In Voltmeter impedance, I talked about measuring the input impedances of the bench AC voltmeters by using a pair of voltmeters. Today I worked out a way to measure the impedance of my cheap RadioShack pocket voltmeter using just the voltmeter itself, my new FG085 function generator, and a 3.3MΩ resistor. The idea is simple: at a number of different frequencies, measure the voltage of the function generator with the voltmeter, then put a 3.3MΩ resistor in series with the function generator and measure the voltages at the same frequencies. This technique doesn’t even require a breadboard—just clip leads.
The idea is that the voltmeter and the 3.3MΩ resistor forms a voltage divider, and what we are measuring is the output of the divider. Because the impedance of the voltmeter is always high enough that it provides a negligible load for the 50Ω-output of the function generator, we can safely estimate that the input to the voltage divider is the voltage we measured without the resistor. So we can use to estimate , where . We can then fit the magnitude of the impedance to a model of a resistor and capacitor in parallel.
I’m not surprised that the fitting had to be stopped at 10kHz, as the plot I did in FG085 function generator output impedance showed the voltmeter having a 2nd-order low-pass filter with a cutoff frequency around 9.3kHz. Although the low frequency at which the measurements start failing is annoying, the >10MΩ and <20pF input impedance are actually quite good.
I would have measured my Fluke 8060A multimeter, but it seems to have died—I might try opening the case and doing the troubleshooting suggested in the manual, but I suspect that replacement parts are needed, which may be difficult to obtain.
I tried measuring the DT-830B $5 voltmeter I have, but it is rather difficult, as it does not have a low-voltage range for AC—its lowest voltage range is 200V, with a precision of 0.1V (though who knows how bad the accuracy is). I ended up using a 180kΩ (measured at 178.6kΩ) series resistor, in order to still have a measurable voltage over a reasonable frequency range.
The very flat line up to 1kHz for the DT-830B may be a bit misleading—all the measurements were the same because of the limited resolution of the multimeter. Changing the least significant digit by one count would make a huge difference in the estimated impedance.
This is a simple enough lab that I think I’ll add it to the book—I don’t know whether I’ll add it to the class, because we’re already running short on lab time.