## Impedance Measurement

REW can make measurements of impedance by using both inputs of the soundcard. Impedance measurements of drive units can be used to calculate the Thiele-Small parameters. The general connection arrangement for impedance measurements is shown below:

The sense resistor, which must be non-inductive, is used to measure the current flowing into the load, which will be (Vleft - Vright)/Rsense. The voltage across the load is Vright, so the impedance is voltage/current = Rsense*Vright/(Vleft - Vright). Note that the accuracy of the result is only as good as the accuracy of the value entered for the sense resistor.

Good results can be obtained using a headphone output to drive the load, with a 100 ohm sense resistor. If a line output is used the sense resistor typically needs to be much larger as line outputs have high output impedance and limited drive capability, try 1 kOhm but note that the results will have much higher noise levels.

An alternative is to drive the load via a power amplifier, which can deliver the lowest noise levels and most accurate results, but great care must be taken as the levels a power amplifier can generate can easily damage soundcard inputs. If using a power amplifier the sense resistor can be much lower, 33 ohms or less, but the soundcard inputs should be connected via a resistive divider providing around 20dB of attenuation and ideally the inputs should also be protected by back-to-back zener diodes to clamp the input to less than 5V.

The soundcard input connected to the load must be the same one which has been chosen as the input in the REW soundcard settings. In the diagram above that is the right input, but if the left is being used simply swap left and right in the diagram. If the left and right connections are the wrong way around your impedance measurements will show curves that are shifted up by approximately the value of the sense resistor. It does not matter which headphone output is used as REW puts the test signal on both outputs.

### Making an Impedance Measurement

- Press the Measure button (or Ctrl+M) to bring up the Measurement panel and select the Impedance button
- Enter the exact value of the sense resistor. This must be measured accurately with a good quality, calibrated multimeter or impedance bridge, or a very high precision resistor (0.1%) should be used. Any error in the value of the sense resistor directly affects the measurement results.
- Click the expand button if necessary to show the measurement settings
- Set the Start Freq to the lowest frequency for which you wish to see the response and End Freq to the highest. If measuring a drive unit to determine its Thiele-Small parameters measure up to 20 kHz. The sweep will span the range from half the start frequency to twice the end frequency (with an overall limit of half the soundcard sample rate) to provide accurate measurement over the selected range.
- Level controls the rms signal level at which the sweep is generated, relative to digital full scale. The maximum value is -3 dBFS, unless the View preference Full scale sine rms is 0 dBFS has been selected, in which case the maximum is 0 dBFS. Using the maximum level places the peaks of the signal at digital full scale - some soundcards may distort at the maximum level. For impedance measurements it is best to use a high sweep level, e.g. -6 dBFS, but if you are using a power amplifier beware of excessive levels.
- Length controls the duration of the sweep signal, specifying the number of samples in the sweep sequence. The default is 256k. Dividing the number of samples by the soundcard's sample rate gives the sweep duration in seconds. The overall duration includes silent periods before and after the sweep.
- If Sweeps is more than 1 REW uses synchronous pre-averaging, capturing the selected number of sweeps per measurement and averaging the results to reduce the effects of noise and interference. The pre-averaging improves S/N by almost 3dB for each doubling of the number of sweeps. Averaging is particularly useful if the measurements are contaminated by interference tones, whether electrical or acoustic, as they typically will not add coherently in the averaging and hence will be suppressed by the process.
- Total Time shows the overall duration of the sequence of sweeps
- The Check Levels button generates a few seconds of a pink noise signal that spans the frequency range selected for measurement and checks that the input level is not too high or too low. Pressing Cancel while the pink noise signal is playing will turn it off (it turns off automatically after 3 seconds). The rms level that was measured is shown on the measurement panel, with a warning if the level is too high or too low.
- Press Start Measuring to make a measurement, progress is shown on the measurement panel along with a display of the measurement headroom.

The result of the measurement is displayed in the graph area, information about the measurement appears in the Measurements Panel. Measurements are given a default name of the date and time at which they are made, a more appropriate name can be entered in the box at the top of the measurements panel.

When the mouse cursor is within the graph panel of the Impedance & Phase graph the equivalent series resistance + inductance or resistance + capacitance and parallel resistance||inductance or resistance||capacitance of the impedance at the cursor position is shown at the bottom left corner of the graph, this is useful when making measurements of inductors or capacitors to check their value. For capacitor measurements the values are most accurate at frequencies where the total impedance has dropped below a few hundred ohms.

For details of the various ways of viewing the measured data, including averaging multiple measurements, refer to the Graph Panel help.

### Calibrating the Impedance Rig

Small gain differences between the soundcard input channels cause incorrect calculation of the load current and the impedance. These can be calibrated out by making a measurement with the load disconnected and the sense resistor shorted out. N.B. both soundcard inputs must be connected to the same output signal.

- Press the Measure button (or Ctrl+M) to bring up the Measurement panel, select the Impedance button and set the sense resistor value to zero
- Press Start Measuring to make a measurement. The completed measurement shows the level of the measurement channel (usually right) compared to the reference channel, where a reading of 100 Ohms corresponds to 100%, 99 Ohms would be 99% etc. If the difference between the 2 channels is too large (more than a couple of percent) the calibration is abandoned as it is likely there is a connection error, re-check the connections and try again.

The calibration factor used for impedance measurements is shown in the measurement info panel next to the measurement thumbnail, along with the sense resistor value.

### Resistance of Test Leads

When measuring very low impedances the resistance of the test leads may become significant. To measure this, first calibrate the rig as above, then make a measurement with the leads shorted together at the point the load is attached (set the RLEADS value to zero before making the measurement). The measurement should be fairly uniform, perhaps showing variation at very low frequencies depending on the low frequency limitations of the headphone drive stage. If the result is more than one or two tenths of an ohm, check that the connections are tight and the leads are not too flimsy. Enter the resistance from the flat part of the measurement into the RLEADS box on the measurement panel.

### Input Channels Swapped

If the input channels have been connected the wrong way around the impedance measurements will be too high by approximately the value of the sense resistor, make a test measurement of a resistor (of less than 100 ohms) to check that everything is wired correctly.

### Impedance Measurement Quality

The main source of measurement noise is acoustic noise and vibration during the measurement. Loudspeakers act as microphones, generating small voltages in response to sounds and vibrations that are picked up as part of the load voltage. To minimise this effect use high drive levels, low sense resistor values, avoid noisy environments and isolate the loudspeaker from vibration. Using a power amplifier to drive the speaker provides high signal levels and allows a low series resistor to be used.

Another source of error is the input impedance of the soundcard, which is in parallel with the load. This limits the accuracy of the measurement of high load impedances, the method is most suitable for impedances below a few hundred ohms.

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