| Products > Test Equipment |
| Siglent SDS1x04X-E BodePlot II (SFRA) features and testings |
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| Electro Fan:
Hi rf-loop, This is kind of off topic, but first thanks for all of your many contributions to the forum. Your explanations are very helpful and your documentation is very impressive. Any chance you can share what editing program you use to annotate screen shots? Thanks! |
| Performa01:
--- Quote from: KungFuJosh on July 03, 2019, 03:41:17 pm --- --- Quote from: Performa01 on July 02, 2019, 03:07:50 pm --- --- Quote from: KungFuJosh on July 02, 2019, 02:31:04 pm ---How can I reconcile the 50R terminated load on CH1 with a HiZ load on CH3? --- End quote --- This is an oversight in the Bode Plot module. The next version will allow to specify separate load impedances for the reference and output channels. Always specify the output impedance. In your setup it would be high-Z (as it is represented by the x10 probe) and for frequencies that low you could even connect the reference channel without a 50 ohms inline termination. The result will be correct as long as the reference channel sees the same amplitude as the DUT input. --- End quote --- Considering this oversight, it doesn't seem functional at all in my current setup. --- End quote --- Quite obviously, the load impedance is irrelevant for voltage/current measurements, so this is only applicable when dBm units for power measurement (mainly in RF applications) are used. For professional audio, dBu is standard, but Vrms, dBV or even arbitrary dB might be just as useful, depending on the actual task. This means, you will of course not see any difference as long as you don’t select dBm units for the measurement. It is also not a problem if you just look at the DUT output (Pout) – you just need to specify the load impedance to get a correct power measurement. To cut a long story short, there is only a problem when measuring the power gain, i.e. Pout/Pin and the load impedance of input and output are not the same. This could yield a maximum error of 3dB. As stated before, this is a known issue and will be fixed at some point. --- Quote from: KungFuJosh on July 03, 2019, 03:41:17 pm --- --- Quote from: Performa01 on July 02, 2019, 03:07:50 pm --- --- Quote from: KungFuJosh on July 02, 2019, 02:31:04 pm ---For the bode plot to be accurate, do the waveforms need to match well? I tried offsetting the phase, which was about 120 degrees off, but that was never perfect, and the amplitude never matched. The closest was setting the amplitude to 90mVpp. --- End quote --- It normally should be just a single signal, fed into both the reference channel of the scope and the DUT input. For frequencies above some 10MHz, proper 50 ohms termination and a resistive power splitter is highly recommended. You can also use the dual channels of the signal generator instead, but then it has to operate in Channel Coupled mode for amplitude, frequency and phase. For higher frequencies, matched cable lengths are important to minimize the phase error. --- End quote --- I'm reading that as "yes, they should match." I'm using both outputs from the AWG as described earlier in this thread. One is direct to the scope CH1 with a 50ohm adapter, and the other output goes to my DUT, which then goes through a 10x probe to CH3 on the scope. With AWG CH2 locked to CH1, it doesn't appear that there's a way for me to have the waveforms match phase and amplitude on the scope. --- End quote --- Sorry, but I don’t quite understand your problem? Are you saying you can’t get a perfect channel tracking for amplitude, frequency and phase on your AWG? --- Quote from: KungFuJosh on July 03, 2019, 03:41:17 pm ---Am I wasting my time with this? Is there any real benefit to the Bode Plot for audio equipment (specifically microphones and tube amplifiers)? --- End quote --- Bode Plot is perfectly suitable for audio applications – it is just a matter of the frequency range. Here is an example for a simple ~20kHz RC lowpass filter: SDS1104X-E_FRA_CTC_100nF_Mylar Here’s another example; this is a discrete transistor amplifier with 60dB gain (measurements shows less because for this test there is also a ~14dB attenuator at the amplifier input). This is with the SDS5000X, but the Bode Plot application has the same core as on the SDS1004X-E: SDS5104X_FRA_R41_-A14+G60dB_Ch2 So this is perfectly suitable to analyze and document the frequency/phase response of a tube amplifier. Your question about the microphone is a different story. Measuring the frequency response of a microphone is about the most demanding task of all and at least as much of a challenge as antenna measurements in telecommunications. The Bode Plot of the SDS1004X-E is not ideal for such tasks, because it cannot apply a correction curve, hence cannot be calibrated. The requirements for the test setup are a true challenge and very expensive on top of that. The DUT would then usually look like this: Mic_FRA Since a microphone is a transducer that has no electrical input, we need another transducer to convert the electrical signal to sound pressure (some sort of reference loudspeaker). The latter will certainly not have a flat response, so you need to know its detailed frequency/phase response in order to apply the required correction to the measurement results. Such a calibrated sound pressure source together with the anechoic chamber would be prohibitively expensive for a hobbyist I think. |
| rf-loop:
In some special cases perhaps also this kind of principle is possible? (as my nickname tell I'm more RF.... ;) ) (of course it need well known ref Mic etc... and very carefully istalled so it do not mess Microphone under test and it get as perfectly same as Mic under test get.) |
| Performa01:
--- Quote from: rf-loop on July 04, 2019, 06:13:51 pm ---In some special cases perhaps also this kind of principle is possible? (as my nickname tell I'm more RF.... ;) ) (of course it need well known ref Mic etc... and very carefully istalled so it do not mess Microphone under test and it get as perfectly same as Mic under test get.) --- End quote --- Thanks for pointing this out. It is of course viable, but now requires an calibrated measuring microphone, which makes the setup even more expensive. Just like reference sound pressure sources, measuring microphones do not have a perfectly flat response, but are calibrated, i.e. their response is precisely known and the corresponding correction can be applied to the measurement results. EDIT: I think your point is that the sound pressure source needs not be calibrated, but it still needs to be high quality to come close to a point shaped source without partial oscillation of the membrane to give a uniform sound field. |
| KungFuJosh:
--- Quote from: Performa01 on July 04, 2019, 08:58:14 pm --- --- Quote from: rf-loop on July 04, 2019, 06:13:51 pm ---In some special cases perhaps also this kind of principle is possible? (as my nickname tell I'm more RF.... ;) ) (of course it need well known ref Mic etc... and very carefully istalled so it do not mess Microphone under test and it get as perfectly same as Mic under test get.) --- End quote --- Thanks for pointing this out. It is of course viable, but now requires an calibrated measuring microphone, which makes the setup even more expensive. Just like reference sound pressure sources, measuring microphones do not have a perfectly flat response, but are calibrated, i.e. their response is precisely known and the corresponding correction can be applied to the measurement results. EDIT: I think your point is that the sound pressure source needs not be calibrated, but it still needs to be high quality to come close to a point shaped source without partial oscillation of the membrane to give a uniform sound field. --- End quote --- Very interesting. It appears that you're suggesting that the test should use a live capsule on the microphone. I often test the mic prior to installing a capsule. So the mic would have a ~72pF dummy load instead of the capsule, and then I insert a signal directly to the mic circuit. Shouldn't this be significantly simpler to test the circuit that way? Thanks, Josh |
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