[amsat-bb] Re: NIST Phase Noise Measurements Group Noise Figure kTBcorrection
i8cvs
domenico.i8cvs at tin.it
Thu Jan 17 12:49:23 PST 2013
----- Original Message -----
From: "Joe Leikhim" <rhyolite at nettally.com>
To: "amsat-bb at AMSAT.Org" <amsat-bb at amsat.org>
Sent: Thursday, January 17, 2013 5:52 PM
Subject: [amsat-bb] NIST Phase Noise Measurements Group Noise Figure
kTBcorrection
> This came to my attention yesterday.
>
> NIST presents interesting 3 dB correction to historic value assumed
> for kTB to -177 dBm/Hz . I always assumed -174 dBm/Hz based on past
> publications.
>
> http://tf.nist.gov/phase/noisemeas.html
>
> I wonder how this affects metrology and calibration of noise figure
meters?
> --
> Joe Leikhim
> Leikhim and Associates
> Communications Consultants
> Oviedo, Florida
> JLeikhim at Leikhim.com
> 407-982-0446
> WWW.LEIKHIM.COM
>
Hi Joe,
The KTB = -174 dBm/Hz noise floor is based on
K= Boltzmann constant = 1.38 x 10^-23 joule / kelvin
T = 290 degrees kelvin as internationally stated as room
temperature by Friis
and so KTB = 1.38 x 10^-23 x 290 x 1 = 4.002^ -21 watt/Hz
10 log ( 4.002^ -21) = -203.977 dBW / Hz
10
-203.977 + 30 = -173.977 dBm/Hz (or approx -174 dBm/Hz)
Actually all Noise Sources heads of the instruments to calibrate
the value of Noise Figure NF in dB are based on Noise Excess
referred to room temperature of 290 degrees kelvin or +17°C
It comes that if KTB is changed from -174 dBm/Hz to -177 dBm/Hz
than the standard room temperature of 290 kelvin (+17°C) must be
changed to 140 kelvin ( - 133 °C ) wich is a not practical temperature
to be managed in any measurement laboratory.
and infact:
KTB = 1.38 x 10^-23 x 140 x 1 = 1.932^ -21 watt/Hz
10 log ( 1.932^ -21) = -207.139 dBW / Hz
10
-207.139 +30 = -177.139 dBm /Hz (or approx -177 dBm/Hz )
At this point all Noise Sources heads of the instruments to
calibrate the value of Noise Figure NF in dB and Noise Excess
must be changed from 290 to 140 degrees kelvin and this will
strongly AFFECT METROLOGY AND CALIBRATION of
all noise figure meters.
If you actually have a preamplifier with a Noise Figure of 3 dB
than it's Noise Factor F = 10^ (NF/10 ) = 10^(3/10) = 1.99
If you actually want to know how many kelvin correspond a Noise
Temperature T for a Noise Factor = 1.99, than
T = (1.995 -1) x 290 = 288.62 kelvin
On the other side referring T to 140 kelvin instead of 290 kelvin
than a Noise Figure of 3 dB will correspond to a Noise Temperature
of T = (1.995 -1) x 140 = 139.33 kelvin.
Best 73" de
i8CVS Domenico
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