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?