distortion analyzer suggestions? - Page 1
Forum section: electronics / testgear

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 24, 2014, 01:34:03 am
content: I would like to be able to measure THD of an audio signal, I figure that some instrument capable of 0-100KHz would be good.I know nothing about distortion analyzers so I was wondering what they are worth and what is a reasonable deal (so I don't go out and buy a 1970s junk box for 150$).I am looking for the "gem" of distortion analyzers, kind of like the keithley 199 is one of the kings of cheap but good multimeters. Is it worth getting some kind of 1970's bit of kit for this job or is it better to save up for some kind of dynamic signal analyzer for a few hundred?

author: nowlan
author_rank: Frequent Contributor
timestamp: February 24, 2014, 01:51:35 am
content: kEITHLEY 2015 THD.http://www.keithley.com/products/dcac/audioanalyzer/?mn=2015I think HP have older alternatives.diyaudio.com has a few threads.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 24, 2014, 02:03:27 am
content: Quote from: nowlan on February 24, 2014, 01:51:35 amah but the 2015 is only 20khz and the price range makes it so that I would rather just buy a DSA for a few hundred more.

author: nowlan
author_rank: Frequent Contributor
timestamp: February 24, 2014, 02:07:23 am
content: Dave did a tear down.https://www.eevblog.com/forum/blog/eevblog-478-keithley-2015-thd-multimeter-teardown/It does 20-20khz audio sprectrum. I know usually you want 5x sampling to see harmonics.I think this things DSP figures it out.I am no expert. I think Spectrum Analyzers only go down to 9khz.

author: robrenz
author_rank: Super Contributor
timestamp: February 24, 2014, 02:51:40 am
content: How about a Dynamic signal analyzer? Dave has shown 2 in video blogs.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 24, 2014, 03:54:01 am
content: Quote from: robrenz on February 24, 2014, 02:51:40 amright, thats what I meant when i said DSA.do you think that a older ~150-200$ distortion analyzer will do better then a ~600$ DSA?I am not sure how to evaluate a DSA's ability to measure distortionif they are really close then I would just rather save up a bit for a DSA.

author: Lightages
author_rank: Unknown
timestamp: February 24, 2014, 03:55:00 am
content: I know you want a higher bandwidth, but have you tried any of the free software solutions for your computer's audio card? Visual Analyzer is very nice:http://www.sillanumsoft.org/orhttp://audio.rightmark.org/products/rmaa.shtml

author: elCap
author_rank: Regular Contributor
timestamp: February 24, 2014, 05:29:07 am
content: I have an old HP 8903B. I haven't used it much but works quite OK. Freq range 20-100kHz. All manuals, incl. service and calibration, can be found at Agilents webpage. I don't know where you live but here in Japan I got it for about $200. On ebay they cost more.Here is a link to a software for it to do swept measurements.http://www.pmillett.com/hp_8903_software.htm

author: VK5RC
author_rank: Unknown
timestamp: February 24, 2014, 05:39:07 am
content: Some good (read expensive spec analyser) go down to a few hertz and can get distortion analyser 'personalities' again for a price, the reason I would tend to go that route is a SA is a more versatile tool later on. There may be other factors I am not aware of! (unknown unknowns!)

author: agdr
author_rank: Unknown
timestamp: February 24, 2014, 11:59:36 pm
content: Quote from: SArepairman on February 24, 2014, 01:34:03 amWhat you need is a QuantAsylum QA400They go for $200. I bought one myself a couple of weeks ago:http://www.quantasylum.com/content/Products/QA400.aspxI have no business relationship with these folks, just a good product IMHO.These days a high quality PC soundcard plus appropriate software, like ARTA or Rightmark, can greatly exceed the specs of the older analyzers. That is what the QA400 is, essentially a USB soundcard in an external box set up for audio measurements (has a calibration feature that soundcards don't have) with their own proprietary measurement software. The software avoids all the Windows driver craziness one runs into in trying to get ARTA and Rightmark set up correctly. Notice in the QA400 writeup they compare its measurements with a EMU-0204 external USB soundcard (now out of production) which has some of the best results when used with ARTA.For example, that first QA400 screenshot they show at the link is the unit in loopback mode. Notice the THD+N reading at the top, about -90.2dB. The THD by itself shows up at -107dB. Now take a look at those older Keithley 2015's that were mentioned which are pretty common used on eBay right now. Scroll down in the data sheet to the spec tables on page 5:http://www.keithley.com/data?asset=363(opens PDF)The residual THD+N is listed as -65dB. The QA400 beats it by 25dB. Not surprising since there most likely were not op amps even available back in the late 90's with the ultra low noise and distortion figures we have now. That Keithley was a well built machine doing the best it could with the parts at the time. That 2015 is still quite useful and worth the money as a 6.5 digit DMM though. I've been thinking of getting one and just using it for that.The QA400 goes up to 192kHz, which given Nyquist gives you almost a 100kHz range. Scroll down a few screenshots in that link and they have one going out to 100kHz.To make the QA400 even more useful use a notch filter after the device under test to eliminate the 1kHz fundamental and leave the harmonics. I just sent the board below out to fabrication. A 1kHz Fliege notch filter using LME49990 chips followed by a 50X (33dB) voltage gain stage using a OPA211. I mainly created that board for use with scope FFTs, to boost the harmonics levels up to the 500uV vertical minimum of my Rigol. But it should also be useful with the QA400 if de-turned about 4 Hz (that is what the pot is for) so the fundamental is hitting the side of the notch well, rather than the notch bottom, to give more a defined bottom to the notch like -36dB. That way I know how much to subtract off the harmonic levels to get some amount of calibration.That 1kHz notch filter + pre-amp board is all DIY, if anyone is interested. The BOM, LT spice plots, layouts for each of the 4 layers, etc are here:https://drive.google.com/folderview?id=0B67cJELZW-i8ODk3VmNVOUc4MWM&usp=sharingand I'll post the Gerbers so anyone can send the board off to their favorite fab house once I have it back and tested.To go the next level of analysis past a soundcard + software, like the QA400, you would pretty much have to add $8,000+ for an Audio Precision System 2 or a Prism Sound dScope. At these prices that more or less works out to be $8000 / 20dB = $4000 for each 10dB of noise floor improvement over a good PC soundcard.

author: GK
author_rank: Super Contributor
timestamp: February 25, 2014, 12:20:57 am
content: QuoteIf you want to accurately measure the THD of a 100 kHz signal, you will need an analyser with a measurement bandwidth of 1MHz (to include harmonics up to the tenth). I don't know of any commercial "audio" analyser with a measurement bandwidth that high.Most are limited to a measurement bandwidth of 80 kHz, which can already be a limitation when measuring distortion at 20 kHz (unless the distortion is predominantly low order). Measurement bandwidth is where most soundcard options fall flat, although you can still measure high frequency linearity using IMD techniques.I just built my own analyser. Has a better THD+N floor than a AP system 2 and will measure THD+N up to 50 kHz fundamental in a 500 kHz measurement bandwidth.http://www.users.on.net/~glenk/thd/thd.htm

author: G0HZU
author_rank: Super Contributor
timestamp: February 25, 2014, 12:26:28 am
content: That QA400 does look very goodI'm not particularly interested in measuring ultra low THD (I'll leave that to the HiFi geeks as my ears are only standard issue human type) but it would make a very good baseband analyser.I've got a Keithley 2015 here and it is pretty good for quickly looking at AF harmonics to/from a modulator/demodulator and measuring them very accurately.I'm not convinced the QA400 will provide the same degree of absolute accuracy as the 2015 but it will have more range for sure.The best thing about the 2015 is you can throw pretty much anything at it and it will measure the levels of the fundamental and the harmonics very accurately and quickly. With the QA400 I assume you will have to attenuate the waveform to keep within its damage level. I'm not really interested in audio harmonic distortion at very low levels so the Keithley works very well for me. But I'm also tempted to get a decent external soundcard for use as a baseband analyser and that QA400 looks to be the best one I've seen so far

author: agdr
author_rank: Unknown
timestamp: February 25, 2014, 01:44:45 am
content: That 5V input maximum on the QA400 is definitely something to watch out for. I was thinking of adding a 5V clamp to that filter board. I couldn't come up a any effective circuit ideas though that wouldn't add a lot of noise (back to back zeners) and was eager to get V1.0 out the door.Feeding the pre-amp stage - or the whole thing - via +/-5V regulators might work.That Keithley 2015 looks like a solidly designed and built unit. Within its range the results should be very accurate. If nothing else it would be good to have on hand for a double check on the numbers of anything else above the -65dB.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 25, 2014, 05:41:23 am
content: why not just put a follower that is powered by -4/+4 volt supply rails?

author: agdr
author_rank: Unknown
timestamp: February 25, 2014, 02:20:57 pm
content: Quote from: SArepairman on February 25, 2014, 05:41:23 amYeah that probably is the solution. Something like a tps7a3001 + tps7a4901 pair running at +/-4.5Vdc or so.

author: Mark_O
author_rank: Frequent Contributor
timestamp: February 26, 2014, 02:28:57 am
content: Quote from: GK on February 25, 2014, 12:20:57 amWow. That's one heck of a DIY project, Glen! Obviously a labor of love. And definitely impressive results. Kudos!

author: TMM
author_rank: Frequent Contributor
timestamp: February 26, 2014, 08:48:45 am
content: I use a Creative E-MU 0204 USB soundcard (paid US$125 from Amazon) and ARTA (free/demo version). The input starts to be over-driven just above 1Vrms but that's no big deal since you really want to be using a variable attenuator on the input to make use of the dynamic range. It can sample up to 192KHz but the noise floor steadily rises about 20dB from 30Khz to 96Khz.It uses AKM AK4396 DACs and AKM AK5385 ADCs. I think the E-MU 0404 and the PCI/PCI-e card versions might use the same.Here is 0.0002% THD in loopback (single ended) through the audio range, it should do a bit better than this when driven with a differential signal.Quote from: agdr on February 24, 2014, 11:59:36 pmThe EMU ASIO drivers work flawlessly for me right out of the box with ARTA and Windows 7 64bitEven if you need to go through the windows mixer it isn't that bad since Vista/Win7. Windows XP and older were a fail as they re sampled everything to 48KHz if i remember correctly.General rule of thumb is:-Set the shared mode of the audio devices to the maximum bitrate and sample rate.-Set the volume level of the output device to 100-Set the volume level of the input device to 1. This may seem weird, but effectively anything above 1 is a boost.

author: agdr
author_rank: Unknown
timestamp: February 26, 2014, 06:43:07 pm
content: Quote from: TMM on February 26, 2014, 08:48:45 amHey good to know! I continue to troll on eBay for a used EMU0204 to use with ARTA. ARTA - and other external audio analysis software - apparently won't work with the QA400 yet. Has to be their proprietary software, which is pretty good but not as extensive as ARTA.I'm using Win7 64 bit. After years of having Windows OSes borked by Creative drivers - all the way back to Win98 and Soundblaster Live cards - I'm still hestitant to install their software on anything. I would probably run a Windows virtual machine on something and let the Creative software corrupt that, lol.Or maybe just set up a dedicated Win7 64 bit box to use for the audio analysis with nothing else on it, in case the Creative software takes it out.GK: I agree with Mark_O, what you've designed there is absolutely amazing! Looks like you've esssentially reverse-engineered a version of an AP, right down to the relays. Just fantastic.

author: uoficowboy
author_rank: Regular Contributor
timestamp: February 26, 2014, 07:18:18 pm
content: I have an HP 339A distortion analyzer that is pretty nice. I got it on Ebay for $150ish and it was mostly fully functional, except for some rotary switch contacts that were in serious need of cleaning. It can go well past 100KHz.

author: linux-works
author_rank: Super Contributor
timestamp: February 26, 2014, 07:31:59 pm
content: lots of audio guys use the RMAA software and decent sound 'cards'.usually by cards, I mean spdif io boxes or very good cards. the 'juli@' (julia? sigh. stupid marketing name) card is one of the best for unbal and bal analog in and out. also has spdif in and out. that is a pro level audio card and it works with rmaa. use the free demo and I think that's even good enough.emu 0404usb is another good sound 'card' for bal i/o testing.midiman firewire audiophile is another high end audio test-worthy 'card'.if you get an spdif in/out dongle of some kind, you can then pick the a/d and d/a that you think are measurement quality.

author: G0HZU
author_rank: Super Contributor
timestamp: February 26, 2014, 08:25:11 pm
content: In my (non audiophile) opinion the distortion analysers that 'just' measure THD+N are of limited use unless you are into dick waving over having achieved a very small (relatively meaningless?) THD number.i.e. there isn't much info to be had from the number because you don't get to see if the contribution to the number is from harmonics or spurious or noise or how many distortion terms contribute to the number.To me, the most intuitive solution is soundcard based because you get to see the spectrum even though the accuracy of measurement probably isn't great. But it will be good enough for nearly everybodySo I would choose the soundcard solution every time if I was into this stuff. The main downside with the 2015 is that it doesn't give a method of providing a true spectrum analyser display. You only get to measure the THD or individual harmonics or THD+noise. It's much more versatile than the one trick pony meters that just give THD+N and it's going to be miles ahead of anything else in terms of absolute accuracy in terms of measuring fundamental signal voltage and harmonics at sensible (human) levels but the sexy looking spectral display from the soundcard solution has to be the best all round solution IMO

author: robrenz
author_rank: Super Contributor
timestamp: February 26, 2014, 08:44:05 pm
content: I don't know squat about this, but isn't that a spectrum plot in reply 16 above?

author: Lightages
author_rank: Unknown
timestamp: February 26, 2014, 09:03:03 pm
content: Yes it is, but what happens is the software measure the magnitude of the 1kHz signal and then measures everything else in relation to this magnitude thus giving the relative distortion/noise level.

author: robrenz
author_rank: Super Contributor
timestamp: February 26, 2014, 09:12:57 pm
content: Thanks, I can grasp that

author: GK
author_rank: Super Contributor
timestamp: February 26, 2014, 10:43:33 pm
content: Quote from: G0HZU on February 26, 2014, 08:25:11 pmNonsense. With an analog THD analyser you can view with THD+N residual signal along side the source of excitation, and that can be a much more useful diagnostic aid than just viewing the spectrum. And besides, just by looking at the residual on a scope you can often deduce much of the spectrum characteristic of the distortion.And the residual signal can always be fed to a spectum analyser, of course. The spectum anaylser doesn't have to be anything fancy, as the analogue unit does all of the hard work. I just use my PC motherboard sound hardware. The FFT takes the noise out of the equation.

author: GK
author_rank: Super Contributor
timestamp: February 26, 2014, 10:49:18 pm
content: Quote from: agdr on February 26, 2014, 06:43:07 pmThanks. I have actually finished it and one day I'll delete the blog and do a proper write up for it. I got half through the development of a companion 10Hz - 200kHz swept frequency spectrum analyser (synthesised hybrid PLL/DDS LOs, 1Hz bandwidth resolution (final IF) filter + digital sampling board), but that is also sitting on the backburner.

author: G0HZU
author_rank: Super Contributor
timestamp: February 26, 2014, 11:33:21 pm
content: Quote from: GK on February 26, 2014, 10:43:33 pmMy point was that the THD+N figure on its own is pretty meaningless. It's probably pure porn to an audio geek but then I'm not in that category. Your point is that if you add some extra test gear to the meter you get extra test info. (thanks for that)If someone gave me a free THD meter like a HP339 I'd give it back to them and tell them to offer it to someone else. By contrast I'd be quite happy to pay good money for the sound card box because it can do so much more. In my opinion the best solution is the soundcard or a similar platform.I find that the 2015 is quite limited because of the restricted access to the DSP data. As far as I can tell I can only get THD or THD+N or individual harmonic levels via GPIB. But I only really value the ability to measure the harmonics on an individual basis. The THD capability is for the audio geeks. I think the P version can do a bit more but is still isn't going to match a decent spectrum analyser for versatility. But I like the ease of use and accuracy of some of the things it measures and it also has the onboard AF signal generator.QuoteCan you put two (or ten?) test tones into the HP339 THD meter and get anything useful from an IMD test? I would guess not. You can do this with the soundcard. It can generate the tones for you as well. The 2015 can't do anything useful here either which is a real shame. It would be nice to have more access to the DSP data from it

author: GK
author_rank: Super Contributor
timestamp: February 27, 2014, 12:10:03 am
content: Quote from: G0HZU link=topic=27311.msg395970#msg395970Well you also dismissed analogue analysers on the basis that they "just" provide a THD+N number without appreciating the utility of the residual signal output.BTW, "THD+N is meaningless" is the dead-horse-beaten mantra of the audiophile subjectivist crowd. It isn't meaningless at all in a lot of scenarios - just one example; if you know the THD+N performance of some broadcast or telecoms audio equipment, a time-effective and routine THD+N evaluation will quickly tell you with a relatively high degree of confidence that the equipment is still functioning as the manufacturer intended.

author: G0HZU
author_rank: Super Contributor
timestamp: February 27, 2014, 12:31:06 am
content: I'm still bemused by it all... To me the THD measurement isn't giving an amplifier system its toughest type of distortion test anyway. The multitone IMD test is far more telling.The soundcard can do this (far, far better) test of the amplifier's limitations. The THD meter can only sit and watch as it is redundant for this test.QuoteI would think an IMD test would be a far harsher and far more revealing than a rather soft single tone test like THD. You can do this with a soundcard but I don't think you can do this with a typical notch/THD meter...

author: GK
author_rank: Super Contributor
timestamp: February 27, 2014, 12:37:48 am
content: Quote from: G0HZU on February 26, 2014, 11:33:21 pmI don't care about the stoneage HP339, but if you can accurately measure THD from 20Hz to 20kHz with a decent measurement bandwidth (tenth harmonic at all frequencies) multi tone IMD generally has very little utility at all.Multi-tone IMD is just another method of measuring nonlinearity, which happens to be more difficult to implement and interpret. It is mostly an academic curiosity that those “audiophile geeks” that you talk about disparagingly would drool over.Simple twin-tone IMD does have a place in the assessment of HF non-linearity in certain bandwidth limited systems. A good example is Class D, where a post-feedback output low-pass filer rolls off the harmonics above 20 kHz. A THD measurement at 20 kHz on such an amplifier will often be misleadingly good, and the amplifier will indicate much worse HF linearity performance on a standardised 19kHz+20kHz twin-tone IMD test.

author: GK
author_rank: Super Contributor
timestamp: February 27, 2014, 12:42:00 am
content: Quote from: G0HZU on February 27, 2014, 12:31:06 amWell, on this point your're plain wrong in general.

author: TMM
author_rank: Frequent Contributor
timestamp: February 27, 2014, 05:05:51 am
content: IMD testing may work okay for audio amplifiers where the source of distortion is a virtually static transfer function. When you measure time-variant systems (e.g. a loudspeaker) it becomes difficult to objectively compare the results of two systems where the distortion across a wide range of frequencies needs to be evaluated. You'd need to do a large number tests at different combinations of frequencies, each producing another spectral plot... it gets out of hand pretty quickly.Running a sweep across the operating bandwidth and plotting the magnitude of the harmonics at virtually every frequency produces a single very readable plot. The catch is that the HD measurement is only valid for the test output level - i.e. THD of an audio amplifier may be great at 10W but horrible at 0.1W due to crossover distortion. 3 or 4 measurements are generally good enough to characterize a system.The mechanisms which cause IMD and HD in audio systems are almost always identical (if THD is low, IMD will be low), and interpreting HD plots is just far more intuitive imo.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 27, 2014, 05:38:12 am
content: Quote from: GK on February 27, 2014, 12:10:03 amThe classic "N&D"set doesn't give you all sorts of stuff simultaneously,but what it does,it does very well!If you measure THD+Noise,note it down,then terminate the input of the DUT,(with the same Zero calibration).you then get a figure for Noise.If you measure both on the dB scale,it is fairly simple to correct the first reading to give a "Distortion-only"figure.Of course,in normal Broadcasting practice,Noise is very small,so its contribution to THD+Noise may be neglected.Distortion tests are normally done at several levels & at different frequencies,to get a "snapshot" of thecharectistics of the DUT.If you are designing HIFI equipment,probably other methods are appropriate,but for many situations,as GK says:-"a time-effective and routine THD+N evaluation will quickly tell you with a relatively high degree of confidence that the equipment is still functioning as the manufacturer intended."

author: TheRuler8510
author_rank: Regular Contributor
timestamp: February 27, 2014, 05:58:54 am
content: Quote from: G0HZU on February 25, 2014, 12:26:28 amThe problem with the Keithley 2015 is that the dynamic range is not so great. For serious measurements you need the range that the HP8903A gives you. There is also the HP8903B, and HP8903E if you don't need a source. The three HP units go down to about -90 dB, whereas the Keithly goes to -65 or so.The HP339 is almost as good as the 8903 units, but you have to deal with the crazy 1970s style functionality. Another option: Tektronix has the AA501. Boonton has a good vintage unit that is the best of the lot, but is expensive.

author: linux-works
author_rank: Super Contributor
timestamp: February 27, 2014, 08:10:22 am
content: I have a 2015 and I'm not impressed with the low dyn range, at least on paper. I bought it for its DMM feature, not its THD (and the fact that it was hella-cheap, for some reason).the sound card way is the best way. we can do that today. we could not do that 10 yrs ago.

author: G0HZU
author_rank: Super Contributor
timestamp: February 27, 2014, 12:01:22 pm
content: Quote from: GK on February 27, 2014, 12:42:00 amIf you remain faithful to the concept of the THD+N test then the idea is to measure the average power in the 'signal' to the average power of the noise/crud that gets generated. Presumably, this is done in some THD meters with a true Vrms detector.It is much harder (on the amplifier) to communicate 1W of signal via two 0.5W tones than it is to send a single tone at 1W. it gets even harder on the amplifier if you try and send four tones each at 0.25W to make up the 1W of average signal power.

author: macboy
author_rank: Super Contributor
timestamp: February 27, 2014, 03:04:35 pm
content: Quote from: agdr on February 24, 2014, 11:59:36 pmMy good old HP 8903B can operate on fundamentals of up to 100 kHz and has a analyser bandwidth of 750 kHz. The residual disotrtion+noise (with 80 kHz low pass enabled) is around 0.0016% (16ppm) (-96 dB).

author: uoficowboy
author_rank: Regular Contributor
timestamp: February 27, 2014, 03:11:37 pm
content: Quote from: TheRuler8510 on February 27, 2014, 05:58:54 amI prefer the knobs of the 339 over the buttons of the 8903. Much more satisfying. But to each his own!

author: GK
author_rank: Super Contributor
timestamp: February 28, 2014, 05:53:09 am
content: Quote from: G0HZU on February 27, 2014, 12:01:22 pmNo, it isn't. Ironically you are (again) applying a common audiophile intuition. HD and IMD are both products non-linearity. If an amplifier has low measured THDacross the full audio bandit will have no IMD issues and IMD measurements will not be anymore "revealing" than the THD measurements. This fact has been established for about 50 years already.

author: G0HZU
author_rank: Super Contributor
timestamp: February 28, 2014, 08:40:34 pm
content: Quote from: GK on February 28, 2014, 05:53:09 amI can only assume that you are misinterpreting what I mean by average power because I didn't expect anyone to disagree with the example case I gave above. By average power I mean long term average power.If a small amplifier has a tested THD spec of 0.01% with a test tone at 1W average power it doesn't guarantee the amplifier can maintain low distortion if the test signal is changed to four test tones each at 0.25W. The average power is the same at 1W but the amplifier is more likely to produce higher distortion with the four test tones because of the significant change in waveform.

author: GK
author_rank: Super Contributor
timestamp: March 01, 2014, 09:49:06 am
content: Quote from: G0HZU on February 28, 2014, 08:40:34 pmI'm not misinterpreting anything that you wrote - you're just choosing not to listen to what I am telling you. If an audio amplifier is tested/plotted for THD at rated power across the full audio range (20 Hz - 20 kHz), and the THD indication across the full audio range is low, then that DOES guarantee that amplifier WILL maintain low distortion in any IMD test.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2014, 01:40:11 pm
content: Quote from: GK on March 01, 2014, 09:49:06 amWell the evidence so far suggests that you still don't understand the implications of the IMD test I proposed above. It is a harsher test.The 1W single tone test is easy to understand. The long term average power is 1W and the power per cycle is 1W.But if you get four tones at 0.25W the test is harsher. The long term average power is still 1W from the amplifier. But there will be times when the 4 tones will sum together and times when they cancel. When they sum you don't just get 1W delivered to the load as in the single tone case. With the four carriers you briefly get about 4 times this amount of power = 4W.So there is no guarantee that the amplifier can deliver the voltage requirement for this 4W without a small degree of clipping.I think the usual solution to this is to heavily derate the IMD test tone power levels to be fairer to the amplifier. i.e. test it at a much lower average power level from the 4 tones.

author: robrenz
author_rank: Super Contributor
timestamp: March 01, 2014, 01:53:04 pm
content: Quote from: G0HZU on March 01, 2014, 01:40:11 pmI have no skin in this game but how is 4 x .25 = 4 ?

author: TheRuler8510
author_rank: Regular Contributor
timestamp: March 01, 2014, 07:00:01 pm
content: Quote from: robrenz on March 01, 2014, 01:53:04 pmG0HZU, is off with his power arithmetic , but what he may be referring to is the fact that 4 carriers of 1/4 power will have a higher peak-to-peak voltage (when summed,) or think of it as a higher peak to average ratio, than a single carrier. For small signals, the IMD function is the same for one carrier or multiple carriers, but for strongly non-linear functions, i.e. clipping, there can be a new function governing the action. In-other-words, clipping on a multiple carrier signal where there is no clipping on a single carrier for the same power applied. Telecom amplifiers are run well backed off from compression for this reason. Another thought is this is one reason why audio amps need "headroom."The best thing to do is do a real test to know what is really happening, as there are always variables you didn't think of.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: March 01, 2014, 07:40:18 pm
content: Another thought on G0HZU's power arithmetic...it is a matter of terminology. He is referring to "instantaneous power." So 4 signals of 1/4 the power will have 1/2 the peak voltage each. When these 4 signals add up, if they are phase independent, can have sometimes have a peak voltage of 2X times (4 x 1/2) a single carrier or 4X times "instantaneous power" of a single carrier because power is related to voltage squared.I find it is not helpful to use "instantaneous power;" refer to my previous post for an easier way to think about it. In my field people rarely (if ever) refer to "instantaneous power," but in audio, I am not sure what they do.Different fields have different terminology, and it sometimes takes a while for people from different backgrounds to get on the same page terminology-wise.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2014, 08:24:49 pm
content: QuoteNot really...If you take the simplest (to analyse) case where you have four 'very' closely spaced test tones each of voltage V then there will be a moment in time when all four are at their peak voltage. If all 4 tones are close in frequency you will get one summed 'cycle' which has a waveform very similar to a sine wave. The peak voltage of this 'near sinewave' is 4 times greater than V because they all sum together.Because power is proportional to voltage squared the power will be about 16 times greater than 0.25W for this cycle. This is because the voltage has gone up fourfold and 4*4=16. This means the power for this cycle will be about 4W. In this case it's called peak envelope power or PEP because it's only measured as the average power in one cycle.There aren't any rules for tone frequency or spacing so my analysis above for the closely spaced tones is valid. If you had test tones that were a long way apart then you don't get such a good approximation to a sine wave for the biggest cycle. So it makes it harder to compute the PEP at this cycle.But either way, (and far more importantly) the amplifier has to deliver twice the peak voltage for the four 0.25W test tones (compared to the single 1W tone) and there is no guarantee that it can do this without clippingIt's a harsher test

author: TheRuler8510
author_rank: Regular Contributor
timestamp: March 01, 2014, 09:13:08 pm
content: Quote from: G0HZU on March 01, 2014, 08:24:49 pmYou did not look at my whole post, or the next post I made. We are saying the same thing.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2014, 09:32:48 pm
content: Quote from: TheRuler8510 on March 01, 2014, 09:13:08 pmSorry about thatI didn't mean to appear to be disagreeing with you. I don't ever use the term 'instantaneous power' so I was just clarifying the definition of PEP.

author: GK
author_rank: Super Contributor
timestamp: March 01, 2014, 11:14:07 pm
content: Quote from: G0HZU on March 01, 2014, 01:40:11 pmFirstly, when conducting comparative IMD tests the amplitudes of the (non-harmonically related) modulation sources are always scaled such that the amplifier does not clip. Amplifier clipping would render the test invalid.Secondly, if an amplifier delivers maximum rated power with an input signal of (lets say) 2V, and 4-tone modulation is used for an IMD test, each tone can only have a maximum amplitude of 2V/4 = 0.5V. This is because the tones must not be harmonically related and at some point they will all simultaneously vector sum in phase to a maximum of 2V.Thirdly, when you reduce the voltage of the excitation signal the power deliver to the load is reduced by the square of the reduction. 0.5V will not produce only 4 times less power into the load than a 2V signal - it will deliver 16 times less. Therefore, a 4-tone IMD test comparative to a full power (lets say 100W) THD test will be comprised of four tones each at a discrete power level of 6.25W !How you you propose that that is a significantly more difficult test for the amplifier than a single 100W input?However all that is completely besides the point that I was making (which you continue to ignore). HD and IMD are both products of non-linearity and are mathematically related. There is not such thing as a form of non linearity that can produce HD but not IMD; or high levels of one but little of the other.If an amplifier produces low measure THD at rated power (for the third time) throughout the entire audio frequency spectrum it is guaranteed to deliver comparatively low IMD performance. This is an established fact in audio design, not just in "theory" but in practice as well.

author: G0HZU
author_rank: Super Contributor
timestamp: March 01, 2014, 11:32:06 pm
content: Well, now the analysis of the 4 x 0.25W tones vs a single tone at 1W has been done (not just by me) you seem to be singing a different tune now to the "no it isn't" answer you gave below in an earlier postQuote from: GK on February 28, 2014, 05:53:09 amI think it IS harder on the amplifier to send 4 tones at 0.25W compared to a single tone at 1W.You are welcome to argue against this but I somehow doubt that is going to happen nowIt's been fun

author: GK
author_rank: Super Contributor
timestamp: March 02, 2014, 12:01:21 am
content: Quote from: G0HZU on March 01, 2014, 11:32:06 pmSilly games now? Well your post didn't make clear distinctions between average and instantaneous peak power and in the context ofcomparativeIMD and HD testing it didn't make any sense. I simply gave you the benefit of the doubt that you weren't trying to do something so silly as proposing a non-comparative IMD test condition to prove that HD testing is theoretically inferior.I have been quite clear in the points that I have been making on the merits of HD analysis versus IMD analysis and have certainly not learnt anything new here in this debate with you.

author: G0HZU
author_rank: Super Contributor
timestamp: March 02, 2014, 11:41:59 am
content: QuoteQuoteHow about a real world example that violates your first quote above?I can see I'm going to have to be careful how I answer this but I can give you a real world 'system' issue that shows the benefit of two tone testing over single tone for an audio amplifier in a telecoms system. Please realise that this is 'just' one example where IMD testing is more revealing of a fault in the AF amplifier compared to a single tone test. I can give others if you dismiss this one.Let's look at a system issue involving a SSB broadcast transmitter.It gets returned with a fault report that says "extremely high distortion in AF stages. Audio unreadable". So technician #1 fires up the transmitter and does a single tone system test and looks at THD in the transmit AF stage with the transmitter running as well. The AF amplifier in the transmit stage passes with flying colours and gives the expected low level of THD distortion and the RF signal from the SSB transmitter is also fine.So it gets put back into service as fully working. It immediately gets returned as faulty again with the same report and gets sent to engineering to have a deeper look.A two tone IMD test is carried out and the problem with the broadcast is now easily apparent. Due to a fault in the audio amplifier (broken RF decoupling capacitors) the transmitted RF envelope of the SSB signal is able to get into the AF stage and be rectified (detected) in the faulty amplifier. This injects the crudely rectified audio into the AF stage. This sets up a potential feedback path and the system can go unstable and sound awful.The system fault can't be replicated with a single tone test because the RF envelope from the transmitter is constant and so no audio is rectified to corrupt the AF stage.I hope you will accept that this can happen and does happen in SSB transmitters. Sometimes it's down to a fault or even poor design inside the radio and sometimes it can be caused by poor aerial grounding allowing high common mode currents to travel back to the radio and upset its operation.A single tone THD test is a waste of time here because it can't replicate the fault condition that causes distortion in the Tx AF stage.

author: G0HZU
author_rank: Super Contributor
timestamp: March 02, 2014, 12:12:29 pm
content: Getting the thread back on track... If anyone is interested in the 2015's (fairly limited) capabilities I can show a screenshot of the little GUI I produced for my 2015 last year. I run it on a tiny netbook so there isn't much screenspace.It has a marker function to allow each harmonic to be measured using the mouse. See below: You simply click on each green bar to measure the level of that harmonic in terms of -dBc.Here's a link to show how I evaluated the THD function on my 2015 when I first bought it.https://www.eevblog.com/forum/testgear/keithley-2015thd-initial-tests/msg313669/#msg313669At the end of the day' it's still pretty blinkered in what it can measure but I am only really interested in seeing the levels of the harmonics and I'm not interested once the harmonics dip below about -65dBc. The THD figure is of little interest to me. I primarily use it to quickly look at the harmonic levels from signal sources as I vary the signal level of the source. It's the quickest and easiest piece of test gear I have for this if I use this GUI.What's nice is you can simply stuff a signal into the 2015 and get the screenshot result almost instantly despite the high number of SCPI commands I have to send it over GPIB to fill the screen with all the data.

author: GK
author_rank: Super Contributor
timestamp: March 02, 2014, 12:35:24 pm
content: Quote from: G0HZU on March 02, 2014, 11:41:59 amNow you are changing the topic almost completely. Your original claim (specifically on THD Vs IMD testing) was that traditional THD analysis (using traditional analogue analyzers) in audio (as in 20 Hz - 20 kHz "HiFi") design is inferior to IMD analysis for reasons thus far unsubstantiated.I specifically outlined the specific conditions that make your claimgenerallyuntrue, and in a previous post I even gave a specific example (bandwidth limited systems) where IMD testing actually has the upper hand over THD testing (measuring the linearity of Class D amplifiers at the higher audio frequencies).In Class D amplifier operation the relationship between low THD and low IMD still holds true right out to 20 kHz theoretically, however the problem of measurement is a practical one as there is no viable way to measure the amplifiers THD performance prior to the output filter.As for your SSB example, it doesn't violate my text that you quoted as, firstly, I specifically wrote: "a time-effective and routine THD+N evaluation will quickly tell you with a relatively high degree of confidence that the equipment is still functioning as the manufacturer intended" - to repeat - "high degree of confidence", not "exclusively, 100% for sure". Secondly I was talking about audio equipment - not transmitters. IMD is used almost exclusively for testing the linearity of transmitters, as, in a partly similar way to Class D in audio design, they fall into the category of bandwidth limited systems (which I mentioned previously), but that's a whole different topic again.I think that is about all that I am willing to contribute to this thread. I'm not interest in further "debate" where basic points go ignored and unacknowledged and the topic of discussion simply get changed to something completely different instead.

author: G0HZU
author_rank: Super Contributor
timestamp: March 02, 2014, 01:08:23 pm
content: Actually what I think is happening here is a chronic misinterpretation of each other's posts.QuoteWhat kicked off the IMD claim was your reference to a health check of a broadcast transmitter. My immediate thought process with respect to this is that (in my experience) the IMD test is a harsher SYSTEM test of the broadcast system.I think you are only interested in the test of the amplifier in isolation. In my experience there are other things that can generate terms that look like distortion in an audio amplifier. See my example above.So partly why this thread is derailing is because you are sticking to your idealistic opinion about the maths behind distortion and I'm looking at the system limits. eg system issues that can mean that IMD testing can be a harsher test.I also proposed a multi tone test system using the same average signal power sent to the listener/load which you clearly misinterpreted.Also, I think you need to lighten up a bit...Also, I expect you to misinterpret this post as well

author: GK
author_rank: Super Contributor
timestamp: March 02, 2014, 01:20:55 pm
content: Quote from: G0HZU on March 02, 2014, 01:08:23 pmI think the case of "chronic misinterpretation" is on your side. I never said "broadcast transmitter". I said "some broadcast or telecoms audio equipment". There are plenty of examples of equipment under that category that are not actual transmitters (nearly everything from between the studio mic. and the exciter). Ask vk6zgo, he might be able to give you some examples.I only cited such examples of "professional" audio equipment as you were at pains to inform everyone that you are not an "audiophile" and are not interested in the evaluation of extremely low distortion equipment.Nighty night.

author: G0HZU
author_rank: Super Contributor
timestamp: March 02, 2014, 01:47:24 pm
content: QuoteOK, but it was a reasonable assumption of mine that that radio is involved in telecoms equipmentBut my point is that you are misinterpreting my posts too.Edit:Rest of post citing example deleted as it isn't productive.