Impedance measurement How-To

mirosol · Post by **mirosol** » 28 Aug 2012, 06:22

Good morning everyone.

I've been searching the web for simple step by step way to measure input and output impedance of my builds. Needless to say, i didn't find any documentation that i could understand and use straight away.

Gaussmarkov has written a nice article about this: http://gaussmarkov.net/wordpress/though ... impedance/

But. My experience and know-how is still quite limited regarding basic/advanced electronics. From the article above: "All we need is a steady AC source. Don’t have one? Guess what? You probably have the parts to build one." I couldn't even find any usable tutorials about this. All i could find were about inverters pushing 120-220 volts AC out of 12V DC. I assume 120V AC is not healthy voltage for measuring impedance of 9V circuit.

So. I would like to know (and maybe see detailed step by step "how to") how to measure input and output impedance of my circuits.

Thanks.
+m

lolbou · Post by **lolbou** » 28 Aug 2012, 08:02

mirosol wrote:"All we need is a steady AC source. Don’t have one? Guess what? You probably have the parts to build one."

See this: viewtopic.php?p=169310#p169310

I guess there are vero layouts online for this too...

deltafred · Post by **deltafred** » 28 Aug 2012, 08:33

mirosol wrote: All i could find were about inverters pushing 120-220 volts AC out of 12V DC. I assume 120V AC is not healthy voltage for measuring impedance of 9V circuit.

You are quite correct in thinking that 120VAC is way too high for testing circuits, maybe ok for all tube designs but even then it seems too high.

I am sure there are Apps for iPhones/Androids.etc for signal generators, or there are programs to use a PC audio card as a signal generator. Here is one example for a PC

http://www.natch.co.uk/downloads/SigJenny/SigJenny.html

I've never used it as I have a couple of signal generators on my bench but it was recommended on another forum I visit.

mirosol · Post by **mirosol** » 28 Aug 2012, 08:57

So if i understand correctly, something like Sabro's layout for ValveWizard's sine wave generator could be used to generate the "ac" signal?
(http://www.sabrotone.com/wp-content/upl ... erator.gif)

Then for in/out impedances is should just follow Gaussmarkov's guide, armed with a DMM?

deltafred · Post by **deltafred** » 28 Aug 2012, 09:08

mirosol wrote:So if i understand correctly, something like Sabro's layout for ValveWizard's sine wave generator could be used to generate the "ac" signal?
(http://www.sabrotone.com/wp-content/upl ... erator.gif)

Then for in/out impedances is should just follow Gaussmarkov's guide, armed with a DMM?

That looks ideal.

One thing to remember is that your DMM will have a frequency range where it will measure AC voltage with reasonable accuracy, outside that range it's accuracy will not be guaranteed so it may appear that the circuit under test is responding differently to what it actually is.

mirosol · Post by **mirosol** » 28 Aug 2012, 09:41

Heh. So when i post my measurements anywhere, i'll just put tilde in front of the resistance, like this: Input impedance: ~200Kohm
+m

deltafred · Post by **deltafred** » 28 Aug 2012, 10:57

mirosol wrote:Heh. So when i post my measurements anywhere, i'll just put tilde in front of the resistance, like this: Input impedance: ~200Kohm
+m

I was thinking more if you check the impedance at different frequencies.

But yes all measurements have an accuracy or tolerance limited by the test gear and methods used. Most people ignore the fact that there is a tolerance and just give the results (unless they have worked in a calibration lab). To be strictly correct you need to specify your test method and have you test gear calibrated and traceable back to an accepted standard reference. All this is way to expensive and complicated for a hobby so use what you have.

You can always say 200Kohm (measured with (Make and model number) DMM).

mirosol · Post by **mirosol** » 28 Aug 2012, 12:03

I still do think there should be "easier to follow" -type of How To about this.. Plus i think my fuzz won't let that sine pass through the circuit in tact. So i think something else is needed to measure the output impedance..

I don't mind writing one - if i ever understand the process and the tools needed well enough...

deltafred · Post by **deltafred** » 28 Aug 2012, 16:12

I haven't read the article you linked so cannot comment on how easy it is to understand.

For the input - The basic principle is to measure the voltage drop across a known impedance (a resistor) in series with the input (when driven with a low impedance output sig generator). This impedance will form a potential divider with the input impedance.

Possibly then easiest way is make the resistor variable and adjust until the input voltage (or the voltage across the resistor) is half the sig gen voltage. At this point the resistor will be the same value as the input impedance. Switch off, isolate the resistor and measure it to find the input impedance.

For the output - The way I would do it would be to measure the output voltage with no load, then using a variable resistor as a load reduce the value until the voltage across it was half the open circuit voltage. Switch off, isolate the resistor and measure. This assumes that the output stage can supply sufficient power. If it is a very low output impedance then this method may fall down as you are now into power amp territory.

(If anyone can see any flaws in this method please shout up because I have no interest whatsoever in measuring the input and output impedances of pedals.)

You are right that your fuzz will not pass sine waves very well, unless they are below the clipping threshold. You would need a scope to monitor the output so you could tell where that point was.

Or you could possibly remove the clipping diodes or bypass the clipping stage because you are only interested in the output stage.

Or just build up the output stage on a breadboard and drive that from your sig gen.