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| Measuring RMS voltage for PWM-like signals at 150 kHz |
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| siealex:
Can any "true RMS" multimeters measure, for example, the filament voltage coming from the voltage converter in a vintage VFD calculator? Typical characteristics of this voltage: 2.5 V RMS (in some models 3.15 V), no DC component (directly from a transformer), 80..150 kHz, pulsed PWM-like shape. My new ZT-702s displays something only in scope mode, the "true RMS" multimeter mode shows all zeros. And even the scope mode displays 3.3 V RMS, a voltage that would burn the VFD filament in a couple of minutes, on a perfectly working VFD with 2.5 V filament voltage. |
| bdunham7:
Scopes are a good way to get RMS readings on higher-BW AC, but you need to have a decent scope and know how the gating works--it isn't hard to get a wrong result. As far as DMMs, better bench meters might be able to get you a good enough reading on a signal like this. The only ones I have that I'm reasonably confident would be pretty accurate are very old, the Fluke 8506A Thermal RMS and the 8505A (regular) RMS but with a 1MHz BW. Analog meters like the HP3400B would work too. |
| TimFox:
The -hp- 3400 (analog readout) and 3403 (digital readout) true-RMS meters have sufficient bandwidth for your measurement. Note that the input connector for each one is a BNC with grounded shell. If you worry about crest factor (peak divided by RMS), you can always set the meter to a higher full-scale input value to see that the reading remains the same. The 3400A is good up to 10 MHz, only AC-coupled (above 10 Hz). The 3403C is good up to 100 MHz, and can be AC or DC-coupled (to include the DC component in true power) by switch selection. I like the 3403 series for noise measurements, since it has fast or slow averaging modes (inside the RMS averaging). |
| gnuarm:
--- Quote from: siealex on August 07, 2023, 09:58:11 pm ---Can any "true RMS" multimeters measure, for example, the filament voltage coming from the voltage converter in a vintage VFD calculator? Typical characteristics of this voltage: 2.5 V RMS (in some models 3.15 V), no DC component (directly from a transformer), 80..150 kHz, pulsed PWM-like shape. My new ZT-702s displays something only in scope mode, the "true RMS" multimeter mode shows all zeros. And even the scope mode displays 3.3 V RMS, a voltage that would burn the VFD filament in a couple of minutes, on a perfectly working VFD with 2.5 V filament voltage. --- End quote --- If the Vmax and Vpp values are correct, I can tell you the Vrms will be greater than 3.0V. Vpp - Vmax = ~3.0V. Ignoring the brief time in the transition and the duty cycle, the minimum possible Vrms would be 3.04V and the max possible would be 6.7V. The voltage is at 6.7V about 20% of the time. Considering that the impact is relative to the square of the voltage, this will have about the same impact as being at -3.04V 80% of the time. My seat of the pants guesstimate is about 4.5Vrms. |
| bdunham7:
As the saying goes "You might be an engineer if you debate something for an hour when a 5-minute experiment would resolve the issue". Or something like that. I think the OPs scope is reasonably close to the truth. I tried to replicate that signal with an AWG, I used +6V and -2V with a 25% duty cycle, a fast rise and a 1µs fall time. This resulted in a balanced signal that looks pretty much the same with DC or AC coupling. This gives me ~3.23VRMS on the scope, as shown. As for what meters can read this directly, I tried some. HP 34401A: 3.28xx V (this meter is a bit 'peaky' at higher bandwidths, meaning it is in spec but the error is to the high side at first, then drops later) Fluke 8842A: 2.98xx V (only specified to 100kHz) Fluke 8808A: 3.18xx V Fluke 189: 2.55xx V Fluke 289: 2.64xx V Fluke 8505A: 3.204 V Fluke 8506A: 3.202934 V (this is probably as good as it gets and the output of the AWG isn't nearly stable enough for this resolution) |
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