Re your circuit for checking modulation of amateur radio transmitters on a `scope. Someone who saw my effort at recreating this box of tricks has told me not to use it to monitor transmission on air, as diodes can cause frequency multiplication and cause me to transmit spurious harmonics. I am not sure if germanium diodes can cause this, what's the score with using it to monitor real time transmission on air please Alan? http://www.chriswilson.tv/RF/rf.html is my attempt. Forgive the rusty nuts, I built it at home at the weekend, with no access to my my normal boxes of hardware odds and sods at work (I am in the race car game). Cheers
Yes, he is correct. Any diode, germanium or not, is a non-linear device, and will generate harmonic distortion products as well as intermod products. While these diodes are not "in" the RF path, they are tapped onto it. Thus, there will be a small amount of harmonic components injected back into the RF line - both through the coupling cap as well as from simple stray pickup through the air. The latter can be reduced by shielding the detector circuit from the main RF thru-path. Lowering the load impedance seen by the detector will also help. In my tests, these distortion products were 45-50dB down or more from the main signal. So, technically, he is correct, and you should be careful regarding this. You can check it yourself by listening for your third harmonic (3x your operating frequency), which will typically be the largest product generated. Maybe have a buddy listen to your main signal, then listen for you at 3x that frequency. It is likely that you will not be heard at 3x. However, it is definitely worth checking out to be sure that you are being compliant with spurious emission levels.
The simple resistive divider used to monitor the RF envelope (the most useful thing anyway) will not introduce any distortion effects at all.
Note also that the demodulator circuit is typically used for linearity measurements of an amplifier. Thus, the demodulator would be connected to the RF before the amp, and the resistive monitor connected after the amp. The only reason that I have both of these circuits connected to the same RF line was to demonstrate the function of each circuit - they'd never be used simultaneously in this way.