Joe - I went back and did the calculation again for both conditions. I had made a small mistake in calculating the combination previously.
K3 external preamp on, internal off, Noise Figure for combination = 3.3 dB Which should be good for most work. K3 external preamp on, internal on, Noise Figure for combination = 1.5 dB Which is probably overkill for most people. Here are the details of the calculation. I hope there are no more mistakes: The noise factor for stages in parallel is Ftotal = F1 +(F2-1)/G, where these are all ratios, not expressed in dB where F is the noise factor, F1 the PR6, F2 the K3, and Ftotal the noise factor for the total system. The noise factor is a measure of how much noise a stage adds to a signal F= SNRin/SNTout Where SNRin is the signal to noise ratio (power) of the signal into the stage and SNRout is the signal to noise ratio (power) out of the stage. So F is a measure of how much noise a stage adds. NF=10log(F) so F =10**(NF/10) for the K3 with the preamp off, NF =18 dB, F2 = 63.1 for the PR6 NF=0.7 dB, F1= 1.17, G= 63 (18 dB) so Ftotal = 1.17 + (62.1)/63 = 1.17 + 0.98 = 2.15 which is 3.3 dB Noise figure with the internal preamp on, NF = 12 db; F2= 15.8 F = 1.17 + 15.8/63 = 1.42 NF= 1.52 dB The 0.7 dB noise figure is the preamp spec, it also says that typical values run 0.5 dB which would improve things even more. Sorry for the miscalculation in the combined noise figure the first time. - Duffey On Apr 2, 2010, at 4:58 PM, Joe Subich, W4TV wrote: > > >> The Elecraft external preamp with 0.7 dB noise figure and 18 >> dB gain helps a lot: >> >> K3 with PR6 external preamp >> >> preamp on 0.9 dB >> >> And the preamp is pretty robust to strong signals. >> > > Is this calculated with both preamplifiers on (cascaded) or > with the PR-6 only? -- James Duffey KK6MC DM65tc Cedar Crest NM < jamesduf...@comcast.net > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
