REW/Impedance Rig

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I'm not sure if this is the best sub-forum, but I have been having trouble getting my DIY Impedance Measurement rig to work.

I have followed the circuit/design here:
Impedance Measurement

Using a Behringer UCA-202 and the Line Outputs (with a 1k Ohm Resistor) I get the calibration stage OK, Bypassing the resistor gives 100-Ohms as below.


Imp Rig Calibration by Robert Seymour, on Flickr

When I come to measure anything from a single 10-Ohm resistor or a speaker driver, it just comes up with a blank measurement for impedance and a solid waveform (180 Degree to -180 Degree) for the phase.


Imp Trace 1 by Robert Seymour, on Flickr

I have double checked all the wiring (and my other half too) and tested that which all seems fine.

I have read that the UCA-202 is not the best, and using the headphone output gives better results but I am getting confused why I am not getting anything at all...

Is there a setting I am missing in REW that would cause this?
 
I was able to make simple resistance and inductance measurements using the REW jig and an EMU0404 USB soundcard.

That soundcard has an amplified headphone output, and the 1/4" jack inputs are 1 Megohm impedance so don't load the DUT. That soundcard requires the headphone output pot, and Hi-Z input pots to be set for acceptable signal levels when directly interconnected.

The impedance calibration routine can be done as directed by setting 0 ohm sense and shorting the sense resistor with no load - the input pots can be tweaked to get both channels reading about the same 100% level. I could then get a fairly accurate 100 and 470 ohm resistance measurement when using a 100 ohm sense resistor.

I could also get power supply choke measurements (samples from 1H to 32H) where the REW calculated inductance was consistent with what I'd measured by other techniques. The plot window only goes up to 10kohm, so the plot doesn't adequately show the self-resonance (unfortunately) for the chokes I was looking at.

It does a very good measurement of a second grade standard 100uH Sullivan-Griffiths inductor, plotting the response out to 96kHz.

It also does a very good job with valve amplifier output transformer leakage inductance - I was able to see how half-primary leakage calculated inductance varied with frequency, and could determine a resonance wobble at 42kHz in a Williamson grade OT.

That soundcard made it pretty easy to make up a custom jig with screw-in terminals or alligator clips for parts - so it's a winner for the diy bench.
 
Last edited:
Just another Moderator
Joined 2003
Paid Member
It's basically the same as the circuit you linked to but I find Claudio Negro's diagram a bit more intuitive. Cables

I'd double check your connections. I've not had issues with REW. Do note that there is a thread that has been active lately about the beringer and problems at low frequency in the impedance measurements, might be worth searching for it.

Tony.
 

Attachments

  • claudio_cable.png
    claudio_cable.png
    11.8 KB · Views: 1,014
Last edited:
Hi Robert:

You obviously have a good handle on this. You're using an audio interface with a very weak output (low current capability) that has a 400Ω resistor on its output to protect it. Mfg's place "current limiting" resistors of this type on outputs to protect the device from the worst case - a short to ground. Better interfaces have much lower output impedances, but you can do well with what you have in this application.

Just use a 1kΩ "reference" resistor in your impedance measurement jig. The REW manual states that you'll see high noise, but that is not true with good quality audio interfaces. It's possible that 1kΩ will be too noisy with yours - you'll just have to try it. I used 1kΩ resistors for years with excellent driver impedance measurement results.

I've attached a PDF of my notes back in the day when I used Smaart to make impedance measurements. I even tried a 50kΩ reference resistor to measure audio component input impedances, but found that 1kΩ worked well enough for that at lower noise, so I changed the function of the switch to deal with measuring Pin 2 and Pin 3 to ground of balanced interfaces.

View attachment Smaart_ZBox.pdf
 
The DATS has a 100Ω current limiter resistor on its output and is absolutely excellent in my experience with usable accuracy up to 5kΩ DUT's. Excellent accuracy under 100Ω's where it counts for driver measurement. UI takes a little getting used to like anything else.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.