General > General Technical Chat
AC Voltage Calibration Source - Not Super Accurate/Expensive
EPAIII:
Thanks to all.
Yes I am watching E-Bay. I looked at that Boonton RFL 828. It looks like it would do nicely. I set up an Ebay watch for them.
Adjustable source and known good meter. Yes, I thought of that.
Kleinstein:
--- Quote from: mawyatt on December 24, 2022, 03:54:09 am ---One idea we've been bouncing around for some time now is to use both the Q and Qbar from the CMOS FF and a CMOS driver to create an "H Bridge" configuration. This will produce the same RMS Voltage as the Reference VDD voltage, but with a DC average of zero. We would have done this way back when we developed the original PCB but wanted something that could check our DSOs as well as DMMs, and DSOs don't have "floating" inputs, so decided on just single ended as shown.
Believe a fast FF like a 74AC74 driving a fast CMOS inverter like a 74AC04 with the outputs paralleled, or resistively summed to create a given Z out, like 50 ohms differential with 3 inverters with 75 ohms series R on FF "Q" and the other 3 inverters with 75 ohms on "Qbar". Use the extra FF D Input to load the used FF "Q" output to equalize the loading on "Q" and "Qbar". VDD for the FF could be from a 5 volt 78L05 or something, and the VDD for the 74AC04 is from the Precision Voltage Reference.
Anyway, something to think about.
Best and Happy Holidays,
--- End quote ---
The bridge circuit would create a higher output voltage, but also no langer has a well defined ground or lots of common mode signal. This is inviting problems with EMI and shielding effects.
For most pasts it is not about a near perfect square wave, but more a near sine wave that can be varried over a large range. What voltages are acutally needed depends on the meters.
It would be nice to also have one higher votlage, like 200 V, at least with a low frequency.
The square wave could be used for a crude AC/DC transfer at low voltage (e.g. 5 V DC), but there are still limitations: A meter working well with a square wave may still have a bad RMS circuit (e.g. averaging cap dead) and failing on the square wave could be just an effect of limited slew rate with a perfectly fine, though limited BW meter.
The square wave could still be usefull in combination with a known (e.g. calculated) filter to slow down the steep slopes and limit the BW.
It still makes sense to check the response at a low (e.g. 50 Hz) and higher frequency (e.g. 10 kHz range if the meter supports this), as the AC divider usually have a resistive and capacitive part to get in theory independent gain for the lower and higher frequency part. Cheaper meters may not support 10 KHz with higher voltage - so one may not need this somewhat more tricky point.
mawyatt:
--- Quote from: Kleinstein on December 28, 2022, 06:27:24 pm ---
The bridge circuit would create a higher output voltage, but also no langer has a well defined ground or lots of common mode signal. This is inviting problems with EMI and shielding effects.
For most pasts it is not about a near perfect square wave, but more a near sine wave that can be varried over a large range. What voltages are acutally needed depends on the meters.
It would be nice to also have one higher votlage, like 200 V, at least with a low frequency.
The square wave could be used for a crude AC/DC transfer at low voltage (e.g. 5 V DC), but there are still limitations: A meter working well with a square wave may still have a bad RMS circuit (e.g. averaging cap dead) and failing on the square wave could be just an effect of limited slew rate with a perfectly fine, though limited BW meter.
The square wave could still be usefull in combination with a known (e.g. calculated) filter to slow down the steep slopes and limit the BW.
It still makes sense to check the response at a low (e.g. 50 Hz) and higher frequency (e.g. 10 kHz range if the meter supports this), as the AC divider usually have a resistive and capacitive part to get in theory independent gain for the lower and higher frequency part. Cheaper meters may not support 10 KHz with higher voltage - so one may not need this somewhat more tricky point.
--- End quote ---
Yeah the bridge would double the RMS value and better "fit" within most DMMs we have which have a 10V range, but is a bipolar waveform with an average of zero.
The fundamental problem with a "filter" is that it's response needs to be known better than the measurement. We prefer to use as close to an ideal squarewave as possible, with a precise known peak voltage and keep the frequency low enough so the DUT meter can have a good chance of representing the True RMS value, as most of the waveform harmonics will fall well within the DUT bandwidth. Of course if we could have a filter of precise known response and/or also a precise known frequency response of the DUT DMM then we could use just about any waveform, of which the sine wave is ideal since it has no harmonics.
Again, the benefit of the squarewave is the ease of creation, ease of establishing a precise amplitude, and of course the simple relationships between peak, rms and average values. If we were still teaching we would get a grad student to formulate the relationship between RMS value and a "squarewave" with unequal exponential RC rise/fall times, the more simple solution is with a linear rise & fall.
Of course YMMV.
Best,
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