[Argus25]

There are a couple of other methods that are equally clever.

As noted by H&H in their textbook, one way to overcome the non-linearity of a diode is to drive it with a current rather than a voltage source. In this instance, the signal of interest is converted to a current by a simple transistor circuit. This drives the diode rectifiers. What happens is the actual diode drive voltage rises to whatever value is required to program the correct diode current proportional to the input voltage through the diodes. I have posted this before on another thread, but I think the whole notion was under appreciated, because the circuit just looked like some sort of amplifier driving the diodes, but it was much more sophisticated than that. Their circuit can be improved a tad by adding a Fet at its input, to reduce any loading of the circuit under test.

Also, if you are dealing with very low level RF signals, as often is the case in Radio Astronomy, there is another very interesting trick. If a Tunnel diode is used in reverse (called a back diode) it acts like a diode with zero forward voltage drop, but it only works for low level signals because its effective PIV rating is quite low.