Variable resolution capability for multichannel filter spectrometers

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(f) is (approximately) a frequency-expanded replica of its input spectrum Sin(f): Sout(Ef) approximately 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< or =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 approximately 0.1 dB lower than at its input.