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+ | ====== Noise Figure Calculations ====== | ||

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+ | //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 // | ||

+ | |||

+ | ---- | ||

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+ | 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 | ||

+ | > | ||

+ | |||

+ | ---- | ||

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+ | 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... | ||

+ | |||

+ | ---- | ||

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+ | Ed Cole Edward Cole kl7uw added: | ||

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+ | 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 | ||

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+ | |||

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