Christopher Bartram <cbartram@...>
Are you sure that amplifier noise is AM noise? I always thought that amplified thermal noise, which this is, was an equal mix of both AM and PM noise.I suspect that this is a situation where on one level we're both right! Of course, thermal noise will have both varying phase and amplitude components, and at a quantum level, I believe thermal noise has a low-pass characteristic. (Physicists please comment - I'm just a horny-handed engineer... ) But when I say thermal noise or AM noise, I refer to noise defined by kTB, which has no practical frequency dependence. Phase noise, though, is generated by the action of an oscillator, and can be thought of effectively as angle modulation of that oscillator by a noise source.
It's possible to measure the amplitude and phase noise generated by an oscillator independently - amplitude noise is surprisingly easy to measure. Phase noise, though, is primarily dependent on the oscillator's amplitude (kTB) noise and the Q of its resonator, and it's measurement requires a bit of subtlety. I could give you a reference to Leeson's classic paper on oscillator analysis and phase noise if you are interested.
The term 'phase noise' tends to bandied about without much thought - that's true of both amateurs and many professionals - and the real meaning of the term has become muddied in many people's heads.