NIST Technical Note 1337: Characterization of Clocks and Oscillators

i. PHASE :-JOISE AND AM NOISE MEASUREMENTS 255III. Phase-Noise Measurements Using the Two-Oscillator *TechniqueA functional block diagram of the two-oscillator system for measuringphase noise is shown in Fig. 8. NBS has performed phase noise measurementssince 196i using this basic system. The signal level and sideband levels canbe measured in terms of voltage or power. The low-pass filter prevents localoscillator leakage power from overloading the spectrum analyzer whenbaseband measurements are performed at the Fourier (offset) frequenciesof interest. Leakage signals will interfere with autoranging and with thedynamic range of the spectrum analyzer.The low-noise, high-gain preamplifier provides additional system sensitivityby amplyfying the noise signals to be measured. Also, because spectrumanalyzers usually have high values of noise figure, this amplifier is very desirable.As an example, if the high-gain preamplifier had a noise figure of3 dB and the spectrum analyzer had a noise figure of 18 dB, the system sensitivityat this point has been improved by 15 dB. The overall system sensitivitywould not necessarily be improved 15 dB in all cases, because the limitingsensitivity could have been imposed by a noisy mixer.NOISY SIGNAL(THIS IS THE QUANTITYTO BE MEASURED)SMALLFLUCTUATIONSNOISE ONLYMIXERLOW-NOISEAMPLIFIERFIG. 8 The two-oscillator technique for measuring phase noise. Small fluctuations fromnominal voltage are equivalent to phase variations. The phase shifter adjusts the two signalsto quadrature in the mIxer, which cancels carriers and converts phase noise to fluctuating devoltage... See Appendix Note # 6TN-206