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Figuring out the problem with frequency counter readouts:
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RF amplifier box:
The frequency counter was setup to read the beat frequency after amplification of the RFPD signals. The output as seen on the frequency counter screen as well as the epics values was intermittent. Also, locking the arms and changing the end laser frequencies did not change the frequency counter outputs. It looks like the RF amplifier is oscillating (Oscillating frequencies were ~107 MHz and 218 MHz) and the input circuit needs to be modified and the connections need better insulation.
As of now, I have connected the RFPD outputs to the frequency counter sans any amplification and we are able to get a readout of the beat frequency at > 40MHz (Frequency counter requires much higher amplitude signals for frequencies lower than that).
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The frequency counter has 4 ranges of operation:
Range 1 : 1 - 40MHz
Range 2: 40 - 190MHz
Range 3: 190 - 1400MHz
Range 4: 1400 - 6000MHz
I set up the beat frequency readout in various configurations to troubleshoot and have recorded my observation (attachments). The amplifiers in all cases is just a single ZFL-500LN.
There seems to be a problem with the RF amplifier and the frequency counter when they are set together. I am not able to figure it out and stuck right here 
Attachment 1: schematic for readout and corresponding observations
Attachment 2: Oscilloscope screen snapshot for schematic 3
Attachment 3: Spectrum analyzer screen snapshot for schematic 3
More info: RFPD used is Thorlabs FPD310 and frequency counter is Mini Circuits UFC-6000. The RF amplifier has decoupling capacitors soldered across the power supply.
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