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ham_radio:noise_figure_calculations [2013/01/08 19:00] (current)
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| | ====== Noise Figure Calculations ====== |
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| | //Loren Moline WA7SKT wrote: // |
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| | //Is it possible to calculate the improved signal to noise ratio between a 1.5db NF LNA and a .6db NF LNA? // |
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| | // Loren WA7SKT // |
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| | ---- |
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| | Dave Sublette answered: |
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| | > 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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| | ---- |
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| | Luis Cupido added: |
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| | Loren, |
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| | I'm not so sure that AapCad computes with a non room temp antenna target.... |
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| | Well, for you to understand it before using software, here it goes. |
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| | The NF is a measure of the degradation of the signal to noise in systems referenced to room temperature. |
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| | 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 |
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| | Rapid calculations with NF can be done in cases like this with simple addition and subtraction: |
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| | 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. |
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| | NF= 10*log(T/290 + 1) |
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| | - your system NF of 1.5dB equals 120K noise temp. |
| | - your system NF of 0.5dB equals 35K noise temp. |
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| | 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. |
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| | If you had a 6dB s/n in 1) you will have 9.6dB s/n in 2) |
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| | So when pointing to lower temp places the improvement in s/n is bigger than the reduction in the NF. |
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| | So it is possible to calculate providing you know what is the noise temp. the antenna is pointing to. |
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| | Luis Cupido, ct1dmk |
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| | 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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| | ---- |
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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. |
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| | Then it enters the sky temp the antenna is looking at to produce the standard Pn=KTB noise power number. |
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| | 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). |
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| | The program also calculates pathloss to the Moon and S/N, but you can use the first section alone for receiver noise performance. |
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| | Here is the link to the excel spreadsheet: http://www.kl7uw.com/emelink.xls |
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| | Ed |
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