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Entry  Wed Nov 5 01:58:54 2014, ericq, Update, LSC, 3F RFPD RF spectra Nov52014_3fPD_DRMIspectra.zipREFL165_DRMIspectrum.pngREFL33_DRMIspectrum.png
    Reply  Wed Nov 5 11:09:44 2014, Koji, Update, LSC, 3F RFPD RF spectra 
       Reply  Wed Nov 5 22:25:42 2014, ericq, Update, LSC, 3F RFPD RF spectra REFL165_demod_DRMIspectrum.png
          Reply  Thu Nov 6 01:58:55 2014, Koji, Update, LSC, 3F RFPD RF spectra 
             Reply  Thu Nov 6 11:49:58 2014, ericq, Update, LSC, 3F RFPD RF spectra 
                Reply  Thu Nov 6 14:41:49 2014, Koij, Update, LSC, 3F RFPD RF spectra 
                   Reply  Fri Nov 7 02:21:12 2014, ericq, Update, LSC, 3F RFPD RF spectra 
                      Reply  Fri Nov 7 14:41:18 2014, ericq, Update, LSC, 3F RFPD RF spectra 165_ampSaturation.png165_HPatPD.png
                         Reply  Fri Nov 7 16:15:53 2014, Jenne, Update, LSC, 3F RFPD RF spectra 
                            Reply  Sat Nov 8 11:35:05 2014, rana, Update, LSC, 3F RFPD RF spectra 
                            Reply  Mon Nov 10 18:11:57 2014, ericq, Update, LSC, 3F RFPD RF spectra 165signals.pdf
Message ID: 10685     Entry time: Fri Nov 7 14:41:18 2014     In reply to: 10683     Reply to this: 10686
Author: ericq 
Type: Update 
Category: LSC 
Subject: 3F RFPD RF spectra 

Quote:

 After some enlightening conversation with Koji, we figured that the RF amplifier in the REFL165 chain is probably being saturated.

The measurements I took yesterday bear this out. However, even putting the high-pass directly on the PD output doesn't reduce the signal enough to avoid saturating the amplifier.

We need to think of the right way to get the 165MHz signal at large enough, but undistorted, amplitude to the demod board. 


 The current signal chain looks like:

AS Table                                  LSC RACK
[ PD ]----------------------------------->[ AMP ]------>[ 100MHzHPF ]----->[ DEMOD ]
      (1)                                        (2)                 (3)

I previously made measurements at (3). Let's ignore that. 

Last night, I took measurements with a directional coupler at points (1) and (2), to see the signal levels before and after the amplifier. I divided the spectrum at (2) by the nominal gain of the amplifier, 23.5dB; thus if everything was linear, the spectra would be very similar. This is not the case, and it is evident why. There are multiple signals stronger than -20dBm, and the amplifier has a 1dB compression point of +3dBm, so any one of these lines at 4x, 6x and 10x fMod is enough to saturate. 

 165_ampSaturation.png


I also made a measurement at point 4 in the following arrangement, in an attempt to reduce the signal amplitude incident on the amplifier.  

AS Table                                           LSC RACK
[ PD ]->[ 100MHzHPF ]----------------------------------->[ AMP ]--------->[ DEMOD ] 
                                                                (4) 

 Though the signals below 100MHz are attenuated as expected, the signal at 110MHz is still too large for the amplifier. 

165_HPatPD.png


Minicircuits' SHP-150 only has 13dB suppression at 110MHz, which would not be enough either. SHP-175 has 31dB suppression at 110MHz and 0.82dB at 160MHz, maybe this is what we want.

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