Measuring output impedance of a function generator - Page 1
Forum section: electronics / projects

author: sentry7
author_rank: Regular Contributor
timestamp: February 11, 2017, 01:14:13 pm
content: Hey guys,I'm doing a project involving a function generator, and in order to do some analysis, I need to model it as an ideal VS with a series impedance. The specified impedance (as labeled on the unit) is 50 ohms, but when I did my own measurements, I get anywhere from 200-300 ohms.I measured the impedance by first measuring the open circuit voltage. Then, I put a load resistance across the terminals and measured the load voltage. Finally, I calculated output impedance: Rout=Rload(Vo/Vload-1)Just to verify the results, I also divided the open circuit voltage by the short circuit current and got close to the same results.Is there an error in my methods, or is there something amiss in the function generator?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2017, 01:40:21 pm
content: The calculation looks correct. One point one might miss is, that output impedance can depend on frequency. If there is AC coupling at the output the output impedance might be higher at low frequencies. So this would be a kind of weak point on the generator but still not that bad.Some of the very cheap generators might have an extra current limit, but they should be still capable of driving a matching 50 Ohms load.

author: slurry
author_rank: Regular Contributor
timestamp: February 11, 2017, 02:23:50 pm
content: W2AEW, Alan, made a really good video about the subject a while ago,here it is:

author: German_EE
author_rank: Super Contributor
timestamp: February 12, 2017, 10:01:58 am
content: Connect the function generator to an oscilloscope via a BNC T-PieceTurn up the output power until the waveform fills the screenWire a resistor across the third connection on the T-Piece, the waveform on the scope screen will decrease in height. When you find a resistor that gives a 50% decrease in output volts then that's your output impedance.Note: The scope will have an impedance of 1M ohm, you may have to factor this into your calculations

author: sentry7
author_rank: Regular Contributor
timestamp: February 13, 2017, 03:19:58 pm
content: Ok guys I got some interesting results.I once again measured the output impedance but this time, I used two different loads and used the difference in load voltage to find an internal impedance. The test signal was a sine wave at about 3kHz.Under a 1k load, the load voltage was 4Vpp. With a 4.7k load, the voltage was 4.68Vpp. Using the equation, I got about 230 ohms output impedance.However, when I tried to build this RC network and measure the peak-to-peak voltage across the capacitor, I didn't measure what I expected to. There is more than 10% difference in the theoretical and measured.Here is my work.Could it be that the internal impedance is a function of the load applied?

author: akos_nemeth
author_rank: Contributor
timestamp: February 13, 2017, 05:55:21 pm
content: Quote from: sentry7 on February 13, 2017, 03:19:58 pmHi sentry7,I think your calculations are correct, what is unknown yet is the output capacitance of the signal generator.I have created two simulations with hit and miss to model the "hidden" capacitance:Output impedance:http://everycircuit.com/circuit/5022037274001408Low pass filter:http://everycircuit.com/circuit/6238414425030656So roughly if there is about 500nF capacitance and 150resistance at the output of the signal generator, your measurements are plausible.Maybe you could:-calculate the output capacitance using the current measurement data?-measure the output capacitance with a capacitive divider?Regards,Ákos

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 13, 2017, 06:05:32 pm
content: At 3 kHz the output capacitance shouldn't be a factor, but you don't give any info about what the generator is, so it well could be if the thing were AC coupled. If you're going for accuracy, you can't round things like the reactance. Use the digits you've got. Did you measure the 4.7 uF capacitor accurately? If it's a film it will be stable at whatever value you measure, but if it's an electrolytic all bets are off. You'd have to account for dissipation factor as esr at 3 kHz. Seems like a simple thing, but still some hidden pitfalls. BTW, what signal generator ever made has a 230 ohm output impedance? They're usually 50 ohms or 600 ohms.

author: sentry7
author_rank: Regular Contributor
timestamp: February 13, 2017, 06:37:19 pm
content: Hmm, didn't consider the output capacitance. Unless I missing something, I couldn't find a output capacitance in the user's manual [http://www.gwinstek.com/en-global/products/Signal_Sources/Analog_Function_Generators/GFG-8200A]The FG is a GW Instek GFG-8216a.This particular cap was indeed an electrolytic, but I used a 100n ceramic cap earlier, and I don't remember getting any better results. I'll try again later today.

author: bktemp
author_rank: Super Contributor
timestamp: February 13, 2017, 06:48:02 pm
content: It says Impedance: 50ohm±10%.What happens if you connect a 50ohm resistor to the output? If the voltage drops by much more than 50% something is wrong with the function generator or your measurement setup.

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2017, 06:54:35 pm
content: Quote from: sentry7 on February 13, 2017, 03:19:58 pmThe original calculation was correct but capacitor calculation is incorrect because it didn't take account of the voltage on the capacitor is not in phase with the voltage on the resistor.Z = (XC2+ R2)0.5

author: sentry7
author_rank: Regular Contributor
timestamp: February 13, 2017, 08:02:22 pm
content: Quote from: Hero999 on February 13, 2017, 06:54:35 pmAC Circuits 101 anyone?The only question that remains is why the actual output impedance is almost 4x what's specified. I have three other identical FGs and they all give around the same impedance.

author: akos_nemeth
author_rank: Contributor
timestamp: February 13, 2017, 08:09:22 pm
content: Quote from: sentry7 on February 13, 2017, 08:02:22 pmHi,Do you use the same cable for the different generators? Maybe there is a crack in the cable...Regards,Ákos