Hello John
> 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.
73
Chris
GW4DGU
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