Local Oscillator Noise Affects Noise Figure Measurement Accuracy

Noise figure measurements are affected by amplitude modulated noise from local oscillators used in noise figure test systems. When testing microwave amplifiers or mixers/receivers, an external local oscillator(LO) is needed. For microwave amplifiers, the LO downconverts the noise to the noise figure meter´s frequency range; while for mixers/receivers, the LO provides a drive signal to the mixer´s L port. The LO adds noise to the testing in the form of AM noise. This AM noise elevates the system noise figure when testing amplifiers causing increased measurement uncertainty. For mixers and receivers, the LO noise adds directly to the measurement and causes erroneous results. The AM noise is reduced by placing a filter between the LO and the mixer. This filter limits noise reaching the noise figure meter. The LO AM noise affects the microwave amplifier noise figure measurement uncertainty and falsely elevates mixer/receiver noise figure measurements.