Bandpass filtering with lowpass filters

✍ Scribed by Harry Urkowitz

• Book ID: 103081972
• Publisher: Elsevier Science
• Year: 1963
• Tongue: English
• Weight: 539 KB
• Volume: 276
• Category: Article
• ISSN: 0016-0032
• DOI: 10.1016/0016-0032(63)90305-3

✦ Synopsis

Lowpass filters may achieve the effects of bandpass filtering by heterodyning the bandpass wave to obtain the "in-phase '~ and "quadrature" components and filtering these with suitable lowpass filters. The impulse responses for the lowpass filters are determined in a simple way from the desired bandpass impulse response. When the filtering is to be arbitrary and the signal robe filtered has arbitrary phase modulation, four lowpass filtering operations are necessary. In the special cases where (1) the signal has only amplitude and/or binary phase modulation, or (2) the transfer function of the filter has "conjugate symmetry," only two lowpass filtering operations are necessary. The outputs of the lowpass filters may be combined simply to give:

1. The instantaneous envelope of the filtered bandpass wave. 2. The instantaneous phase or frequency of the filtered bandpass wave. 3. The filtered bandpass wave itself at almost any carrier frequency.

The techniques described above can be extended straightforwardly to linear timevariable filters.