Reduction of Adjacent-Channel Interference from On-Off Keyed Carriers

The well-known reduction of sideband interference by filtering in the keying circuit of transmitter is examined in detail to determine the amounts of sideband reduction which can be obtained with practical filters and still retain desirable keying waveforms. It is found that reductions in interfering bandwidths by 1OO to 1 or more over unfiltered keying at sideband levels of 70 db or more below the carrier are possible by employing filters having specified frequency response characteristics in the keying circuit of linear transmitters. Design criteria are given for a filter which results in the greatest practical interference reduction and also retains a very desirable transient response. The overshoot when employing this type of filter is less than two per cent of the mark to space voltage and at the same time a high out-of-band rejection ratio is obtained. A practical design of a relatively simple 4 section low-pass filter which meets these requirements is presented. The effects of receiver linearity on the amount of bandwidth reduction realizable are also considered.