## Bandwidth limited ASK

### Bandpass filtering method

In order to minimize the occupied bandwidth of the transmitted ASK signal, filtering or pulse shaping is required either prior to or after modulation onto a carrier. The switching method of ASK generation does not allow any pre-filtering of the modulating baseband symbol stream, as the switch is a non-linear process and does not transfer the pulse-shaping information onto the carrier envelope. In this case, any filtering to constrain the bandwidth must be performed on the modulated bandpass signal. |

For example, if we wish to filter a 30 kHz wide modulated signal superimposed on a carrier at 900 MHz (typical of some digital cellular systems in the USA), this would imply a filter with a Quality factor Q = 900 x 106 / 30 x 103 = 30000. At present, this Q can only be achieved using crystal filters, which have very very poor amplitude ripple and group delay distortion in the passband. They certainly do not allow the designer to achieve the root raised cosine bandpass response required for zero intersymbol interference.