Since the Frequency counters have not been a reliable error signal for FOL PID loop, we will put together an analog delay line frequency dicriminator as an alternative method to obtain the beat frequency.
The configuration will be similar to what was done in elog 4254 in the first place.
For a delay line frequency dicriminator, the output at the mixer is proportional to where
L - cable length asymmetry, fb - beat frequency and v - velocity of light in the cable.
The linear output signal canbe obtained for
For our purpose in FOL, if we would like to measure beat frequency over a bandwidth of 200MHz, this would correspond to a cable length difference of 0.5 m (assuming the speed of light in the coaxial cable is ~ 2x108m/s.