Last night I tried searching for a beat note signal with two different PD trans impedance gains.
Although I didn't find a beat note signal.
- (1. trans impedance gain = 2400)
I started with a trans impedance resistance of R=2.4k, which is 10 times bigger resistance than the original.
The total PD gain should be about 960 [V/W] theoretically if we assume the responsibility of the PD is 0.4 [A/W].
Then I checked the bandwidth of the RFPD using Jenne laser.
The bandwidth was about 30MHz, which is 3 times narrower than the original. And it agrees with our expectation.
As Koji and I mentioned at the last weekly meeting, the cut off frequency of an RFPD follows inverse square root of the trans impedance resistance R.
where C is a capacitance of the photo diode. (See this)
I was expecting the signal level of -50 dBm / rtHz with a 23dB RF amplifier, assuming the line width of the signal is 10kHz.
- (2. trans impedance gain = 240)
I also tried it with the original trans impedance gain (see this entry).
R = 240 [Ohm]
G = 96 [V/W]
BW = 100 [MHz] (I didn't measure it in this time)
expected signal level = -70 dBm/rtHz
Still I didn't see any beat note signals..