Today I calibrated the QPD on the ref cavity. The armlength from AOM to the quad is 2.1 m.

The calibration for x and y channel on the QPD in radian unit

QPDX:  radian = [dV]* 10^-3 / (3.36 * Vo)

QPDY: radian = [dV] * 10^-3 / (3.689 * Vo)

where [dV] is reaout voltage in Volt, Vo is the sum voltage from 4 quadrants in Volt.

If the prefer units are in micro radian and milli volt

QPDX:  urad = [d_mV] / (3.36 * Vo)

QPDY: urad = [d_mV]/ (3.689 * Vo)

where [d_mV] is reaout voltage in milliVolt, Vo is the sum voltage from 4 quadrants in Volt.