NIST Technical Note 1337: Characterization of Clocks and Oscillators

274 A. L. LANCE, W. D. SEAL, AND F. LABAARSIN h, r II:>AM LEAKAGEFrequency (Hz)FIG. 19Phase detector output (AM-PM crossover); T, delay time.mixer output for phase (PM) and amplitude (AM) noise in the singleoscillatordelay-line FM discriminator system. It is noted that the phasenoise and AM noise intersect and that the AM will therefore limit the measurementaccuracy near the carrier. Even though AM noise is much lowerthan phase noise in most sources, and even though the AM is normallysuppressed about 20 dB, there is still AM at the mixer output. This outputis AM leakage and is caused by the finite isolation between the mixer ports.The two-oscillator technique does not experience this problem to thisextent because the phase noise and AM noise maintain their relative relationshipsat the mixer output independent of the offset frequency from thecarrier.B. CALIBRATION AND MEASUREMENTS USING TIlE DELAYLINE AS AN FM DISCRIMINATORThe block diagram of a practical single-oscillator phase noise measurementsystem is shown in Fig. 20. The signals in the delay-line channel of thesystem experience the one-way delay of the line. With adequate sourcepower, the system is not limited to the optimum I Np (8.686 dB) previouslydiscussed for a power-limited system. Measurements are performed usingthe following operational procedures.(1) Measure the tracking spectrum analyzer IF bandwidths as set forthin Section III.C.I.(2) Establish the system power levels (Section IV.B.l).(3) Establish the discriminator calil'>ration factor (Section IV.B.2).(4) Measure and plot the oscillator characteristics in the automaticsystem used (Section IV.B.3).(5) Measure the system noise floor (sensitivity) (Section IV.B.4).TN-225