High-resolution submillimeter-wave spectroscopy is based on the heterodyne principle, where the incident signal is down-converted to a low intermediate frequency (IF) by nonlinear mixing with a local oscillator (LO) signal. The IF difference frequency output is discrete Fourier transformed into ≈1,000 frequency channels to measure the spectral power dependence of the signal. Unfortunately, the LO system cannot generate pure tones: the signal has a “skirt” of additional power in the vicinity that generally decreases in spectral power density as the frequency difference from the center increases. This extra signal is known as phase noise.
The effect of local oscillator phase noise on spectrometer response has been characterized by simple equations reported in the literature. However, there is little actual measurement to verify that simple equations (as opposed to a full numerical simulation) can be used to determine LO synthesizer phase noise requirements from the spectrometer science sensitivity goals. This can be an acute problem in the submillimeter-wave/ terahertz regime because the LO is generally derived from a microwave synthesizer source and multiplied up to the high signal frequency.
In the past, the microwave sources have either been low-noise but fixedtuned, such as Gunn-diode oscillators, or complex wideband synthesizers, typically massive and requiring much power. For planetary instruments, a source with better size, weight, and power (SWaP) characteristics that still has wide tuning range is desired.
The measurement technique developed in this work utilizes one LO source with known high purity that can be fixed in frequency, and the LO source under test. The outputs from these are mixed together in a mixer/receiver, and the result fed to the spectrometer part of the instrument. By scanning the tunable source through the spectrometer, the response of a single channel can be measured to determine the impact of the LO on the spectrometer measurements.
This is a straightforward way to quickly evaluate the compatibility between submillimeter-wave LO effects. Previously, a gas cell has been used to perform a spectral measurement; this requires more substantial laboratory equipment, expertise, expense, and time.