Giacomo, Matt, Lisa

In the last few days we tried to use the Laser Head Calibrator (4mW @ 980nm, D1201258, S/N S1300150) to check the RF chain of the IMC LSC RFPD.
The idea is to use this type of LHC for all the ISC RF sensors to make sure that the signal chain from the sensor to the ADC works properly.

Before using the LHC on the LSC RFPD, we used it on an 1811 to calibrate the RF input of the laser head. We got about 13 mW/V.

We then used the LHC on the IMC LSC RFPD (24.078 MHz) installed on IOT2. We drove its RF input at 24.078473 MHz (100 Hz off from the IMC modulation frequency) to produce a 100 Hz sine in the I and Q demodulated channels. However, the amplitude of the sine was much smaller than what we expected (something like a factor of 50), given the RF input calibration factor and the diode responsivity measured with the DC response.

Since this measurement was supposed to be straightforward, we took this defeat personally and decided to bring the LHC back to the electronics shop and do more testing.
After using it on another 1811, a spare LSC RFPD and one of the BBPDs, we finally realized what the problem was.

One end of the R1 50 Ohm resistor which couples the RF into the laser head was not soldered .. so the level of RF which we were sending in was correct only if the cable was bent in such a way to make the connection...soldering that end helped :-)

So, for a spare LSC RFPD for 24.078 MHz (S/N S1203398) tested in the lab we get:

V_DC = 4 [mW] x Resp [A/W] x 100 [V/A] = 180 mV ==> Resp [A/W] = 0.45 (this also takes into account the overlap factor with the diode)

Vout_RF/Vin_RF @ 24.078 = 8.5 dB => with V/A = 476 and the Resp A/W = 0.45, this is consistent with the 13 mW/V calibration of the LHC RF input.


 The message 

We can now test again the laser calibrator on the IMC LSC RFPD, and hopefully we will get something that makes sense.