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