NIST has developed a new state-of-the-art AM and PM system for noise measurement and calibration for 100 GHz oscillator signals, amplifiers, and components. Two primary uses at this time are in clocking high-speed digital signal processors (DSP) and broadband telecommunications signal structures. The system relies on a cavity stabilized 100 GHz Gunn oscillator that serves as a low-noise reference signal, a key development effort at NIST. It is phase-locked to a 10 GHz laboratory reference by low-noise mixing and development of microwave regenerative frequency dividers, another ongoing NIST program. The broadband phase noise of the 100 GHz Gunn oscillator is limited by mixer/divider noise, since the 10 GHz reference is a sapphire-loaded cavity oscillator (SLCO) at ultralow phase noise. At the same time, mid- and long-term environmentally induced fluctuations in phase throughout the chain are far lower than those in traditional synthesis approaches by the elimination of multipliers, narrow band filter stages, and amplifiers.

Phase noise at 100 GHz is measured by two nearly identical phase-locked-loops whose loop phase deviations are analyzed simultaneously by means of a cross-spectrum analyzer. Much of the noise generated in the PM or the AM detector of one channel is independent of the noise generated by the detector and its amplifier in the other channel. The broadband noise generated in the mixer and amplifier for one channel is independent of the broadband noise generated in the mixer and amplifier of the other channel. Averaging the cross spectrum causes the incoherent noise contributions of both channels to average towards 0 as 1/N^0.5, where N is the number of averages. For example 1000 averages reduces these incoherent noise contributions by 15 dB.

The figure below shows a simplified block diagram of the two PLL�s, one with signal path in blue and the other with signal path in red. At the bottom of the figure is the 100 GHz reference whose phase is referenced to a 10 GHz SLCO.

Dave Howe and Fred Walls have developed a system for calibrating the level of phase and amplitude noise at 100 GHz in oscillators, amplifiers, and other components. At this time the two key applications of the system are the clocking of high-speed digital processors and evaluation of the performance of broadband telecommunications signals.

The system, shown schematically in the above figure, relies for reference on a cavity-stabilized Gunn oscillator that serves as a low-noise reference signal. The oscillator is coupled and locked to an ultra-low-noise SLCO through several regenerative frequency dividers, and the broadband noise of the system is then limited by the noise from the mixer and divider. Measurement of phase or amplitude noise at 100 GHz is accomplished by two nearly identical phase-locked loops whose phase deviations are analyzed simultaneously using a cross-correlation spectrum analyzer. Much of the noise generated in the PM or AM detector in one channel is independent of the noise generated in the same components in the other channel, and thus does not show up in the correlation measurement.