Heterodyne frequency down-conversion is a commonly used technique for performing high-resolution spectroscopic radio astronomy. It combines an incident radio frequency (RF) astronomical signal with a local oscillator (LO) tone in a highly non-linear mixing device such as a Superconductor-Insulator-Superconductor (SIS) tunnel junction. The mixer generates an intermediate frequency (IF) output, which is substantially lower in frequency than the RF and LO input signals; this allows subsequent processing and reproduction of the astronomical signal power spectrum. The mixer, LO and IF components combined form a heterodyne receiver.
The LO is a key device within the receiver system. It must provide sufficient power to "pump" the mixer; be stable in frequency and amplitude; and possess a broad tuning range. Additionally, the LO signal must be spectrally pure; i.e., spurious harmonic content is of sufficiently low amplitude to avoid spectral contamination and system noise degradation.
The LO spectral purity is difficult to specify for near quantum limited receiver systems and is often overlooked as a parameter when defining the LO chain. However, determining the LO purity is important as the harmonic content can mix with the input signal resulting in unwanted parts of the RF spectrum being down converted into the IF band which degrades receiver sensitivity and may also introduce confusing spectral artefacts. We have used a commercially available 230 GHz LO source to investigate LO harmonic content and have correlated unusual features identified in an SIS mixer LO pumped I-V curve with direct measurement of frequency tones from the source.