[time-nuts] Thermal noise contribution to phase noise

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jan 16 04:41:28 UTC 2013


Bob Camp wrote:
> Hi
>
> The noise spectrum is composed equally of AM and PM noise. Together they add
> to -174.
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Graham / KE9H
> Sent: Tuesday, January 15, 2013 3:46 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Thermal noise contribution to phase noise
>
> Bruce:
>
> The last time I looked, the thermal noise floor was still -174 dBm/Hz
> (at 300 Kelvin).
>
> http://en.wikipedia.org/wiki/Thermal_noise
>
> Are you saying Boltzmann's constant is off by 3 dB, or are we mixing
> apples and oranges here?
>
> Is there a 3 dB adjustment between noise floor (at room temperature) and
> the "single side band" phase noise measurement, which only looks at half
> the noise, since it only looks on one side of the reference signal?
>
>    
The upper and lower sidebansd are coherent so their combined power level 
is 6dB higher when translated to baseband as is done in a traditional 
analog diode mixer based phase noise measurement system.

Bruce
> --- Graham / KE9H
>
> ==
>
> On 1/15/2013 1:38 PM, Bruce Griffiths wrote:
>    
>> I've noticed a disturbing tendency to quote the thermal noise
>> contribution to phase noise as -174dBm/Hz instead of the corrent value
>> of -177dBm/Hz as verified by measurement by NIST:
>> http://tf.nist.gov/phase/noisemeas.html
>>
>> This error occurs in papers from Spectrum Microwave, Wenzel Associates
>> and others.
>> Blindly propagating the results quoted in the early literature isnt
>> particularly helpful given that the definition of SSB phase noise has
>> changed in the intervening decades.
>>
>> Bruce
>>
>>      
>
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