The article Is a receiver test with different resistors connected it its input terminals meaningful? ended with the question Can we find the noise power contributed by a 50Ω antenna using a receiver of known sensitivity?
The technique depends on the fact that the audio output power of a traditional SSB communications receiver is linearly related to the RF input power (including the equivalent internal noise power) up to the onset of AGC action, which is typically more than 20dB above the equivalent input noise power. Be warned that disabling the AGC does not usually provide a significant extension to the linear range, some stage will overload but you have no warning you have reached that threshold.
For this article, I will use Equivalent Noise Temperature as the sensitivity metric, the reason will become obvious. You can convert various receiver ‘sensitivity’ metrics using the handy Receiver sensitivity metric converter.
Example
Let’s say that a certain receiver has sensitivity specified as 50Ω input, 0.1µV for 10dB S/N. The specifications probably do not give the Equivalent Noise Bandwidth, let us assume it is 2000Hz for this example (appropriate to an SSB telephony receiver but the actual bandwidth is critically important, it can be measured).
Let’s convert that specification to Tr.
Above, Tr is 434K. Continue reading Can we find the noise power contributed by a 50Ω antenna using a receiver of known sensitivity?