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Need help about receiver noise figure measurement

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rf_usn

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Dear all,

Could anybody help me about the method of NF measurement at low frequency. I need to measurue NF of a low IF receiver with IF frequency of 2MHz.

Thank you very much in advance.

Best regards,

rf_usn
 

jiripolivka

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As with any other frequency, you need a calibrated noise source and a good power meter. Then you can follow the Y-factor method or the "twice the output power" as explained in any textbook.

For frequency like 2 MHz, I would recommend to use the "old" saturated-diode noise source; it has the upper frequency limit but it can be used down to ~ DC.
Instead of a low-frequency power meter, a good audio or video voltmeter can be used to measure the POWER calculated from this voltage across a typical load resistor at IF output of your receiver. If you use 50 Ohms load, some spectrum analyzers allow a memory mode and 1-dB per division resolution; I used such instrument to measure the power < 10 kHz.
If your receiver is "low-noise" (radios for ~2 MHz are typically not low-noise type as they receive artificial signals on galactic-noise background, quite high there), a suitable noise source can be a good 50-Ohm resistor connected to a 50-Ohm coaxial line (flexible cable , best with TEFLON insulation.) You can take one output reading at this resistor under ambient temperature, another reading when you immerse the resistor in boiling water.
Finally, the "desperate" method described in old textbooks required a good calibrated signal generator and the output power meter. You first calibrate the receiver frequency response, take the 1-dB bandwidth to calculate the noise power density without an input signal. Then inject a center-frequency signal to obtain twice the output power. After a simple calculation, you can get the NF.
 

jgruszynski

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Use the source (not the force), Luke:

Agilent Fundamentals of RF and Microwave Noise Figure Measurements

cp.literature.agilent.com/litweb/pdf/5952-8255E.pdf
 

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