NIST Technical Note 1337: Characterization of Clocks and Oscillators

7. PHASE ~OISE AND AM NOISE MEASVREMENTS 261System sensitivity can be increased by driving the mixer with high-levelsIgnals that lower the mixer output impedance to a few ohms. This ~resentsa problem in establishing the calibration factor of the mixer, because itmight be necessary to calibrate the mixer for different fourier frequencyranges.The equation sensitivity = slope = beat-note amplitude does not holdif the output of the mixer is not a sine wave. The Hewlett-Packard 3047automated phase noise measurement system allows accurate calibrationof the phase-detector sensitivity even with high-level inputs by using thederivative of the Fourier representation of the signal (the fundamental andits harmonics). The slope at 4J ;;:; 0 radians is given byA sin 4J - B sin 34J + C sin 54J = A cos 4J - 3B cos 34J + 5C cos 54J= A - 3B + 5C + .... (67)Referring to Fig. 9, the carrier-power reference level is obtained as follows.(1) The precision IF step attenuator is set to a high value to preventoverloading the spectrum analyzer (assume 50 dB as our example).(2) The reference and test signals at the mixer inputs are set to approximately10 dBm and 0 dBm, as previously discussed.(3) If the frequency of one of the oscillators can be adjusted, adjust itsfrequency for an IF output frequency in the range of 10 to 20 kHz. If neitheroscillator is adjustable, replace the oscillator under test with one that canbe adjusted as required and that can be set to the identical power level of theoscillator under test.(4) The resulting IF power level is measured by the spectrum analyzer,and the measured value is corrected for the attentuator setting, which wasassumed to be 50 dB. The correction is necessary because this attenuatorwill be set to its zero decibel indication during the measurements of noisepower. Assuming a spectrum analyzer reading of -40 dBm, the carrierpowerreference level is calculated ascarrier power reference level = 50 dB - 40 dBm = 10 dBm. (68)3. Phase Quadrature of the Mixer Input SignalsAfter the carrier-power reference has been established, the oscillator undertest and the reference oscillator are tuned to the same frequency, and theoriginal reference levels that were used during calibration are reestablished.The quadrature adjustment depends on the type of system used, Threepossibilities, illustrated in Fig. 9, are described here.(1) If the oscillators are very stable, have high-resolution tuning, and arenot phase-locked, the frequency of one oscillator is adjusted for zero deTN-212