Noise measurement on spectrally pure oscillator output - utilises narrow band filter tuned to difference between two oscillators

摘要

In an oscillator with a very pure spectral output the amplitude or phase is very low. These characteristics are measured using a narrow band filter tuned to the difference frequency between two oscillators. This increases the noise to signal ratio to permit analysis. The output of an oscillator under test (1) and a second oscillator (4) are mixed (7) and the resultant output amplified (8). The oscillators are tuned to slightly different frequencies and the amplifier output is applied via a coupler (10) and phase shift circuit (11) to a narrow band filter (12). The signal is demodulated (13) and amplified (17) to provide a nul signal on are superimposed the phase fluctuations from the oscillator under test. These signals are then spectrally analysed (3).