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I know how to calculate Noise figure of an amplifier on the paper but how can measure Noise figure of a LNA in real world?

Our lab doesn't have a fancy Noise figure meter but I can readily access to VNA, Spectrum analyzer and signal generator. As none of these instrument are perfect I'm wondering if it's possible to measure accurately NF of a DUT without having a noise source.

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  • \$\begingroup\$ Spectrum analyzer should be enough. Just terminate the input with 50 ohms and measure the output noise. Compare with the noise expected from a 50-ohm resistor x gain. There may be some tricks to setting up the spectrum analyzer, but unfortunately I don't do this kind of measurement often enough to remember off the top of my head what to watch out for. \$\endgroup\$ – The Photon Apr 24 '17 at 23:08
  • \$\begingroup\$ @ThePhoton Thank you, Unfortunately I'm not quite sure If I understand what you said. Did you meaning terminate the LNA input with 50 ohm resistor and speculate LNA output through Spectrum Analyzer? Doesn't this method involve spectrum analyzer internal noise to the measurement and leading to loose of accuracy? \$\endgroup\$ – pazel1374 Apr 25 '17 at 0:02
  • \$\begingroup\$ No, terminate the input of the amplifier, and measure its output noise with the spectrum analyzer. Yes, if the amplifier noise is not very high, you might need to account for the spectrum analyzer noise contribution. \$\endgroup\$ – The Photon Apr 25 '17 at 1:07
  • \$\begingroup\$ Input a signal at 3dB SNR. Insert a 3dB 50_Ohm pad before the LNA. Use the SA to measure the change in SNR. \$\endgroup\$ – analogsystemsrf Apr 25 '17 at 6:59
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Here is a page that describes some basic techniques for noise figure measurement: https://www.maximintegrated.com/en/app-notes/index.mvp/id/2875

The simplest technique, mentioned by The Photon, is called the 'gain method'. In this method, you need to know the gain of the amplifier (which you can measure with a spectrum analyzer and a signal generator) and the output noise power when the input is terminated. The 'magic number' for noise power is -174 dBm/Hz at room temperature. Take the difference between the measured noise power and the room temperature noise power (-174 dBm/Hz) and subtract the gain to get the noise figure. It should be possible to add some more gain after the amplifier in question to get well above the spectrum analyzer noise floor, then figure out what the noise figure of the first state is. Essentially, you build a relatively high gain amplifier, measure the gain and noise figure of that, then stick your amplifier on in front of that, measure the gain and noise figure of the combination, then use Friis' formula to get the noise figure of the first stage (F = F1 + (F2-1)/G1 + ...).

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  • \$\begingroup\$ That's it.... use of second stage to get past to the noise floor of spectrum analyzer seems to be the answer. But if it's ok with you I'm waiting for possibly a better answer as this method require to already know second stage noise figure. \$\endgroup\$ – pazel1374 Apr 25 '17 at 1:15
  • \$\begingroup\$ As a second thought two same amplifier can be cascaded and assuiming NF are the same for both, an exact value can measured without concerning for noise floor? \$\endgroup\$ – pazel1374 Apr 25 '17 at 1:19
  • \$\begingroup\$ You have to measure NF of both. However, if the second part has high gain, then you'll be well above the spectrum analyzer noise floor, even with just that amplifier alone (and it is perfectly fine to use a chain of off the shelf amps to get enough gain) \$\endgroup\$ – alex.forencich Apr 25 '17 at 1:23

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