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| Which DMMs can reasonably measure RMS? |
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| W6EL:
I was surprised the other day when I tried measuring the RMS value of a rather standard TTL square wave on several "True RMS" meters. The waveform was 0 to ~4.1 volts, 50% duty cycle. I ran the test at several frequencies (100Hz, 2 KHz, 200KHz) just to make sure. Every meter I had basically failed to even get close to the correct RMS value, which should be ~2.90 volts (Vpp * sqrt(D)). The only gear I have which measured it correctly was my HP 54645D scope (not really surprising). I figured a TTL waveform has to be one of the more common waveforms people would measure using a handheld meter. After all, TTL appears in many low-speed digital circuits. I wouldn't expect to measure more complex waves accurately or higher-speed waveforms (especially with a handheld unit), but come on! Of particular disappointment is the more modern Owon meters, which surely could take a few readings and do an actual calculation? One would think? Even the scope mode on the HDS272S (a great handheld scope) lacks an RMS readout, providing only Vpp, Vax, Vmin, and Vamp (which reads the same value as Vpp). The HP 400EL is excused of course, since it is calibrated to read RMS only for a sine wave (like many analog meters of its time). Somewhat ironically it provided a closer measurement than most of the other equipment though. I get that RMS requires a calculation. But come on. How much trouble is it to take a few consecutive samples at 5 bits resolution and do the calculation? Surely modern meters can do such a thing? Does anyone know of a good meter that would pass the TTL test? Maybe one of the EEVBlog models? Agilent? Fluke? Here are my results at 2 KHz. I have measured using both DC and AC since this is a fully-positive signal: * HP 4645D scope: 3.035V RMS, 4.094Vpp * HP-400EL: 2.22V (3V scale used, read 74% of full scale) * Tenma 72-410A True RMS: 2.224 (AC), 2.289 (DC) * Owon B35T TrueRMS meter: 2.076 (AC), 2.278 (DC) * RadioShack TrueRMS meter (can't find the model): 2.064 (AC), 2.278 (DC) * Owon HDS272S: 1.761 (AC), 2.281 (DC) I'm quite disappointed. I had thought the so-called "True RMS" would be a bit closer than this. How do the expensive meters stack up against my hobby-lot? |
| nctnico:
You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only. But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside. |
| W6EL:
--- Quote from: nctnico on April 25, 2023, 05:43:45 pm ---You have to be very carefull when selecting a DMM that can measure odd waveforms and other frequencies than 50Hz. Most handheld meters are 50Hz only. But you don't have to buy extremely expensive meters. Vici VC8145 is one that can measure RMS in the audio frequency range because it has a dedicated RMS converter chip inside. --- End quote --- Have you actually tested this with something other than a sine wave? I tried down at 60 Hz as well, same results. I believe at least one of these meters I mentioned has one of those "RMS chips" inside. |
| Kleinstein:
The TTL signal is a mixed AC+DC signal. For the RMS reading there are 2 ways to handle this: 1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero). 2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading. A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell. So the Tenma, Owon B35T and radioshack meters don't look that bad. Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading. |
| W6EL:
--- Quote from: Kleinstein on April 25, 2023, 06:10:48 pm ---The TTL signal is a mixed AC+DC signal. For the RMS reading there are 2 ways to handle this: 1) have AC coupling and thus only show the AC part. For the 50% square wave this would be the same AC as DC reading (50% of peak voltage if the low voltage is at zero). 2) a combined DC+AC RMS values and thus the AC part + DC part as the geometric sum. In this case 1.41 * the DC reading. A few meters offer both version and many meter offer only the AC coupled case. Usually the manul will tell. So the Tenma, Owon B35T and radioshack meters don't look that bad. Many handheld DMMs don't work to very high frequencies. So 2 kHz square wave can already be a bit on the fast side and thus a lower than expeced reading. --- End quote --- It's not the frequency. I tried even 60 Hz, same results. I'm actually surprised how well all these meters handle higher frequencies. The Tenma bench meter works well above 200 KHz, and the HP 400 really does work fine at 10 MHz! The others are pretty flat in the usual audio range at least, tapering off some as you get higher. While I can understand RMS being different for AC and DC coupled measurements, the RMS calculation should be conceptually a differential measurement from Vmin to Vmax at duty cycle D. What would an HP-3400 (a meter that converts the energy to thermal energy) read for a square wave about zero versus one with DC bias? |
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