Take the limit you are measuring to, and subtract the transducer factor and any cable losses. That yields the receiver signal in dBuV which corresponds to a spec level signal. Subtract off a factor for how much you want the receiver noise floor to be below the spec level, typically 6 dB. Call this level N. Then basic statistical thermodynamic theory says that N = kTBF, where K is Boltzman's constant (the ideal gas law constant divided by Avogadro's number), T is absolute temperature in Kelvin, B is bandwidth in Hertz, and F is the noise figure. At room temperature, 25 degrees Celsius or 298 Kelvin, N = -67 dBuV/Hz, with the bandwidth dependency going as 10 log (BW) because the basic equation is for noise power. Now given your bandwidth, say 120 kHz, you can solve for the necessary noise figure. If your receiver/analyzer noise figure is higher than that needed, there is another simple equation that will allow you to specify a preamp that will allow the combination measurement system to perform as required.
---------- >From: "KC CHAN [PDD]" <kcc...@hkpc.org> >To: <emc-p...@majordomo.ieee.org> >Subject: noise figure >Date: Fri, Dec 21, 2001, 6:42 PM > > > Hi all > > It may not be purely EMC question, actually it is RF related, but I am sure > the experts here can answer my questions. > > We all know that we need to have a pre-amp. that is as lower noise figure > as possible, but how low it is enough or how it is related to the noise > floor viewed by a receiver or spectrum analyzer. > > Thank you > KC Chan > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old > messages are imported into the new server. > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
