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ham_radio:noise_figure_calculations [2013/01/08 19:00]
ham_radio:noise_figure_calculations [2013/01/08 19:00] (current)
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 +====== Noise Figure Calculations ======
 +
 +//Loren Moline WA7SKT wrote: // 
 +
 +//Is it possible to calculate the improved signal to noise ratio between a 1.5db NF LNA and a .6db NF LNA? //
 +
 +// Loren   ​WA7SKT //
 +
 +----
 +
 +Dave Sublette answered:
 +
 +> Loren
 +
 +> Somewhere on the Agilent website is a free download of AppCAD. It is a 
 +> wonderful design tool. One of the features is a "​signals and Systems"​
 +> section that allows you to do exactly what you are talking about. ​
 +> Every segment of a system is entered with its associated loss or gain 
 +> and noise figure, where appropriate. Then the complete system noise 
 +> figure, gain and S/N is calculated. Any component can then be altered ​
 +> to see what effect that alteration will have on the system performance.
 +
 +> It might be a bit difficult to find where that download is on the 
 +> website. It is a very busy website.
 +
 +> 73,
 +
 +> Dave, K4TO
 +
 +
 +----
 +
 +Luis Cupido added:
 +
 +Loren,
 +
 +I'm not so sure that AapCad computes with a non room temp antenna target....
 +
 +Well, for you to understand it before using software, here it goes.
 +
 +The NF is a measure of the degradation of the signal to noise in systems referenced to room temperature.
 +
 +If all is at 290K then you have 6dB s/n with a 1.5dB NF system and you will have 7dB s/n with a 0.5dB NF system
 +
 +
 +Rapid calculations with NF can be done in cases like this with simple addition and subtraction:​
 +
 +If you are pointing antennas to the sky and have non 290K target then the story is completely different. You need to calculate it all.
 +Best is to work with noise temperature equivalents than all adds up quite nicely.
 +
 +NF= 10*log(T/​290 + 1)
 +
 +  - your system NF of 1.5dB equals 120K noise temp.
 +  - your system NF of 0.5dB equals 35K noise temp.
 +
 +So if you're pointing to the sky to a spot with about 30K of equivalent noise temperature,​ your system in 1) sees a total noise 120+30=150K your system in 2) sees a total noise 35+30=65K, so your system in 2) is 150/65 times quieter, that is 2.3 times better, that is it has a 3.6dB better S+N/N ratio.
 +
 +If you had a 6dB s/n in 1) you will have 9.6dB s/n in 2)
 +
 +So when pointing to lower temp places the improvement in s/n is bigger than the reduction in the NF.
 +
 +So it is possible to calculate providing you know what is the noise temp. the antenna is pointing to.
 +
 +Luis Cupido, ct1dmk
 +
 +P.S. now calculate if you're at microwaves and point to a 5K spot in the sky... \\
 +P.P.S. note that I wrote system NF and not preamplifier NF !!! You must compute the total system NF where your preamp is, must include second stages feed losses etc...
 +
 +----
 +
 +Ed Cole Edward Cole kl7uw added:
 +
 +My EME pathlink calculator does this in the first section: ​ It calculates cascade NF and NT for a three stage receiver including feedlines and antenna temp.
 +
 +Then it enters the sky temp the antenna is looking at to produce the standard Pn=KTB noise power number.
 +
 +By comparing Pn (0.5dBNF) to Pn (1.5dBNF) you will have the improvement factor in dB you are seeking.
 +There are three columns so you can compare three examples side by side (I have sample numbers entered).
 +
 +The program also calculates pathloss to the Moon and S/N, but you can use the first section alone for receiver noise performance.
 +
 +Here is the link to the excel spreadsheet: ​ http://​www.kl7uw.com/​emelink.xls
 +
 +Ed
 +
 +
 +
  
 
ham_radio/noise_figure_calculations.txt ยท Last modified: 2013/01/08 19:00 (external edit)
 
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