NIST Technical Note 1337: Characterization of Clocks and Oscillators

7. PHASE NOISE AND AM NOISE MEASUREMENTS 265The configuration shown in Fig. 12b is sometimes used and does notgreatly degrade the noise floor because the reference signal of 10 dBm islarger Ihan Ihe signal frequency. See Sections IV.B and IV.C.4 for additionaldiscussions related to system sensitivity and recommended system eva·luation.Proper selection of drive and output termination of the double-balancedmixer can result in improvement by (5 to 25 dB in the performance of phasenoisemeasurements, as discussed by Walls et al. (1976). The beat frequencybetween the two oscillators can be a sine wave. as previously mentioned,with proper low drive levels. This requires a proper terminating impedancefor the mixer. With high drive levels, the mixer output waveform will beclipped. The slope of the clipped waveform at the zero crossings, illustratedby Walls et al. (1976), is twice the slope of the sine wave and therefore improvesthe noise floor sensitivity by 6 dB, i.e., the output signal, proportionalto the phase fluctuations. increases with drive level. This condition of clippingrequires characterization over the Fourier frequency range, as previouslymentioned for the Hewlett-Packard 3047 phase noise measurement system.An amplifier can be used to increase the mixer drive levels for devices thathave insufficient output power to drive the double-balanced mixers.Lower noise floors can be achieved using high-level mixers when availabledrive levels are sufficient. A step-up transformer can be used to increasethe mixer drive voltage because the signal and noise power increase in thesame ratio, and the spectral density of phase of the device under test is unchanged,but the noise floor of the measurement system is reduced.Walls et al. (1976) used a correlation technique that consisted primarilyof two phase-noise measurement systems. At TRW the technique is used asshown in Fig. 13. The cross spectrum is obtained with the fast Fouriertransform (FFT) analyzer that performs the product of the Fourier trans-·form of one signal and the complex conjugate of the Fourier transform ofDUAL­CHANNELFFTFIG. 13Cross-spectrum measurement using the two-oscillator technique.TN-216