Selectivity system for a direct conversion receiver

A process for filtering the baseband signals in a direct conversion receiver having a plurality of baseband signal channels, comprising the steps of:

• filtering said baseband signals using matched active low-pass filters cornered at a frequency 1.2-5 times the audio bandwidth of the receiver which are operational for substantially supressing off-channel signals and thereby helping to preserve the dynamic range of the receiver;
• filtering said baseband signals using matched quasi-digital or digital filters cornered so as to define the audio bandwidth of the receiver and which are operational for suppressing all off-channel signals including signals proximate to said bandwidth but which are characterized by limited dynamic range;
• and
• filtering said baseband signals using matched LC filters cornered at a frequency 10-100 times the audio bandwidth of the receiver which are operative for suppressing RF mixer products and off-channels signals;
• wherein said active filter is cornered at approximately 33 kHz and said quasi-digital or digital filter is cornered at approximately 10 kHz.

In a direct conversion receiver having a plurality of baseband signal channels carrying separate baseband signal components, the improvements comprising:

• digital signal processing means controlled in accordance with a real time low-pass filtering software program characterized by sample weighting factors for processing said baseband components and defining the audio bandwidth of the receiver;
• means for varying said audio bandwidth by adjusting said weighting factors in accordance with the operational mode of the receiver;
• and
• a plurality of active low-pass filters one of which is installed on each of said signal channels prior to said digital signal processing means and which are cornered at a frequency greater than said audio bandwidth and are operational for suppressing near off-channel signals.