50s Japanese am/fm tube dual speaker radio

[bob91343]

The internal modulation oscillator on most low cost signal generators doesn't generate a low distortion sinusoid; in fact most of them are terrible. That's why I suggest modulating the generator with an external audio oscillator.

When you check distortion with such a setup, you need to use enough signal to cause limiting; otherwise the AM will interfere with the FM and you only want FM.

IM and harmonic distortion are related. If the system is linear, both will be low. If it's nonlinear, both will increase. Which parameter you measure isn't usually important. You can measure harmonic distortion with a single source but need two sources to measure IM. You won't, in general, find a system where one reading is high and the other low. Further, in a low end receiver, you can't expect low distortion, although distortion will increase with modulation level. Most low end units work fine with small deviation but when you get full 75 kHz deviation they won't be able to handle it.

Of course, if you listen to rock n roll you won't be able to tell whether the distortion is from the radio or in the source. Classical and jazz are better sources, since the idea in both cases is to present the musical characteristics rather than the tonal characteristics.