33622A - Used to Measure Capacitors - The Sine method

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I have already demonstrated how to use the Agilent 33622A to measure a capacitor or Inductor by leveraging the fast rise time square wave output of the AWG see here: 33622A used to measure Capacitors and Inductors - Curve method

In this blog I will repeat some of the test using a different approach. Feed a sine wave into the RC network and adjust the frequency until the voltages across the resistor and Capacitor are the same

at this point we know the Impedance of the Capacitor is the same as the known value resistor and now we have the frequency, and Impedance we can calculate for C

this is the test circuit

because the 33622 has floating outputs it is possible to connect the ground of the oscilloscope to the center of the resistor and capacitor, this then allows both channels of the oscilloscope to be set to the same range improving the chances of getting an accurate balance.

Not all AWGs have this ability but the Agilent 33622A does and on both channels.

Cautionary tale :

A small lets say "Feature" came into play during the testing and that was the fact that the 33622A has a permanent 50 Ohm output (Source) impedance. and if you change the load impedance in the menu it does not change the source impedance. this needs to be factored in if you do not have a floating output on the generator as the voltage supplied to the DUT will vary depending on the R and C, and if your going to test a large capacitor in the uF range then the resistor will be in the 10-100 Ohms thereby becoming a significant load on the generator.

the other factors that caused me initial problems was the test setup. I was using a bread board, two scope *10 probes and of course the parasitic capacitance of the leads from the generator. Another surprise and not always expected factor was the internal protection capacitor from the floating signal source to real ground. I performed a test without a test capacitor but everything else connected and to my surprise there was 72nF before I even began (WOW). in the following table I adjusted the calculated reading to factor this out and it helped considerably to bring the accuracy up a notch or two.

the other factor you need to consider is using an 8 bit device to measure the voltages (My Scope and many other low cost scopes are only 8 bit accurate at best, when it comes to measuring the amplitudes it can be worse). Unfortunately accuracy comes with a price and I was unable to pay the price when I bought my current scope. The source on the other hand is very accurate so it can be relied on for both the frequency and source voltages.

OK, the math

Zc = 1/(2*PI*f*C)

would give you the impedance of the capacitor at a specific frequency

re format to derive C we get

C = 1/(2*PI*f*R)

R is in here because we will be balancing the two channels of the oscilloscope by varying the frequency of the 33622A. Once balanced the Zc = R

I will not be providing a video for this one, you have seen plenty of that for the previous C and I measurements

here are the results from the curve methods for review (I have color coded the results to make it easy to compare)

now using the Sine method (The red marked value is that obtained without a test C even in circuit

So while this was not as accurate as the Curve method, it does provide a reasonable result. If the frequencies are within the range of your DMM (On AC) and you have two meters then you could improve the result accuracy. You need two in order to balance the effects of the measuring device on the circuit and trust me when I say, the test equipment does and will have an affect on the result.

A nice thing about the 33622A is that it is very easy to sweep the frequency through many decades of range with nothing more than the dial and a pair of buttons

Pleas provide comments and feedback, I will do my best to answer them