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| Measuring Distortions with the Scope:What you see is not what you really have.. |
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| Martin72:
--- Quote --- for reliable measurements always double check is my advice. --- End quote --- Neutrik mentioned it in the manual ( using earth-free connection) so I thought ok give it a try. As written, nothing changed on the 2122X, but 1062X. Neutrik also explaining in the manual, how THD will be measured in the A1. A notchfilter filters out the fundamental frequency. |
| rf-loop:
Martin, you can try one small thing with SDG1000X Load 50ohm. 1kHz Set next SDG displayed output levels exactly These are only some examples. 111.8 mVrms* look what THD you get. Voltage level band change here (you can hear relay) 111.9 mVrms* look THD 353.4 mVrms* look THD Voltage level band change here (you can hear relay) 353.5 mVrms* look THD 1.118 Vrms* look THD Voltage level band change here (you can hear relay) 1.119 Vrms* up to 3.536Vrms* * Listen relay! for make sure V band border. I do not have any idea why it change some times example 353.4 or .5 and 353.5 or .6 and so on (in other V band change same, some times just tiny amount different) Example just with my Keithley 2015 THD what is not so stellar but better than nothing. Yes I have many tables done long time ago but they are only on paper notebook. (Setup for this Kethley 2015 THD: Mode Just plain THD (not THD+N) , freq.set manually 1kHz (instead of auto tune), Harmonics: up to 9th, SFIL:NONE (available NONE, A, C, CCITT, CCIRARM), Average 100rdgs repeating (front panel button Filter, Keithley input feed thru 50ohm. Examples 353.4 Vrms* my Keithley display 0.0196% (this voltage band bottom and lowest THD) one V band top value 353.5 Vrms* my Keithley display 0.0918% (this voltage band top and highest THD ) next V band bottom value This THD changing behavior repeats through all level bands (but naturally with bit different numbers). So THD is not just some value... it is many different values depending many things. If user know his generator he can perhaps some times select better signal quality. Just example if want use 4dBm. Use 3.97dBm and THD is better. THD is better when use internal voltage bands top values. And most bad when use just internal voltage bands bottom values. |
| blackdog:
Hi rf-loop, Thank you very much for explaining about the difference in behavior due to the output level of a function generator. I wanted to do this in the third part based on the aberrations that occur on the flanks of the square wave signals from the function generators I have here. The variation in THD also occurs with analog sine wave generators when additional gain stages are turned on or off there. But the differences are usually much smaller. My setup for the many measurements I have chosen (2V RMS at 50 Ohm load) where possible so that you have a bit of certainty, that 2V RMS you can then externally with a potentiometer or with 50 Ohm attenuators back to a lower level, but the "Sweet Spot" of every function generator can and wil be different. Look at my Rigol DG4162 chanel 1 and 2 distortion levels, same level, frequency but 40% different THD, probably has at to do with the linearity of the DA converters used in the DG4162 function generator. This can happen with all function generators with two or more outputs. In the measurements I showed here I assumed the audio frequency range. The distortion that occurs with the multiple function generators I have is fairly constant over the audio range. This is because the design of most function generators covers a much larger frequency range than the audio band and the opamps used i n the amplification stages are very broadband. Distortion meters that cannot show a residual always give uncertainty about the indicated value of THD. All THD meters implemented with some form of NOTCH filter almost always show the THD and many other forms of residue that have no relation to the test signal. Many analog THD meters therefore have a number of filters built in to provide more certainty about the displayed THD value. Often it is also referred to as THD + N which stands for Total Harmonic Distortion and Noise. I purposely showed the original signal in yellow in my measurements on the scope photos with the output of my Audio Precision THD function in green. So that a reasonable estimate can be made of what the residue looks like. I am aware that a scope picture of the residue can poorly show that e.g. a lot of 1/F noise is present. That is what the measurements I did last with my measurement computer and the proper sound card are for. The user of one function generator will have to look carefully at what the datasheet says in terms of distortion at a given frequency range. Almost always the distortion data are better than specified. And as rf-loop points out, by playing with the output level you can often get the distortion lower because then there are less amplifier stages/relays in the signal way and the amplifier stages are more optimally in their working range. But be aware that if you generate e.g. signals around 10MHz, then the DA and the amplifier stages will have more errors and the distortion will be much higher because of e.g. the slew rate of the opamps used in the output section of the function generator. As almost always, figure out what your good and not-so-good points are of your used equipment and think about those points when you start taking measurements. Several times I have done measurements on amplifier stages driven by the HMF2525 function generator which is very clean for a 14Bit model, see computer FFT for this. And I was expecting during the measurements I was doing what a THD around 0.002% or less and I was stuck at 0.02% THD. I had forgotten several times that I was using the THD meter in the Audio Precision, but the HMF2525 as a generator, before I made the THD measurements, I had tested my D.U.T with a square signal from the HMF2525, for measuring low THD, I had forgotten to switch generators.... Brief conclusion regarding audio measurements with a sine signal. The Siglent SDG1032X is quite useful for tube equipment, this is because with most tube equipment the THD is quite high. For significantly lower THD, it is better to choose a Siglent SDG2042X, because this generator has a 16Bit DAC. But please note that I am talking about sine wave signals, the SDG1032X has a very good point and I will soon show it in measurements of pulse signals from the function generators. This time i do not remove the text below, now you now why "some times" mijn Englis is that bad. :-DD Translated with www.DeepL.com/Translator (free version) Kind regards, Bram |
| rf-loop:
Here is one example. Level do not change nearly anything but SDG change internal "voltage band" (whole range have many voltage bands) I take here just one example. Previous one level band ends (highest in this band) at 353.5mVrms and next step go to next band most low level what is 353.6mVrms. (both are very close 1Vpp and roughly 4dBm (bit under). I do not have good and steep filters for audio frequencies so my SA noise level and max input level give some limits what I can do. If I have good filter I can go more down because of course there can find more harmonics and also some spurs. Also Spectrum analyzer mixer level need keep enough low for keep its own harmonics down enough. So in this case -110dBm level is border. After generator output there is external attenuator set for drop SA input level to -25dBm. And because SA internal Att is 0dB "mixer level" is -25dBm (this SA is specified for -30dBm mixer level but with my cross check this 5dB did not give any readable difference in harmonics levels in this case. It can use higher mixer levels but I want keep SA own generated things low) It is fun to see how much harmonics levels change. Yes it is natural if look how SDG works and not go deeper inside now. But fore some users this behavior is good to know. And, this around same happen between every internal voltage band change. This is just one of them. Note that these THD % are calculated using only 1st (fundamental), 2nd and 3th harmonic levels |
| Martin72:
--- Quote from: rf-loop on December 30, 2022, 07:12:20 pm ---Martin, you can try one small thing with SDG1000X Load 50ohm. 1kHz Set next SDG displayed output levels exactly These are only some examples. 111.8 mVrms* look what THD you get. Voltage level band change here (you can hear relay) 111.9 mVrms* look THD 353.4 mVrms* look THD Voltage level band change here (you can hear relay) 353.5 mVrms* look THD 1.118 Vrms* look THD Voltage level band change here (you can hear relay) 1.119 Vrms* up to 3.536Vrms* * Listen relay! for make sure V band border. I do not have any idea why it change some times example 353.4 or .5 and 353.5 or .6 and so on (in other V band change same, some times just tiny amount different) --- End quote --- Setup: SDG1062X with 50 ohm load termination connected to the A1. Values with * = relay switch point 111.7mVrms 0.044% 111.8mVrms* 0.117% . . 353.4mVrms 0.043% 353.5mVrms* 0.116% . . 1.116Vrms 0.042% 1.118Vrms* 0.114% . . 1.2Vrms 0.110% 1.4Vrms 0.100% 1.6Vrms 0.091% 1.8Vrms 0.083% . . . 2.9Vrms 0.050% 3.0Vrms 0.048% 3.056Vrms* 0.047% 3.535Vrms 0.041% On every "relais switch point" THD increases, then decreasing until the next switch point with one exception: The last switch point won´t be affected(or it is a different relais from a different circuit). Interesting things, thank you rf-loop :-+ |
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