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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jan 2 17:34:18 EST 2018

```Attila,

Since we talk background noise and white noise, the amplitude is the
same for AM and PM. This is part of the AM/PM lecture of NIST that I
know you have participated in at least once.

Under the assumption of low modulation index, which is fair assumption
for background noise compared to most carriers, both the AM and PM noise
of a certain side-offset has two side-band peaks, a lower and an upper.
The big difference is that for AM they have the same polarity and for PM
they have opposite polarity. Thus, they are as orthogonal as common mode
and differential mode. Noise on both absolute frequencies will combine
and contribute to the same AM and PM levels. It's that simple.

So, for that scenario you know one you know the other.

Now, for actual sources this is no longer true. The AM noise can be much
higher, which is why it can be a real danger to the PM noise if there is
a AM to PM noise conversion. One source of such conversion can be the
amplification stage, but another could be a mistuned filter, which have
different amplitudes of the side-bands, which can create conversion as
the balance does not balance the same way anymore.

Also turns out that the nitty-gritty of cross-correlation spectrum
analysis also occurs in AM-to-PM conversion and cancellation. This I had
not paid full attention to, but got reminded off at the workshop. Thus,
they are tied together and should be measured and understood together.

Cheers,
Magnus

On 01/02/2018 08:55 PM, Attila Kinali 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
>
```