Item 1 on our commissioning for SPI is to figure out why we are not getting the same results for heterodyne efficiency in CDS as we did with an oscilloscope chamber side.

To evaluate the heterodyne measurement made by the measurement and reference interferometers we use the efficiency measurement of Vpp/(2*V_mean).

Ths is calculated by low passing the demodulated signal from each ifo in CDS and then multiplying by 2, befre dividing the demodulated signal by this mean. Shown in this photo here in the spi medm screens.

Jeff's firwst suspect was the low pass filter we use to get the mean demod signal.

The attached picture shows the raw demod signal on the left and the mean signal on the right. The mean for both looks roughly correct.

Jeff has put in a 4th order butterworth at 10Hz, see LHO alog #90638.