These are not easy things to design.
There was a book published by Artech house called Logarithmic Detection or something similar. I don't know if it is still in print and I don't have a copy, I just read a very small part of it many years ago- not the DLVA part.
If I remember correctly you need a tunnel diode for the detector or a zero biased schottkey. The video amplifier was I tink just a log converter function., similar to an opamp with a diode in the feedback.
Not much to go on, but a start I hope.