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Leif Asbrink leif at sm5bsz.comHi again John, > Leif, one other question... how do you use a noise figure meter to > measure an SDR? I have an HP 8970A with noise head available, but > haven't figured out how to do a measurement where there's no RF output > to connect the meter to. You could open the unit and connect the signal that goes to the A/D converter to the NF meter. Not very practical and probably very difficult because of spurs. You can use the noise head and your SDR program to measure power over the widest spur free frequency range you can find. Just use the SDR to measure by how many dB the noise floor changes when you switch the noise head on and off. That is exactly what the NF meter is doing although it makes several more things to compensate for the noise it generates itself which is important if you measure low gain amplifiers. Now, you do not want those complications because you want to measure the system NF of the entire SDR. What you have is like figure 3 here: https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1ma178/1MA178_2e_NoiseFigure.pdf Your SDR is a spectrum analyzer and the power difference you observe is the Y-factor in dB. Convert it to linear power scale and use equation 12 in the document. In case the NF of your SDR would be very near 0 dB you would find the difference that you measure equal to the excess noise you can read on the noise head. 73 Leif