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| Measuring Distortions with the Scope:What you see is not what you really have.. |
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| pdenisowski:
--- Quote from: Martin72 on January 02, 2023, 08:01:28 pm ---To me it sounds more plausible to measure everything than "only" the harmonics - Or do I have a thinking error? --- End quote --- No, that's absolutely correct: measuring "everything" over a bandwidth (minus the fundamental), i.e. THD+N, is relatively easy: just notch the fundamental out and integrate over the bandwidth, so to speak. THD (just the harmonics, not noise, spurs, etc.) was a lot harder to measure before modern spec ans with automated measurement personalities. You first have to measure the power of the fundamental - this is usually done in zero span mode with a RBW just slightly wider than the signal. Then you have to change frequency to each harmonic (calculated based on the frequency of the fundamental) and measure those in zero span. But since the width of the harmonics grows with harmonic order, you have to scale up RBW at each harmonic to make sure you're measuring the entire (wider) harmonic signal. But again, modern spec ans with an automated harmonic distortion measurment function can do all of this very easily -- measuring THD "by hand" can be very time-consuming and error-prone. I would say that THD is actually an easier measurement than THD+N (at least with a spec an). THD+N requires a tunable notch filter, whereas THD with a modern spec an doesn't require any additional hardware. It might also be worth mentioning that THD+N is essentially the inverse of SINAD, which is a measurement often supported on audio and some radio testers, but not common on RF spectrum analyzers. |
| Performa01:
--- Quote from: Martin72 on January 02, 2023, 08:01:28 pm --- --- Quote ---For THD+N, you essentially measure all power (harmonics, spurs, noise, etc.) --- End quote --- To me it sounds more plausible to measure everything than "only" the harmonics - Or do I have a thinking error? --- End quote --- I guess so… ;) Of course it used to be a lot easier to measure THD+N, as it just required an RMS voltmeter and a notch filter. This is how the traditional distortion meters work. No way to measure (and calculate!) THD without a spectrum analyzer. But just because something was much easier to accomplish in the past, it is not automatically more plausible as well. You might think so as an end user ("I don't care what it is, it just doesn't belong there!"), but even as an end user you are affected differently by the different unwanted signals. Consider the most popular area where distortion is a big thing: Audio. So you have, say, -50 dBc THD+N and you think this is fairly bad and it does not matter where all the unwanted signals stem from. But: • If it's pure noise, then your signal is essentially distortion-free but the signal to noise ratio is just 50 dB. Some folks won't even notice that noise, at least not with low dynamic pop/rock music… • If it's pure harmonics, then you might notice it because of a slightly altered sound. If the harmonics happen to be predominately even numbered, then the audiophools will rave about the "warm and fuzzy" sound as they are used to it from their single ended tube amplifiers - using a (highly linear) light bulb as a pullup resistor :palm: • If it's only spurs, then it will sound rather disturbing and folks who wouldn't have taken notice in one of the two previous cases might start to complain all of a sudden. So even the end user would prefer separate specifications for THD, S/N and spurs. For the designer of such gear it's of course all the more important to know where the unwanted signals come from. Needless to say that the investigation as well as the final countermeasures look very different for the three different types of unwanted signals mentioned before. |
| rf-loop:
THD is Total Harmonic Distortion. It is just it, nothing else. It do not include any other things but harmonics (naturally there is never absolutely ideal meters). When we measure other things, example harmonics and non harmonics and noise this is not anymore THD. It is THD+N where N include everything (if not more specified) so why we even talk then about THD (if not just for make things messy), it is more like TD (total distortion ;) ). Problem is that if we have measured THD+N and result is example 0.1 % and then we tell "THD" is 0.1%. It is then just basic lie. Also there is other things. One important is band width we have used for measurement or example what all harmonics we have included to measurement. Also some other things what make this whole "THD" thing quite messy - if we do not use right name and include other parameters. Some may example use also some weighting filter what may affect quite lot. Many times peoples talk just about THD. I will repeat this. After someone tell to you that some equipment, example audio amplifier or signal generator have THD x%. If not told anything more than "THD", then many times may need ask this question: "What THD or is it something else?" |
| Performa01:
--- Quote from: rf-loop on January 03, 2023, 06:28:22 am ---THD is Total Harmonic Distortion. It is just it, nothing else. It do not include any other things but harmonics (naturally there is never absolutely ideal meters). When we measure other things, example harmonics and non harmonics and noise this is not anymore THD. It is THD+N where N include everything (if not more specified) so why we even talk then about THD (if not just for make things messy), it is more like TD (total distortion ;) ). --- End quote --- I like this term (TD)! Very appropriate! :-+ --- Quote from: rf-loop on January 03, 2023, 06:28:22 am ---Also there is other things. One important is band width we have used for measurement or example what all harmonics we have included to measurement. --- End quote --- Yes. If a more sophisticated spectrum analyzer calculates THD for us, it takes he number of the highest harmonic as a parameter. The traditional distortion analyzers usually have a fixed bandwidth, around 80 kHz as far as I remember. But with the many alternative solutions today, like soundcards plus software, we get rather dubious conditions. A soundcard that can provide 192 kSa/s could maintain an 80 kHz analysis bandwidth, but we don't know what analog (AA) filters are there in place to reduce the bandwidth to something more suitable for audio. Ultimately, there are many soundcard solutions where the attempt of measuring THD at 20 kHz would just be a joke because of insufficient bandwidth. Yet serious designers of audio gear want to know the distortion at 20 kHz, because they have to specify THD over the whole audio range – and at 20 kHz distortion figures usually don't look that good anymore (if measured correctly)… --- Quote from: rf-loop on January 03, 2023, 06:28:22 am ---Also some other things what make this whole "THD" thing quite messy - if we do not use right name and include other parameters. Some may example use also some weighting filter what may affect quite lot. --- End quote --- I hate weighting filters in general. No wonder, they are supposed to make measurements representative for the user experience, but are confusing and even misleading for the designer. They'll just get abused by the manufacturers of audio gear in order to make the banner specs look good. Back in the mid-seventies of last century I was looking for a decent tape recorder. There have been many brands back then and I compared the specs as I wanted the highest possible S/N performance. Of course I had a favorite as well, and this machine specified about 55 dB S/N at 19 cm/s. Of course this was a weighted value after some standard of the DIN 45500 family. One year later, in the newest catalog, they suddenly specified 62 dB under the same conditions. Only in the fine print you could find that it was now measured after IEC-A or something –I don't recall exactly anymore. In any case, I was a young teenager back then, overlooked the fine print and thought "wow, they have improved their machine by quite a bit!" – and you guess it, the new numbers went into the comparison table and made this machine the winner. |
| Martin72:
Thankyou guys, interesting stuff as always.. :-+ Seen through the test field glasses, the matter would be relatively simple: Target : Max. 0.15% THD Actual : approx. 0.04% THD(+N) Approx. 0.02% THD only Test passed... 8) But it is of course very great to look behind the scenes at what is behind the measurements. But the thing with the 0.15% is still suspect. Sure, rather too much than too little - But that the measured values are so drastically below.... |
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