Hi Dick,

Darrel gave me a copy of your draft on the full-wave/half-wave question. The problem I see is that the one-way square law device, taken as the diode model, is much more complicated than the exponential law device, which is supposedly a very accurate description of a semiconductor diode:

I(V) = Is(T)[exp(eV/kT)-1].

For small V, the current has a linear component, a much smaller quadratic component, and still smaller V^3 , V^4 ..... components. The diode is run at voltages small enough that the 4th order term is negligible. But the linear term is still there, larger than the desired quadratic term. As the voltage makes its excursions, both positive and negative, the linear and quadratic components of the current do their thing, neither switching off when the voltage is negative. If you make a full wave circuit, the quadratic terms are equal (redundant). But the linear terms are opposite and cancel, so the full wave circuit would be suitable for square law detection of a base band signal - or any signal whose spectrum extends down into the post detection bandwidth. Of course the diodes would have to be carefully matched and maintained at the same temperature. In practice it may be simpler to use an i.c. Gilbert cell multiplier as a true squarer. That's what we use to measure the power of the baseband signals going into our correlators.

The single diode circuit as a base band power detector is noisy because of the linear feed through. As a detector of IF power it's fine (can't be beat!) because the feed through is up at IF frequencies and is totally eliminated by the post detection filter.

Too bad you didn't come for the Single Dish Workshop.