W-band Frequency Synthesis and AM/PM Noise Measurement System
The Time and Frequency Metrology Group has developed a state-of-the-art frequency synthesizer and dual-channel AM/PM noise measurement system at W-band. The measurement system is designed to make low-noise PM or AM measurements of either pulsed or CW amplifiers or other devices. Archita Hati’s leadership in design and measurements has led to a 1000-fold improvement over the prior state-of-the art in the field. This is a critical set of measurements needed to aid future designs in electronic surveillance, secret communications, and remote weapons detection.
The system relies on a Gunn oscillator that is phase-locked to a low noise air-dielectric cavity stabilized 10 GHz reference by low-noise multiplication and mixing. The stabilized Gunn serves as a reference source that is tunable in discrete steps of 100 MHz over the widest possible range available in a single Gunn oscillator centered at 94 GHz, as much 4 GHz. The air-dielectric cavity stabilized 10 GHz reference is a key invention of NIST* and its development is led by Craig Nelson. The broadband phase noise of the W-band Gunn-oscillator reference is the lowest ever attained. At the same time, mid and long-term environmentally induced fluctuations in phase throughout the chain are far lower than those in any other synthesis approach.
Dual-channel W-band AM/PM noise measurement system
The figure shown above shows the simplified block diagram of a dual-channel W-band AM/PM noise measurement system for an amplifier in pulsed mode. The 92 to 96 GHz signal is pulsed ON and OFF for a duty cycle of 10 % to 100 % at a given pulse repetition frequency (PRF) by use of a PIN diode switch. One part of the pulsed W-band signal is then fed to the device under test (DUT) and another part to the delay element. These two signals are further split and fed to a two-channel system composed of two separate phase-noise measurement systems that operate simultaneously. The phase shifters establish true phase quadrature between two signals at the mixer inputs. The output (after amplification) of each mixer is fed to a two-channel cross-correlation fast Fourier transform (FFT) spectrum analyzer. The advantage of this technique is that only the coherent noise, i.e., noise of the DUT, that is present in both channels averages to a finite value. The time average of all incoherent noise not due to the DUT approaches zero as √N, where N is the number of averages used in the FFT. For example, 1000 averages reduces these incoherent noise contributions by 15 dB. As a time-delay measurement, the resolution, or “noise floor,” is substantially better than any such measurement system, representing spectral length detection in the DUT that is equivalent to less than an angstrom. For emerging W-band applications, NIST’s world-record resolution is crucial to testing components for improved kinds of enemy and threat detectors, high-resolution medical imagers, and long-distance positive-identification scanners.
1. A. Hati, C.W. Nelson, J.F. Garcia Nava, D.A. Howe, F.L. Walls, H. Ascarrunz, J. Lanfranchi, and B.F. Riddle, “W-band dual channel AM/PM noise measurement system - an update,” Proc. 2005 Joint Mtg. IEEE Intl. Freq. Cont. Symp. and PTTI, pp. 503-508 29-AUG-05.
2. A. Hati, C.W. Nelson, J.F. Garcia Nava, D.A. Howe, and F.L. Walls, “W-band dual channel PM/AM noise measurement system,” Proc. 2004 Joint Mtg. IEEE Intl. Freq. Cont. Symp. and UFFC Conf., pp. 298-302, 23-AUG-04.
*United States Patent US007075378B2. High spectral purity microwave oscillator using air-dielectric cavity, D.A. Howe, A.S. Gupta, C. Nelson, F.L. Walls, July 11, 2006.