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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 6 17:28:27 EST 2018

Hi Joe,

On 01/06/2018 10:26 PM, Joseph Gwinn wrote:
> On Fri, 05 Jan 2018 21:54:58 -0500, time-nuts-request at febo.com wrote:
>> May I just follow up on the assumption there. The Bessel series is the
>> theoretical [basis] for what goes on in PM and also helps to explain one
>> particular error I have seen. For one oscillator with particularly bad
>> noise, a commercial instruments gave positive PM numbers. Rather than
>> measuring the power of the signal, it measured the power of the carrier.
>> Under the assumption of low index modulation the Bessel for the carrier
>> is very close to 1, so it is fairly safe assumption. However, for higher
>> index the carrier suppresses, and that matches that the Bessel becomes
>> lower. That's what happened, so a read-out of the carrier is no longer
>> representing the power of the signal.
>> However, if you do have low index modulation, you can assume the center
>> carrier to be as close to full power as you want, and the two
>> side-carriers has a very simple linear approximation.
> Yes.  This is exactly right.  There is a modulation index for which the 
> carrier is totally suppressed.  

Yes, the first of a series of zeros is at 2,4048.

> That must have been a very bad oscillator.

It was. None of mine, and not my measurement, I just helped triage it.

> You mentioned elsewhere that we now have to consider AM, not just PM.  
> This has been my experience as well, especially with power supply noise 
> fed to a final RF power amplifier, especially if that final amplifier 
> (or its driver) is not fully saturated.

Indeed, and the risk of AM-to-PM conversion makes huge difference in AM
and PM levels allow circuits to ruin the PM properties. If you consider
it as an act of isolation and the need for balance to ensure the
isolation, it becomes easier to understand conceptually.


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