David, I'm not a practicing engineer, but I always thought Noise power=10*log BW. So the noise power in a 10hz filter was 10 times that in a 1Hz reference. Tell me what I am thinking of.
Chas, W1CG At 03:05 PM 6/15/2007, Dave Haupt wrote: >You're getting very close to instrumentation norms. > >Noise, by its very nature, varies in apparent strength >by the bandwidth in which you measure it. When >measuring truly random noise, the power (in watts) is >in direct proportion to the bandwidth of the filter >used in the measurement. Man-made noise often >consists of thousands of semi-correlated carriers, and >the noise power can grow faster than simply being >proportional to BW. > >To be accurate, an engineer specifies noise either as >"dBm/Hz" which means "dBm measured in a one Hertz >bandwidth" or "watts/hz" or something like that. It >would also be equivalent to say "dBm measured using a >200Hz filter" or something like that. Audio-frequency >guys tend to measure amplitudes in volts, at which >point a square root gets into the picture, and you >will read things like "nanovolts per root Hertz". > >Sine waves and noise are described using different >mathematical terms and as an interesting result, to >make a truly accurate spectrum analyzer, >instrumentation manufacturers often use an algorithm >to attempt to distinguish a sine wave from a noise >function. Anybody who's used the digitally-enhanced >analog spectrum analyzers by HP (8566/68, 8590 and >8560 series) has probably encountered the "noise >marker". The instruments are calibrated for accuracy >with sine wave signals (or sums of sine waves - any >repetitive waveform), and are in error, fundamentally, >for noise. When you turn on the "noise marker", the >instrument makes additional calculations to make the >readings accurate for random noise. > >When you get down to the nitty-gritty of noise, it's >not at all a simple subject. > >73, > >Dave W8NF > > > >>From: "Doug McCormack" <[EMAIL PROTECTED]> > >>Subject: Re: [Flexradio] Noise Floor S meter vs >Panadapter > > >> Thanks everyone for the quick and detailed >explanation. I now understand > >> the S meter displays one sum across the entire >width of the green filter, > >> while the panadapter displays hundreds of sums >across the width of each > >> individual pixel. When I set the filter very >narrow (approaching one pixel) > >> the S metter reading approaches the panadapter >reading. > > >> My old Kenwood was similar in that selecting the >narrow CW filter caused a > >> drop in S meter noise floor. This behavior makes >perfect sense for any radio. > > >> I guess if someone asks about the noise floor, they >need to specify at what > >> filter width. > > >> 73, Doug, VE3EFC > > > >____________________________________________________________________________________ >Yahoo! oneSearch: Finally, mobile search >that gives answers, not web links. >http://mobile.yahoo.com/mobileweb/onesearch?refer=1ONXIC > >_______________________________________________ >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/ _______________________________________________ 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/
