At 3/21/2008 15:01, you wrote: >Here is a cascade noise figure calculator. Play around with different gain >figures and noise figures for the first amp to see how it affects the >total noise figure. Consider the second amp your receiver. Put in an 8 db >or so figure for the second amp to simulate your receiver. > > > >You will see that as you increase the first amp gain the overall noise >figure does drop some but not by as much as the gain increase. You can >simulate a pad between the preamp and your receiver by increasing the >noise figure of the second amp by the amount of pad you would use. It is >not quit the same as reducing the gain of the preamp (first amp) but close. > > > ><http://www.minicircuits.com/pages/mcl_nf_calc.html>http://www.minicircuits.com/pages/mcl_nf_calc.html
In order to calculate the actual effect of a pad between your preamp & RX, a 3rd calculation stage is necessary. I just uploaded to the files section of this group an Excel spreadsheet (Cascade noise figure.xls) that allows calculating the total noise figure of up to 6 cascaded amps and/or attenuators in any order. The gain of the final stage can be omitted for cascade noise figure calculations (you'll see that it only affects the total gain). Since the "gain" of a NBFM RX is meaningless for this calculation, you can just enter the RX noise figure & leave that last gain cell empty. With this tool you can see how much extra noise figure an attenuator between the preamp & RX input adds to the overall system. You can also factor in the duplexer loss by entering it as an attenuator (1st stage NF=duplexer loss, 1st stage gain=-duplexer loss). Bob NO6B
