| 摘要 |
Communications systems employing frequency modulation have noise threshold levels that practically limit the minimum received signal level. This limit is particularly significant to satellite earth stations. A method that automatically and advantageously trades improved threshold levels for increased distortion by reducing the noise bandwidth of the receiver as the carrier to noise ratio approaches the threshold level is described. This method senses when noise peaks approach, equal or exceed the FM carrier level, converts the rate at which these noise peak events occur into a control voltage whose average magnitude controls the bandwidth of a filter inversely to the noise peak event rate. When this method is applied to FM receivers with threshold extending FM feedback loops, the control voltage also increases the loop feedback factor thereby reducing distortion and better optimizing the distortion-threshold trade-off. Illustrative designs for a voltage controlled filter, a noise peak event indicator and controller are described.
|
