The Radar Display as a Linear Filter

When an intensity-modulated cathode ray tube is fed by an electronic gamma-correction circuit to compensate for the tube power law, the over-all system may be interpreted as a linear filter. It is shown that if the electron distribution within the beam is Gaussian, then (for at least some applications) the corresponding equivalent filter has a Gaussian amplitude characteristic and zero phase shift. Thus, a recent radar tube, the 5FP14A, has a frequency response given by exp [¿7.07f²] when used on a range of 30 nautical miles. (Here f is expressed in megacycles.) A similar result applies to this tube when used as a flying-spot scanner. If the radar video amplifier is to produce no degradation of this display it must be flat to a frequency 1.2v/w, where v is the sweep velocity and w the spot size. The desirable pulse duration is equal to the time required to sweep past a spot. Simple modifications of these relations are necessary if ground-range sweeps are used.