Calculating output impedance

Sometimes it’s useful to know how to find the output impedance of unknown devices such as that of my cheap Chinese signal generator (MingHe MHS-5200A) and today I saw a youtube video from W2AEW which explains how it’s done.

What we need to do is take two measurements of the output voltage. In my case I set my signal generator to a 2v sine wave at 5kHz. We want a low frequency sine wave so as the coax doesn’t start to affect our measurements with reflections and such. Likewise we want a sine wave because we don’t want the high frequency harmonic content which would be generated by a square or triangle wave which would cause the same issues.

First we need to measure the open circuit voltage. I tweaked my Oscilloscope so that at 0.5v per devision this was exactly four devisions and so 2v peak to peak.

Next we need to measure the voltage with a known load across it. Using a BNC T-piece and a test lead with crocodile clips on the end, the signal generator on one side and the oscilloscope on the other I first clipped a 1k OHM resistor across the wires but while this had an effect on the measured voltage it was too small to measure with my scope so I decided to go for a much lower value of 22 Ohms given that I expected the output to be around 50 Ohms. This had a much greater effect and I was able to alter the scale on the scope to 0.1v per devision and the result was about 0.61v.

We then take those measurements and stick them into the following formula:

Rout = Rload ( Vopen / Vload -1 )

So 22 ( 2 / 0.61 -1) = 50.131 Close enough to 50 Ohms for me to believe that.

Have a look at the original Youtube video. I’ve learned a lot from this guy and highly recommend you subscribe to his channel.

Ben Shephard avatar
About Ben Shephard
A radio amatuer based in Nottinghamshire, UK. IO93ic