At 08:54 AM 6/14/2007, Philip J Gentile wrote: >i would terminate the antenna connection into a 50 ohm load to measure >noise. > >phil AB2JL
When making noise power measurements one has to be aware of the fact that the instantaneous power can vary quite widely. The noise is a normally distributed (gaussian) process. The power you would measure, averaged over a very long time, corresponds to the variance (or in voltage terms, the voltage is the standard deviation). In any given instantaneous measurement, though, you could be 2,3, 4, or more standard deviations away from the mean. What you're really measuring is the average of the variance of the incoming samples, and that average has a variance too.. With an analog meter with time constant much greater than the measurement bandwidth (the usual situation... meter might have a "video" bandwidth of 10-20 Hz, measuring the noise out of a 3kHz IF), you won't see the deviations (they'll have "averaged out".. by the ratio sqrt(VideoBW/ResBW)) However, in a FFT type analyzer, you tend not to have this averaging, so the variation in "bin amplitude" from buffer to buffer (frame to frame) can be pretty large. If you have a "peak detector" type system, it aggravates this, because if you wait long enough, eventually, one of those 5 sigma events comes along and bumps that bin way up. James Lux, P.E. Spacecraft Radio Frequency Subsystems Group Flight Communications Systems Section Jet Propulsion Laboratory, Mail Stop 161-213 4800 Oak Grove Drive Pasadena CA 91109 tel: (818)354-2075 fax: (818)393-6875 _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Knowledge Base: http://kb.flex-radio.com/ FlexRadio Homepage: http://www.flex-radio.com/
