[time-nuts] Thermal noise contribution to phase noise

[Bob Camp]

Hi

If you look at them on a spectrum analyzer you can't see the difference between AM noise and PM noise (provided the modulation index is small). If you use an AM detector, it will "see" one, but not the other, so there is a difference. 

The difference is fairly simple, with AM, the sidebands phase is such that they add to each other and the envelope voltage changes at the modulation frequency. WIth PM sidebands, the phase is such that they subtract from each other. The envelope voltage does not change with time. 

When you go to look for AM, you set up the sidebands one way. When you go to look for PM, you set them up the other way. That of course assumes you have an I/Q demodulator that can handle sidebands with independent phase relationships. The fancy new phase noise machines indeed do this.

If you are doing it the "old way" you look for PM with a phase detector (insensitive to AM) and you look for AM with an AM detector (insensitive to PM). A mixer *can* do this, but only if you have it set up just right. Zero volts out is phase sensitive, but amplitude insensitive. Peak voltage out is amplitude sensitive, but phase insensitive. 

So much fun.

Bob

On Jan 19, 2013, at 2:34 PM, Mike Feher <mfeher at eozinc.com> wrote:

> I would think that you would not be able to tell the difference between AM
> or PM noise. 73 - Mike
> 
> Mike B. Feher, EOZ Inc.
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960 office
> 908-902-3831 cell
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Bob Camp
> Sent: Saturday, January 19, 2013 1:07 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Thermal noise contribution to phase noise
> 
> Hi
> 
> If:
> 
> 1)  You have a carrier. Let's say it is an ideal signal with absolutely no
> noise on it.
> 2) That carrier is in a real 50 ohm system
> 
> ..there will be thermal noise at -174 dbm per Hz.
> 
> The model for a carrier includes both AM noise and PM noise. Correct?
> 
> The practical difference for small angle PM vs AM is the phasing of the
> sidebands. Correct?
> 
> Either:
> 
> a) All of the noise is AM noise. 
> b) All of the noise is PM noise.
> c) Half of the noise power is AM and half of the power is PM.
> 
> Pick one, a,b, or c.
> 
> If you pick a or b, *why* and equally importantly *how* does all the random
> thermal noise go only to one process? Since it's (by definition) random
> (amplitude and phase), it's going to be tough to get it all in just one
> bucket or the other. 
> 
> If the noise splits equally, then -177 is what you are going to get.
> 
> Bob
> 
> On Jan 18, 2013, at 11:52 PM, Joe Leikhim <jleikhim at leikhim.com> wrote:
> 
>> I have just sent off an e-mail to David Howe of NIST Metrology requesting
> clarification about this assertion that KTB is revised -3dB.
>> 
>> --
>> Joe Leikhim
>> 
>> 
>> Leikhim and Associates
>> 
>> Communications Consultants
>> 
>> Oviedo, Florida
>> 
>> JLeikhim at Leikhim.com
>> 
>> 407-982-0446
>> 
>> WWW.LEIKHIM.COM
>> 
>> 
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