REW Measurements with Motu M2
Impedance measurement with an amplifier
This amplifier has two cascaded built in series resistors which gives the output impedance of the amp. One is 4 Ohm the other is 29 Ohm connected to different output sockets. This allows matching the impedance of the DUT to the series resistance. The DUT impedance should not be much higher than the series resistor.
The accuracy of the impedance measurement depends directly to the precision of the reference resistor used in the calibration process. the built in series resistors must have low thermal coefficient in order to minimize the drift during the sweep caused by power load. Good cooling and high thermal capacity also helps minimizing this drift. Shorter sweep results in lower thermal drift.