Multiband downconverter for digital receivers

摘要

Disclosed is an apparatus (40) for digitizing an electromagnetic input signal (Sm) having frequency components within at least first and second segregated frequency bands, particularly useful in a wireless communications system. The apparatus includes a filtering circuit (54, 56, 58) arrangement for passing only frequency components of the input signal substantially within the first and second frequency bands, to provide a filtered signal having a spectrum with spectral components below a predetermined power level within a frequency gap between the first and second frequency bands. An analog to digital (A/D) converter (60) samples the filtered signal at a sampling rate sufficient to provide a digitized output signal with a frequency spectrum corresponding to that of the filtered signal and also with a replica of spectral components of one of the frequency bands appearing within the frequency gap. Preferably, the sampling rate used is less than the Nyquist sampling rate for the composite frequency band extending from the lowest to highest frequencies of the two bands. This allows a single A/D converter to be employed to sample the filtered signal, even when a large frequency gap exists between the segregated bands. A single mixer stage (42, 44) can then be used in conjunction with the single A/D converter (60) in a wireless communications receiver application.