A few more observations. A couple of years back I became interested in noise, noise figure, etc. It all started with a noise problem [affecting a repeater of course] I did some research, conducted tests, scratched my head, etc.
I even acquired an Agilent [HP] noise figure meter & a couple of noise sources. Then one day I came home from a hamfest with an ARR UHF preamp. After seeing their ad in QST for many years - the one where they show their preamp hooked to an HP 8970 displaying the gain and a really low noise figure - well - I had to try it. I couldn't match their published noise figure. So I did a little internet research. As it so happens, some of our fellow amateurs [the weak signal folks] are really into this sort of thing. They have organizations [e.g. Central States VHF Society]. These organizations have annual pow-wows to get together and exchange ideas. One of the things they do is test preamps for gain & noise figure. These are usually member's projects but sometimes folks bring commercial units. Each year they publish their results. I went through the results of several organizations and for several years and looked at how the ARR amps tested. They averaged about the same as mine tested. I guess I'd say I'm not real impressed. BTW, the Central States 42nd Annual CSVHFS Conference is July 2427, 2008, at Wichita, KS. But back to the original subject - receiver noise figure - and the suggestion to measure it directly. Maybe not so easy? Let's start with a typical repeater receiver. We put it on the bench. It has power connections, an antenna connection, volume and squelch controls, speaker [and usually also discriminator] audio outputs. Power it up and open the squelch. Hmmm, noise. Hook a 50 ohm load to the antenna input. Same noise? Hook an antenna to the input. Same noise? Hook a noise diode to the receiver's antenna input. Turn the source on & off. Did the noise change? Where do we hook our measuring equipment? The speaker audio? The audio that doesn't change? What goes on here? Aha! FM receivers have limiters! So - if the receiver's noise output doesn't change [or doesn't change much] with different levels of noise input - just how do we measure the receiver's noise figure? We don't. That's not to say that you couldn't get inside the receiver - get somewhere into the IF and make a measurement. But measurement of bandwidth limited noise has its own issues - plus there are so many receiver architectures today. How about receivers whose IF is processed in a DSP. In any case - we don't need to since there is a direct correlation between sensitivity and noise figure - a correleation that is known and published for all types of digital and analog, wide and narrow band systems. See the TIA working group report which can be found at: http://www.antd.nist.gov/wctg/manet/docs/TIAWG88_20.pdf For those who wish to further research noise figure, I suggest that you start with Agilent's application note 57-1 "Fundamentals of RF and Microwave Noise Figure Measurements" which can be downloaded free from their web site at: http://cp.literature.agilent.com/litweb/pdf/5952-8255E.pdf Some other items of interest: "What's All This Noise About Noise?", QEX, November 1996 "An Introduction to Noise Figure", RF Magazine, March 1993.
