rank: Super Contributor
timestamp: January 03, 2022, 05:08:33 pm
content: Quote from: David Hess on January 03, 2022, 04:07:44 pmThat would work well, as you only need to adjust the gain to compensate for the instability of the sine wave variability. So if you have +/-1% amplitude variation then you only need say +/- 2% gain variation. A small value LDR optocoupler as the controlling element(ensure the gain variation is small) works very well.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2022, 05:18:36 pm
content: Quote from: moffy on January 03, 2022, 05:08:33 pmAn operational transconductance amplifier can also be used as the variable gain element, even in a low distortion design.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 03, 2022, 05:20:21 pm
content: Quote from: 2N3055 on January 03, 2022, 03:08:45 pmYeah, I agree. But, depending on the load, they are likely to need an output amplifier after the DDS chip, and the challenge will be to make this amplifier stable at this level.We need to read the requirements: "a generator that stabilises in amplitude (<60uV/min) within ~30 min and has less than 30uVpp noise on a 1v/1KHz signal".

author: 2N3055
author_rank: Super Contributor
timestamp: January 03, 2022, 05:33:12 pm
content: Quote from: SiliconWizard on January 03, 2022, 05:20:21 pmYeah I read that, stability better than 60 ppm and noise less than 30 ppm...I agree with others, a leveling loop from the output is probably the most realistic way to achieve something respectable on output connector.... And by that I don't necessarily mean analog negative feedback. It can be running digital calibration cycles in the background.

author: enut11
author_rank: Super Contributor
timestamp: January 03, 2022, 08:45:07 pm
content: Shown below is typically what I find when I test a low distortion audio signal generator.From a cold start, amplitude drops steadily with stability setting in after about 60min.A close-up of the flat part of the curve shows short-term variations, the worst in this case being about 60uV/min droop at around data point #1230.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: January 03, 2022, 10:10:42 pm
content: Audio distortion analyzers like a Sound Technology 1700 have very stable and low distortion 20-20K sources.[ Specified attachment is not available ]

author: enut11
author_rank: Super Contributor
timestamp: January 04, 2022, 02:56:40 am
content: Thanks. I Like the old world analog audio test gear. HP also made some good ones. I downloaded the 1700 manual but there were no specs included.What I need is better than 0.1dB amplitude stability around 1v, at least at mid audio frequencies.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 04, 2022, 03:16:12 am
content: AFAIK, amplitude stability isn't a requirement for THD measurement. That said, and I've never measured it for stability, the oscillator used in the Bob Cordell THD project has always done very well for me. It's put away right now, but maybe I can do a stability measurement in the near future.http://www.cordellaudio.com/papers/build_a_thd_analyzer.shtml

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 04, 2022, 08:21:15 am
content: 50 minutes soundcard startup drift test:Test equipment: Keysight 34465A and M-Audio Deltacalibrated to 0.000 dBu approx. 1 year ago. Is now off by approx. -0.036 dBMeasurements started approx. 2 minutes after switching the 34465A on and the soundcard output on. I.e no long warmup. Logging programme: Test controller.

author: jonpaul
author_rank: Super Contributor
timestamp: January 04, 2022, 08:52:43 am
content: Tektronix SG505, Audio Precision were Bruce Hofer and Rich Cabot classic designs.N+ THD 0.0018% frew 10..100 kHz.You cant do better.Jon

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 04, 2022, 10:12:01 am
content: 2 hour drift measurement with using the "High definition audio"soundcard on motherboard Asus M5A78L-M.All equipment warmed up this time.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 04, 2022, 04:19:37 pm
content: Use a PCM1794 DAC with a passive attenuator on the output.https://www.ti.com/product/PCM1794A-Q1

author: enut11
author_rank: Super Contributor
timestamp: January 04, 2022, 07:36:18 pm
content: Quote from: Messtechniker on January 04, 2022, 10:12:01 am@Messtechniker. More info on how you did this test please.enut11

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 05, 2022, 07:13:54 am
content: My measurement setup is as follows:Computer with motherboard Asus M5A78L-M with its "High Definition" onboard sound chip. Signal generating software: "ASIO4All"https://www.asio4all.org/and "Audio Tester"https://www.audiotester.de/Set to produce 1 kHz 0.00 dB at the green output socket.Connect green output socket to Keysight 34465A set to AC and use math to display dBs.Connect its LAN to a second computer running "Test Controller"https://lygte-info.dk/project/TestControllerInstalling%20UK.htmlfor logging.Do logging as long as required. Screenshoothttps://app.prntscr.com/en/index.htmlthe results of "Test Controller" "Current values" tab as required.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 06, 2022, 01:49:55 am
content: Quote from: enut11 on January 02, 2022, 11:59:40 pmIf I'm not wrong 60 uV is 60 ppm for 1 VAC signal. This is not 'stable' but this is 'ultra-stable' I think.I worked with a Chinese low distortion audio range generator kit (1 kHz) it was the most stable of all several different simple analog oscillators I had in my hands. It works at 50 Hz in my case. I guess it is as stable as used passive components are (plus some semiconductor drift caused by heating). I had something like 100-200 ppm amplitude stability with that oscillator over time. (I don't have more accurate AC voltmeters to make a deeper investigation, unfortunately).

author: enut11
author_rank: Super Contributor
timestamp: January 06, 2022, 06:17:17 am
content: Thanks @Vovk_ZI came to the same conclusion about that single frequency chinese audio signal generator kit. My requirement is 20Hz-20KHz minimum which can be done with switched caps and variable resistors.The plea for help from Forum members was to see if other solutions existed.enut11

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2022, 10:18:12 pm
content: Quote from: Vovk_Z on January 06, 2022, 01:49:55 amPrecision could be improved by temperature compensating the rectifier circuit better. That would be a good place for a precision monolithic matched pair used as diodes. Jim Williams used the same configuration to stabilize the amplitude of his Wien Bridge oscillators and noise sources. Leveled RF sources use the same type of circuit but with faster diodes.Wouldn't a precision rectifier be more stable?

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: January 07, 2022, 12:32:47 pm
content: Hi,maybe the smallest solution could be the Apex SWR200, impressive datasheet:"SWR200 is a Precision Sine Wave Reference providing an ultra stable sine wave output of +7.071V at±0.5% initial accuracy and temperature coefficient as low as 20ppm/°C over the full military temperaturerange".I guess you have to bring a big wallet. ;-))In the 1980s there was a simimlar hybrid-circuit ROJ20 from Datel but i don't remember the data.

author: enut11
author_rank: Super Contributor
timestamp: January 08, 2022, 07:01:52 am
content: Big wallet indeed - $374 each!

author: enut11
author_rank: Super Contributor
timestamp: January 08, 2022, 07:09:54 am
content: Quote from: David Hess on January 06, 2022, 10:18:12 pmThe Chinese audio range generator is good if you have a very stable +/- power supply.I have also tried this design which has a precision rectifier (U3) plus voltage reference (TL431) to control the output level but it is very very noisy - pity.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 08, 2022, 12:43:42 pm
content: Did Apex get the Thaler designs? They had some nice references, including an AC reference.