Variable resolution capability for multichannel filter spectrometers

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Abstract
In this paper we describe a new electronic instrument called a spectrum expander, which is a two‐port digital signal‐processing device whose output power spectrum Sout(Ef) is (approximately) a frequency‐expanded replica of its input spectrum Sin(f): Sout(Ef) ≈Sin(f), where E is the expansion ratio (usually E>2). When inserted between a signal source and a spectrometer, the spectrum expander improves the effective resolution of the spectrometer, thereby permitting real‐time, high‐resolution measurements to be made. This capability is especially important in radio astronomy, where fixed‐resolution multichannel filter spectrometers are common. Our treatment of the spectrum expander includes a discussion of its basic principles, formulation of design rules, a brief study of random noise and quantization effects, and presentation of data illustrating expander performance. The prototype expander, constructed from Schottky–TTL components, accepts any signal of bandwidth⩽12.5 MHz and expands it to fill a 128‐channel spectrometer with 32‐MHz total bandwidth. Signal degradation caused by processing with the spectrum expander is minimal; the signal‐to‐noise ratio at its output is measured to be ∼0.1 dB lower than at its input.

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