Signal processing bandpass filter

Fig. 9.8. Time-frequency plane described by the STFT. The analysis can be viewed as a series of FTs defined on windowed segments of the signal (vertical bars) or as a filtering process implemented with a bandpass filter-bank (horizontal bars).

The sequence of the processing steps between the beam former and the scan conversion is different among the various commercial systems but the goals of the steps remain the same. The beam former output will be a wideband RF, an IF, or a complex baseband signal, which will usually be bandpass filtered to reduce out-of-band noise contributions. In systems with very wideband processing, frequency diversity techniques (e.g., split spectrum processing) can be brought into play to try to reduce the impact of coherent interference or speckle. 

The rationale for using the two-stage conversion process may be understood by examining the selectivity of the one- vs. two-mixer configuration. In the former case, a single mixer is used with a variable local oscillator and a 70-kHz bandpass filter to the convert amplitude modulation signal (frequency between 100 kHz to 10 MHz) to the IF of 70 kHz. The maximum and minimum frequencies passed by the 70 kHz filter are denoted by... 

Such instruments are non-dispersive and utilize a narrow bandpass filter to monitor one compound of interest, commonly carbon dioxide or carbon monoxide, in a process environment. The bandwidth of the filter is closely matched to the bandwidth (for carbon dioxide the unresolved envelope of a number of rotational sidebands) of the analyte and a reference signal is obtained from another filter which passes radiation from a transparent area of the spectrum. 

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