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AP3302 Pt3 Contents

AP3302 Pt3 Section 1 Contents

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AP 3302 Pt. 3

Section 1



bright spot on the trace instead of as a blip. In this case the receiver output is applied as short positive going pulses of voltage to the grid of the c.r.t. We know that the brightness control of an oscilloscope varies the voltage on the grid of a c.r.t. thus altering the brightness of the display. Therefore a target echo applied as a positive pulse to the grid of the c.r.t. increases the brightness of the spot on the trace for the duration of each pulse (Fig l0b). This 'bright-up' form of display is known as intensity modulation and it can also be obtained by applying negative pulses to the cathode of the c.r.t.

As soon as the spot has covered the full width of the c.r.t. screen it is returned to the left-hand side and remains there until the transmitter 'fires' again. The whole process is then repeated. A radar transmitter may produce many hundreds of pulses per second. If it 'fires' 500 times in one second (a typical figure) the spot moves over the trace 500 times each second. This recurring movement of the spot ensures that a bright trace is built up and maintained. The repetition rate is too fast for the eye to be able to follow the separate movements of the spot and we see the trace as an apparently steady line on the screen, with the target echoes moving slowly along the trace as the range of each target changes. This type of display is known as a type A display and it shows range only. Fig 11 illustrates the sequence of events in each cycle of operation.

Changing the Range Scale

For the display described in the preceding paragraphs the maximum range which can be shown on a six-inch c.r.t. is 30 miles. If, on the same c.r.t., we want to check for echoes from a range greater than 30 miles we reduce the speed of the spot, i.e. the velocity of the timebase is reduced (see later). For example, if the speed is halved the spot travels only one inch in the time taken for an echo to return from a target at ten miles range, and the maximum range which can then be shown on a six-inch c.r.t. is 60 miles.

Notice in Fig 11 that the time during which echoes may be displayed on the c.r.t. is limited to the interval between transmitted pulses. The number of pulses that can be transmitted per second, i.e. the pulse recurrence frequency or the pulse repetition frequency (p.r.f.), must therefore be such that the interval between pulses is sufficiently long to allow the spot to complete each trace and return to the starting point.


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