[MyHosting.com]   [KO4BB Home Page]   [Manuals Home Page]   [KO4BB Wiki]

# Differences

This shows you the differences between two versions of the page.

 — ham_radio:noise_figure_calculations [2013/01/08 19:00] (current) Line 1: Line 1: + ====== 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)