NF measurement using a Spectrum Analyzer
First, this program is just a calculator with the functionality to save the measured data to an Excel sheet.
Noise figure measurement can be done in different ways with different result. If a spectrum analyser is available, it can be done using the gain or Y method. The simplest way is the gain method. And is the method that can generate most uncertainties. It relies on how accurate the gain of a device can be measured. High gain devices (LNA) can be quite accurate measured using this method.
I have only done simple NF measuring, and do not use external pre-amp and do not take account for the SA’s own internal noise or the SA’s input return loss. All these uncertainties will add on, but the measured result can be compared to the manufacturer measured NF of LNA’s and see how close it is. When this is known measuring on home built LNA’s can be fun to compare.
If your SA can display dBm/Hz the noise figure, NF of the device under test can then be computed as: NF = Noise + 174 dBm/Hz – Gain
Why 174dBm/Hz?
The average temperature of the earth's surface is approximately 62.3°F, 16.8°C, or 290°K and provides the energy to the surrounding matter that generates natural background noise. The thermal noise generated due to this temperature equals -174 dBm/Hz or a few nanovolts. So, during normal indoor temperature -174 can be used. At least for us normal amateurs.
If your SA cannot display dBm/Hz the thermal noise must be re-calculated with respect to the SA’s RBW. Example, 1Hz is equal to -174, 10kHz is -134 dBm/Hz. This is selectable in the calculator.
If you are lucky to have a calibrated noise source you should use the Y-method. This is a more accurate method.
Turning the noise source OFF/ON calculate the gain and use the calibrated ENR generates the NF. Here is how good and calibrated noise source you have is the key for accuracy.
By ticking the checkboxes different parameters will be added to the list view. You probably want to test the gain and NF for different frequencies. Then save the result.
A screenshot from the program. Download: nf-sa-gain-calculator.zip
The exported Excel file is saved in: C:\Test-Reports\Noise Figure
No fancy report is made, just the collected data.
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