Whilst looking for test rigs to experiment with tuned circuit filters, I came across the following article (http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1991/12/page29/index.html). The author suggests a test setup including an interesting micro-wattmeter circuit that I've been studying.
I understand most of it, but the biased diode detector is still something of a puzzle. Is it correct to say that the biasing of this diode is critical to how this circuit operates?
It seems like the point is to place the diode in a region below the knee (square law area). Since the op amp meter driver is lifted off the ground by the same diode with the same biasing, zero output from the RF amps produces zero output on the meter (after adjusting the zero POT). Any output from the RF amp(s) will slide along on the diodes characteristic curve and is amplified (a little) by the op amp. In the square law region of the diode the output is proportional to the square of the input, so the meter is detecting power (not voltage).
Do I have that right? I'm intending to place this detector downstream of a step attenuator, but even at that I'm guessing it wouldn't take much to overload the diode detector. Has anyone built anything like this and how useful was it? Are there any potential pitfalls to this design?
EDIT: one other thing, I realize the schottky's in my design wouldn't work well (the biasing on the RHS of the meter is designed for a greater Vf for starters!). They won't be used in any final build - I'll be using proper low capacitance RF schottky's instead (1N5711).