A pre-amp can never increase the measurement system sensitivity (or equivalently, reduce the noise floor) by the full value of its gain. The noise figure of the pre-amp must be considered, as well as the noise figure of the receiver without the pre-amp. In fact, if the pre-amp’s noise figure were no different than that of the receiver’s without it, you would get no increase in sensitivity whatsoever. In order to increase sensitivity, you need a lower noise figure pre-amp, and then enough gain to amplify the low-level signal above the noise floor of the receiver in question. The equation governing this is:
Fsys = Fpa + (Frcv – 1)/Gpa, where Fsys = the resultant noise figure of receiver plus pre-amp, Fpa = pre-amp noise figure, Frcv = noise figure of original stand-alone receiver, and Gpa = pre-amp gain. CAUTION - ALL THE VALUES IN THIS EQUATION ARE NUMERICS!!! You have to convert >from dB to numerics in order to use this equation, then when done convert back to log-space. Ken Javor Phone: (256) 650-5261 _____ From: Grace Lin <[email protected]> List-Post: [email protected] Date: Wed, 19 Mar 2008 07:13:04 -0400 To: <[email protected]> Subject: Internal Preamplifier Dear Members, The EMI receiver in my workplace comes with an internal preamplifier (20dB) up to 3.6GHz. I thought if I turned on this preamplifier, I should lower the noise floor 20 dB. Without the preamplifier on, my noise floor at 30 MHz read about 20 dBuV. With the preamplifier on, the noise floor read about 10 dBuV. There is only 10 dB difference between preamplifier on and off. Where did I miss? Is there any other adjustment (preselector, etc.) I should set as well? Thank you and look forward to your help. Best regards, Grace - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
