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| alm:
--- Quote from: mawyatt on January 16, 2022, 12:48:21 am ---One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!! --- End quote --- You keep talking about this unity factor as if it's something magical that is exact by definition. Say you take your chopped DC circuit, run in through a slew-rate limiter. Now say you do the math, and figure out that for the slew rate, frequency and amplitude, the RMS value is (0.95 +/- 0.01)*Vdc. How is this any less accurate than comparing to (1.00 +/- 0.01)*Vdc? Are you doing a bridge circuit with a thermal RMS converter where you are directly comparing the analogue RMS value of two signals without digitizing them, like with the old Fluke 540B? Or are we not talking about a unity factor, but about a very precisely defined factor that may be pretty much any value within a factor of two or so of unity, just to stay in the same DMM range? --- Quote from: mawyatt on January 16, 2022, 01:13:31 am ---Think you are missing the point of using the squarewave, never said it was the most realistic or best test, just simply the easiest to implement and verify. Doesn't cost much either :-+ --- End quote --- That's fair, but you made it sound like the higher harmonics content was an advantage of this technique, while I think it's a distinct disadvantage, though maybe an acceptable ones given the advantages you mention. I know it's easy, that's why Tektronix used it. But I think you should be very careful to use it on a not previously tested DMM, and might need to characterize the DMM before you can reliably use this method. Especially DMMs with a bandwidth less than the audio frequency range would be a concern. Or if you would want to use a similar technique for current. Many DMMs, like the 34401A, have a much narrower bandwidth for current (5 kHz in the case of the 34401A). But for DMMs with a wide bandwidth it's a neat solution indeed! --- Quote from: mawyatt on January 16, 2022, 01:13:31 am ---Regarding particular frequencies DMMs are "tuned" too, this would be the mains and maybe a few harmonics that come to mind for obvious reasons. Regarding reading rates, some of the newer DMMs like the KS34465A and DMM6500 seem to have higher digitizing rates, although the old HP3458 also had high digitizing rates I believe. --- End quote --- In the case of the 3458A, you have the choice between three different built-in AC modes that change the low frequency and high frequency cut-off and may make assumptions about the repetitiveness of the signal, and in addition there's Swerlein's algorithm which assumes the signal is a low distortion sine-wave with a frequency up to 1 KHz or so. In addition, you could digitize the input signal at up to 100 kS/s (depending on the desired resolution). So before you would do an accurate ACV measurement, you would identify which kind of signal it is. Presenting such an instrument with a wide-band signal would be pretty much a worst-case requiring use of the analog True RMS converter. |
| mawyatt:
--- Quote from: alm on January 16, 2022, 01:36:54 am --- --- Quote from: mawyatt on January 16, 2022, 12:48:21 am ---One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!! --- End quote --- You keep talking about this unity factor as if it's something magical that is exact by definition. Say you take your chopped DC circuit, run in through a slew-rate limiter. Now say you do the math, and figure out that for the slew rate, frequency and amplitude, the RMS value is (0.95 +/- 0.01)*Vdc. How is this any less accurate than comparing to (1.00 +/- 0.01)*Vdc? Are you doing a bridge circuit with a thermal RMS converter where you are directly comparing the analogue RMS value of two signals without digitizing them, like with the old Fluke 540B? Or are we not talking about a unity factor, but about a very precisely defined factor that may be pretty much any value within a factor of two or so of unity, just to stay in the same DMM range? --- End quote --- Having a waveform that has the exact same RMS and Average DC value is a big advantage. One can take a DMM, measure the DC Reference voltage, then measure the Average DC value of the waveform using a simple RC LP using the same DMM. This ratio should be very close to 1/2 or somethings wrong!! Then measure the AC RMS waveform value, it should also be very close to the Average DC value and 1/2 the DC Reference measurement, unless something is wrong or the DMM can't handle the waveform. This is exactly why we wanted to compare the RMS computational DMMs like the KS3465A and DMM6500 and the RMS analog chip DMMS like the 34401A. The fact that they all behaved quite well it a tribute to the design and implementation of these fine instruments, so hat's off to Keysight, Keithley and the older Agilent and HP :-+ :clap: Best, |
| David Hess:
--- Quote from: mawyatt on January 16, 2022, 12:48:21 am ---One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!! --- End quote --- Precision square waves, pulses, and triangle waves are the simplest to create from a DC reference without leveling. |
| David Hess:
--- Quote from: mawyatt on January 16, 2022, 02:04:01 am ---Having a waveform that has the exact same RMS and Average DC value is a big advantage. One can take a DMM, measure the DC Reference voltage, then measure the Average DC value of the waveform using a simple RC LP using the same DMM. This ratio should be very close to 1/2 or somethings wrong!! Then measure the AC RMS waveform value, it should also be very close to the Average DC value and 1/2 the DC Reference measurement, unless something is wrong or the DMM can't handle the waveform. --- End quote --- And an average response calibrated for sine waves should read Pi/(2*Sqrt(2)) high. |
| mawyatt:
Early this morning freed up the KS34465A from the on-going System testing. Forced the CMOS 4060 2^14 divider on the PCB using a 2^14 * 60 signal from an AWG to get a 60Hz Squarewave waveform. Here's the results. Vdd Reference =5.0001813VDC Vsqwave Average = 2.5000648 VDC Vsqwave RMS = 2.5000922 VRMS Set the DMM to mx+b Scaling, Slow Smoothing Filter & Statistics, Auto Zero On, 100 PLC and Input Z Auto. No attempts were made do anything other than hook up the DMM and force the Squarewave to ~60Hz, although the AC wasn't on as things were warming up from the colder night and a storm just passed through. So you could say maybe, the temp change, humidity, phase of the moon or whatever influenced the measurements, but think these are quite good results for the KS34465A IMO, certainly better than expected :-+ Now back to some testing so we can afford another DMM :D YMMV Best, |
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