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Post subject: Receiver sensitivity with temperature
Posted: Fri Nov 16, 2007 6:38 am
Joined: Fri Nov 16, 2007
We have a debate in my company
on how to calculate the receiver sensitivity with
Sensi = kTB + NF + C/N
is demodulator requirement, and does not vary with
temperature for a digital implementation
is the noise figure, and is referenced to 290K according
IEEE standard (Friis proposed this temperature in
1944). NF varies with temperature of course.
My point is that, when you compute the sensitivity
at different temperature, the "T" of kTB should
remain equal to 290K, otherwise you account twice
for the thermal noise change, once in the NF, and
once in kTB.
What is your opinion on that?
Posted: Fri Nov 16, 2007 2:42 pm
Joined: Tue Jun 26, 2007
Location: Dallas, TX
Here is my take on the issue
Looking at the equation
we must first note that the bandwidth and
the required C/N at the input of the demodulator
usually do not change with temperature. Therefore,
the only two terms which are in question are the
NF and Pin_mds(or sensitivity). The NF is defined
as the SNR(dB) at the input of the system minus
the SNR(dB) at the output of the system. The NF
may also be derived from a total integrated input
referred voltage noise relative to the voltage noise
produced by the source resistance (typically 50
Ohm). In the latter case, the integrated input referred
noise does change with temperature, but the reference
voltage noise is still calculated using the 290K
number (~0.895nV/sqrt(Hz) for 50 Ohm @290K). Therefore,
the NF does increase with temperature.
term kT may be assumed only if the input impedance
of the system is matched to the source resistance.
This term should also be treated as a constant much
like the noise of the source resistance. Then, the
input referred voltage noise of the system will
account for the noise floor variation inherently
because of the kT noise associated with it.
In short, you are correct in your statement.