[time-nuts] AM vs PM noise of signal sources

Bob kb8tq kb8tq at n1k.org
Tue Jan 2 15:23:18 EST 2018


Just as with PM noise, AM noise is a “that depends” sort of thing. The first 
dependency is on your test gear. If the AM noise was well below the PM 
noise, would you be able to measure it? Equally, if you are doing something 
like a diode detector - can it measure AM below XXX dbc? If the detector is
only good to -120 dbc/Hz, you aren’t going to “see” -130 ….

Noise on output state supply lines can generate AM. In some cases the stage
is a better AM modulator than PM modulator. Limiting (like with a logic gate) 
is not going to pass AM noise well.  That combination can make for some 
“interesting” noise profiles. 

One “old time” assumption for wide band noise is that there is no process that
will generate AM independent of PM. Thus they will always be in an equal power
relationship. The key phrase here is wide band. Think in terms of notching out the
carrier and looking at the result on a spectrum analyzer in this case. 

One of the classic arguments for an AGC or for certain types of limiters in
oscillators is AM to PM conversion. The concern here is “close to carrier” 
rather than far removed. There certainly are examples tossed around where
AM noise is significantly higher than PM noise at low offsets. 

So, no data that I can share. Maybe a few things in there will be of help.


> On Jan 2, 2018, at 2:55 PM, Attila Kinali <attila at kinali.ch> wrote:
> Hi,
> I am currently looking at noise calculations that deal with AM and PM noise.
> To check whether the calculations make sense, I am looking for some numbers
> of the white noise floor AM and PM noise levels of signal sources.
> Unfortunately, almost everyone only deals with PM noise and hardly
> anyone mentiones AM noise levels. The best I could find sofar is [1]
> which supports the notion that AM noise is so far below PM noise, that
> it is insignificant. Does someone else have more data and would be willing
> to share?
> 			Attila Kinali
> [1] http://www.wenzel.com/documents/amnoise.htm
> -- 
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