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| trobbins:
--- Quote from: nenea dani on July 02, 2023, 02:03:54 pm --- I am attaching a diagram with the output from 4i4 for 20kHz sine somewhere at 1/2 FS with the specification that it is not in the loop. Everything is satisfactory, but higher in frequency we have artifacts. I am attaching two more diagrams, one up to 250kHz and the next up to 2.6MHz. The noise generated by the switching sources can be clearly observed, although it is possible that some artifacts have another origin. The noises in the last diagram do not disappear if the signal is off, but are proportional in any situation to the level of the master potentiometer. These disturbances are best seen with the oscilloscope on a rectangular signal because they have about 12-15% of the useful value. The spectral components used in the composition of the rectangle are stable over time, but the noise of the switching sources wanders continuously. I use an LC filter with fc =136Khz, N=7 at the output of Focusrite to clean all the radio frequency when I need a sine wave 20Hz-100kHz. After all, it's a 200 Euro sound card and for the hobby it's just fine. --- End quote --- Does '1/2 FS' mean you use a sampling frequency of 96kHz with a nominal 48kHz bandwidth? Can you post a spectrum from say 10kHz to 2.6MHz, rather than with separate spectrum widths, to give confidence that the artifacts are shown along with the generated signal and its harmonics. The 240kHz artifact is pretty much at the noise floor level. The 1.1MHz to 2MHz artifacts may be just a result of your measurement system and ground/parasitic loops, given they don't disappear when the signal is off. Can you elaborate on what you mean when talking about a 'rectangular signal'. For use in audio measurements, I can't see what influence any soundcard artifacts in the low MHz range (or even at 250kHz if that is an actual artifact) would have on any measurement accuracy or outcome or ??? If the soundcard was in use at the same time, and in the same measurement setup along with an instrument that has wider bandwidth, then yes the other instrumentation may well be affected (eg. a DVM with 300kHz bandwidth for rms voltage, or an alternate instrument for Bode plotting like a Picoscope), but that seems to be getting into some exotic test setups where one would have to be aware of lots of potential issues. |
| macboy:
I have a Panasonic VP7722A which is a very nice analyser. Being older it does not have a big graphical display that the more modern ones have, but it is simple to use and has a very low THD floor (around 1ppm or 0.00010%). Unlike some pure analog analysers like the HP 8903B (which I owned in the past) this one uses DSP to individually measure the fist 10 harmonics. The front panel can only display the first 5 but you can grab the others over GPIB. The DSP can also average out noise so that you can measure only THD in addition to THD+N. It also does IMD, SNR, and other things. I have also used a sound card plus software (REW, Rightmark audio analyser, others), but I prefer a standalone instrument for basic measurements. There is something about cabling up a DUT to the front panel jacks, pushing a few buttons, and seeing a measurement appear on an LED display. The main benefit of the software solution is the FFT display, plus easily saving the results. The drawbacks are plenty: booting PC and software, using keyboard and mouse instead of buttons (touch screen would be nice but I don't have), and the need to roll your own interface between amplifier output and sound card input so that you don't blow up the latter. |
| nenea dani:
--- Quote from: trobbins on July 03, 2023, 07:16:51 am --- --- Quote from: nenea dani on July 02, 2023, 02:03:54 pm --- I am attaching a diagram with the output from 4i4 for 20kHz sine somewhere at 1/2 FS with the specification that it is not in the loop. Everything is satisfactory, but higher in frequency we have artifacts. I am attaching two more diagrams, one up to 250kHz and the next up to 2.6MHz. The noise generated by the switching sources can be clearly observed, although it is possible that some artifacts have another origin. The noises in the last diagram do not disappear if the signal is off, but are proportional in any situation to the level of the master potentiometer. These disturbances are best seen with the oscilloscope on a rectangular signal because they have about 12-15% of the useful value. The spectral components used in the composition of the rectangle are stable over time, but the noise of the switching sources wanders continuously. I use an LC filter with fc =136Khz, N=7 at the output of Focusrite to clean all the radio frequency when I need a sine wave 20Hz-100kHz. After all, it's a 200 Euro sound card and for the hobby it's just fine. --- End quote --- Does '1/2 FS' mean you use a sampling frequency of 96kHz with a nominal 48kHz bandwidth? Can you post a spectrum from say 10kHz to 2.6MHz, rather than with separate spectrum widths, to give confidence that the artifacts are shown along with the generated signal and its harmonics. The 240kHz artifact is pretty much at the noise floor level. The 1.1MHz to 2MHz artifacts may be just a result of your measurement system and ground/parasitic loops, given they don't disappear when the signal is off. Can you elaborate on what you mean when talking about a 'rectangular signal'. For use in audio measurements, I can't see what influence any soundcard artifacts in the low MHz range (or even at 250kHz if that is an actual artifact) would have on any measurement accuracy or outcome or ??? If the soundcard was in use at the same time, and in the same measurement setup along with an instrument that has wider bandwidth, then yes the other instrumentation may well be affected (eg. a DVM with 300kHz bandwidth for rms voltage, or an alternate instrument for Bode plotting like a Picoscope), but that seems to be getting into some exotic test setups where one would have to be aware of lots of potential issues. --- End quote --- Sorry for rectangular and FS. I had in mind square wave and Full Scale respectively. Now I have calibrated the board, I have disconnected all the unnecessary devices nearby and the LED lights. I use RG316 cables and everything is well shielded. I am attaching two diagrams with a 20kHz signal / around 55mVeff measured with the oscilloscope at the input of the analyzer and then removed. I immediately connect with the generator off and it can be seen that the switching sources are still there or somethings. You are right that devices connected together can influence each other and that radio frequency artifacts do not matter in audio. However, some high-performance amplifiers can exceed 500Khz and I heard that ten times the band limit is needed for tracking tests phase change. In short, to work correctly between 20Hz and 20kHz, it must have minimum attenuation between 2Hz and 200kHz. |
| trobbins:
--- Quote from: nenea dani on July 03, 2023, 07:56:40 pm ---Sorry for rectangular and FS. I had in mind square wave and Full Scale respectively. Now I have calibrated the board, I have disconnected all the unnecessary devices nearby and the LED lights. I use RG316 cables and everything is well shielded. I am attaching two diagrams with a 20kHz signal / around 55mVeff measured with the oscilloscope at the input of the analyzer and then removed. I immediately connect with the generator off and it can be seen that the switching sources are still there or somethings. You are right that devices connected together can influence each other and that radio frequency artifacts do not matter in audio. However, some high-performance amplifiers can exceed 500Khz and I heard that ten times the band limit is needed for tracking tests phase change. In short, to work correctly between 20Hz and 20kHz, it must have minimum attenuation between 2Hz and 200kHz. --- End quote --- Was the last plot made with the same test connections as the first two plots, but just with the soundcard not being asked to generate the 20kHz sinewave tone? Did you do a reference plot with the same equipment and test connections in place, but the soundcard itself unpowered ? Can you elaborate on what "ten times the band limit is needed for tracking tests phase change.", as I'm not sure what 'tracking tests phase change' means. Do you make tests that use the 4i4 to generate signals and apply them to equipment, and have the Sigilent scope connected at the same time for measuring performance of the equipment in the frequency range beyond that of the 4i4 test signals, and is that what you are concerned about? |
| pdenisowski:
Discontinued, but definitely not a toy: https://www.rohde-schwarz.com/us/products/test-and-measurement/audio-analyzers/rs-upv-audio-analyzer_63493-7558.html Edit: looks like you can still pick up a used one from Ebay for 15-20K USD :) |
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