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Entry  Thu Oct 21 05:51:10 2010, kiwamu, Update, Locking, no green beat note 
    Reply  Thu Oct 21 15:59:28 2010, kiwamu, Update, Locking, no green beat note : details bw.png
Message ID: 3754     Entry time: Thu Oct 21 15:59:28 2010     In reply to: 3750
Author: kiwamu 
Type: Update 
Category: Locking 
Subject: no green beat note : details 

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..


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