Spectrum Economy for Filtered and Limited FSK Signals

Abstract

Where a frequency-shift keying (FSK)keyer generating a spectrum that is broad compared to that required for the information is used, considerable spectrum economy can often be obtained by passing the signal through a properly selected symmetrical bandpass filter. The signal can then be passed through a hard limiter which regenerates some of the sidebands filtered off, but not nearly all of them. This limiter renders the signal suitable for amplification by class <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</tex> , switching type, or other high-efficiency amplifiers. For low indices of modulation, an approximation can be made of the effect of a filter by assuming that the modulation signal is filtered by the low pass analogy of the bandpass filter, and that each component of the modulating wave modulates the carrier independently.