The question arises from time to time, is a receiver test with different resistors connected it its input terminals meaningful?
Scope
This discussion applies to linear receivers. A receiver using a diode AM detector, with or without BFO injection is NOT a linear receiver for this purpose, nor is an FM receiver. A good traditional superheterodyne SSB Communications Receiver is a linear receiver for the purpose of this discussion, but for example any techniques designed to reduce / cancel noise will render it non-linear
Noise in resistors
Thermal agitation within a resistor gives rise to broadband noise (Johnson-Nyquist noise, thermal noise), the noise power that can be captured from a resistor in a given bandwidth is given by \(P=k_0 T B\) where:
- k0 is Boltzman’s constant;
- T is the absolute temperature; and
- B is the bandwidth.
Receiver sensitivity
If:
- a receiver is designed for a 50+j0Ω source; and
- has noise figure specifications or specifications that imply a noise figure; and
- and is tested to specification
the measurement is done with a 50+j0Ω source that contributes thermal noise (50Ω @ 290K) and the equivalent internal noise contribution of the receiver can be calculated. Continue reading Is a receiver test with different resistors connected it its input terminals meaningful?