AW: Re: Couple EE questions...

Magnus Danielson magda at it.kth.se
Wed Oct 23 14:35:24 CEST 1996


> 
> > Well, you actually balance two sources of noise with each other, one noise 
> 
> > source is the op-amp input itself and the other is the resistor noise. The 
> 
> > trick
> > is to select the resistor value which results in the lowest noise figure.
> 
> Well, I am not too firm in english technical terminology.
> The context suggests that "noise figure" means the same as the German
> "Rauschzahl", that is "how much worse is the real system compared to
> an ideal system, where only the impedance is a noise source".
> This would be near 0dB in a good system.
> Or is it just a different word for SNR (which would be some -70 ... -100dB
> in a good system) ?
> Just to be sure of terminology before I can go on.

Juergen! Now look what you have done... you have forced me to actually study
the subject by reading text books....

Naw, I don't take it that serious... RTFM migth be a good thing from time to
time :)

The term "Noise Figure" is a dB scale value saying how much _more_ noise the
system has than the "perfect" system. The "perfect" system consist of only
Johnson noise which is the noise which comes from pure temperature on a 
resistor.

Vnoise(RMS) = VnR = (4kTRB)^(1/2)

Where
	k is Boltzmann's constant (1.380658(12) * 10^-23)
	T is temperature in Kelvin (273.15 + degreeds Celsius)
	R is Resistance in ohms (I hope you know what that is!)
	B is bandwidth in hertz

A small detail, it's the SNR and not the NF that should be optimised...
You can optimize the NF but destroy the SNR.

Magnus




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