[Argus25]

G8HQP Dave,

Yes I agree. Due to the fact the RC discharge profile is an exponential, as all the equations indicate it is impossible to demodulate a 100% modulated carrier without distortion.

When I was playing around with AM detectors some years ago I built one where the capacitor was charged and discharged from a split power supply with a current source so the discharge curve approached zero as a straight line (and could cross it). With that I was able to demodulate a close to 100% modulated wave without negative peak clipping of the high frequency modulation content.

Also for a lot of detectors they try to save on parts. With a 455KHz IF for example with the detector time constant short enough for a close to HiFi response, there are serrations from the 455KHz on the signal, and it really needs another RC filter after that. The tendency for most manufacturers was to just hammer that with a bigger detector RC time constant.It was a reasonably big problem with a 262kHz IF's in some early radios for audio.

I can't attach it right now but I built a small detector module to replace the diode detector in one of my Eddystone EC10 Comms radios, so it will work for very low signals without square law distortion, I could attach it when I get home. It converts the IF voltage into a current and uses that to drive a pair of balanced diodes, and it's transfer function is much better than a 1N60 diode for low level signals but it's still relatively resistant to overload. It just has one transistor and a transformer.

One beauty about diode detectors, despite the issues we have been talking about, they operate over a very wide dynamic range and are very resistant to overload. I have built a number of OP amp precision detectors but in this respect they are poor.

Another interesting detector, for very small signals is the Tunnel diode (used in reverse) and in this mode it's called a "Back Diode", They describe this in Howorwitz & Hill, I have made a few of these before.