Richard told (or reminded) us this morning that the resonant circuit of the RFPD for locking the IMC had been adjusted for 21 MHz rather than the actual modulation frequency of 24 MHz. So I went ahead to the IOT2L optical table and measured the RFPD response using the AM diode laser. As shown in the attached figure, the RFPD is indeed configured for 21 MHz. Because the resonance at around 21 MHz is so broad, 24 MHz is well within the resonance with a slightly bigger response than 21 MHz by a few dB. The notch seems to be adjusted to 42 MHz. The ratio of the response at 48 MHz over 24 MHz was found to be about -20dB.

The raw data is also attached.