WE currently use long cables to give us the dispersion that we want for the MFD. A cable gives a long delay - both the phase delay and the group delay.
But we only need the dispersion (group delay). We can get this by just using a very sharp low pass filter and having the corner be above the frequency that we have the beat signal.
For example, the MiniCircuits SLP-200+ has got a corner frequency of 200 MHz and a group delay of ~10 ns (like a 3 m vacuum delay). So we would have to use 10 of these to get the delay we now get. The passband attenuation is only 0.5 dB, so we would lose 5 dB. The cost is $35 ea. We have a few on the shelf.
OTOH, if we tune the beat frequency down to 30 MHz, we can use the SLP-30 which has a group delay of 30 ns around 30 MHz. That's like 9m at light speed. We could easily get a nice result by just using 4 or 5 SLP in series.
So why is Kiwamu using cables?? And how should we really choose the beat note frequency??