Experiments in Pulse Communication With Filtered Sidebands

A ny RF communications system that uses carrier on/off pulses can be designed to have better receiver sensitivity by using negative or zero group delay narrow band filters in the receiver. These filters greatly reduce the bandwidth normally required by removing the sidebands, while retaining the necessary information in the pulsed on/off carrier. SNR improvements greater than 40 dB are being obtained by means of bandwidth reduction alone (Processing Gain). The method is applicable to RADAR, TACAN, IFF, DME, PAM, PWM, UWB and UNB data methods. The information presented here enables any experimenter to duplicate the displayed results. There are no tricks or secrets, but readers should remember that this method only applies to on/off pulse communications, where the side-bands do not contain modulation information. Figure 1 shows the spectrum of a pulsed signal applicable to RADAR and other pulse methods. For all amplitude modulation methods: I t = I m (cos ω c t) + 0.5K(cosω c + F)t + 0.5K (cos ω c – F) (1) Where ω c is the carrier and F is the Fourier transform of the modulating signal. K is the modulation index, which is presumed to be 1.0 for pulse modulation. This is an AM sequence for which all components are required to initially produce amplitude modulation. The carrier is continuous for audio modulation, but is keyed on/off for pulse modulation. The Fourier spectrum for a pulse using rectangular pulse modulation is: F(t) = A peak (t/2T p) [ 1/2 +(2/π)cosπ(nt/2T p) – (2/2π)cos2π(nt/2T p) + (2/3π)cos3π(nt/2T p) – (2/4π)cos4π(nt/2T p) + (2/5π)cos5π(nt/2T p) ... ] (2) which nulls when nt = 1.0, thus the DC component can be ignored. (1/T p) is the repetition rate and " t " is the pulse width. Only the fundamental is of interest because all harmonics are removed by the filters. This is a sinx/x waveform; all portions of equation (2) have a common form: (Y)cos π(nt/2T p). Figure 1 · Sinx/x Pulse spectrum for a pulse width of 400 nanoseconds with a repetition rate of 200 kHz. Most pulse systems would not use a rate this high.