At 06:54 PM 03/20/08, you wrote: >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.
An addition to the "some other items..." classification, all on the repeater-builder "Tech Info" page... Fundamentals of RF and Microwave Noise Figure Measurements - Application Note 57-1 712KB PDF by Agilent Technologies 10 Hints for Making Successful Noise Figure Measurements - Application Note 57-3 967KB PDF by Agilent Technologies Practical Noise-Figure Measurement and Analysis for Low-Noise Amplifier Designs - Application Note 1354 1.7MB PDF by Agilent Technologies Mike
