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| MegaVolt:
--- Quote from: Kleinstein on June 29, 2020, 11:19:53 am ---Using things like 7.5 PLC makes the meter sensitive to mains hum (50 Hz, but not 100 Hz). Due to the time needed for rundown, there is a beat frequency and this would be visible. So the amplitude seen here is likely from the 50 Hz hum. This is why the preferred modes are integer PLC numbers. --- End quote --- Yes, I understand this feature of integrating ADCs. Everyone has it and it seems even 3458a judging by the picture in the application. --- Quote ---For the much faster test in the digitizing mode one may see residues of ripple from an chopper OP too. --- End quote --- You express your thoughts very briefly. And I often cannot understand because the translator often loses some of the meaning. Could you write a little more about the idea of this test. If we talk about the noise that I see with DigiV, then it does not look like 50Hz. I see 14KHz noises I wrote about them above. |
| MegaVolt:
Here are the noise graphs. For the rms value and for the constant component (Vm0). For the DC component and the 0.1 V range, there are curious points of 0.1; 0.35; 0.6 NPLC they give a big shift but in fact there is a periodic process that is longer than 1000 samples. Therefore, we see the error of expectation because we consider it for part of the period. I attached a picture taken for 100 thousand counts. |
| MegaVolt:
Looking at the similarity of the 1 V and 100 V graphs, there is a suspicion that they divide 100V by 1000 and then process it as a 1V signal. For channels 1V and 0.1V, 5 NPLC is really the best, followed by 1 and 2 NPLC. I believe in these ranges software averaging of samples with 1 NPLC will give good results. I'll check it a little later. |
| Kleinstein:
Using a divide by 100 and than use the 1 V range is the normal path for the 100 V range in many meters, so no surprise here. The extra periodic background looks strange with steps in frequency. The frequency is likely some beat frequency with mains hum. With some extra time for signal processing / run-down 10 readings at 0.1 PLC would likely take a little longer than 20 ms and thus some phase shift with every reading. The mains frequency will also have an effect, but I doubt the more discrete frequency ranges would be due to the mains frequency. The periodic signal would also cause the plateau at low PLCs for the 0.1 V range, as different PLC settings would mainly effect the frequency and less the amplitude. |
| MegaVolt:
--- Quote from: Kleinstein on July 02, 2020, 08:05:13 am ---Using a divide by 100 and than use the 1 V range is the normal path for the 100 V range in many meters, so no surprise here. --- End quote --- I understand the essence of this decision. One divider instead of two. This saves 1 precision resistor. And this is expected in a budget multimeter. But in 7.5-digit, I thought there would be no such savings. Especially considering that the amplifier standing on the 1V input spoils the noise characteristics. --- Quote ---The extra periodic background looks strange with steps in frequency. The frequency is likely some beat frequency with mains hum. With some extra time for signal processing / run-down 10 readings at 0.1 PLC would likely take a little longer than 20 ms and thus some phase shift with every reading. The mains frequency will also have an effect, but I doubt the more discrete frequency ranges would be due to the mains frequency. The periodic signal would also cause the plateau at low PLCs for the 0.1 V range, as different PLC settings would mainly effect the frequency and less the amplitude. --- End quote --- Forgot to add. If you disable AZ, then the cunning envelope disappears. And there remains just a certain frequency with a constant amplitude. The signal does not become smaller in amplitude but looks prettier. |
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