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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Timebase Principles

If the distance on the screen between points P and Q is four inches, then on switching from the 0-80 miles range to, say, the 0-20 miles range, the spot must still move four inches across the screen in the course of each sweep; but now the sweep time must be 20 x 10.7 = 215 uS. Thus when we switch from one range to another we must make the spot move at a different speed; as the spot movement is in a specified direction we say that we change the sweep velocity.

At the end of each sweep the spot is on the right-hand side of the screen at point Q (Fig. 5). In the interval between the end of one sweep and the beginning of the next we must make the spot return to its starting point at P. This return movement is called the fly-back. We do not want to see the flyback on the screen and so must apply either a negative-going blanking pulse to the c.r.t. grid or a positive-going pulse to the c.r.t. cathode during the flyback time to cut off the electron beam and blank out the screen.

As we saw in AP 3302 Part lB (p172) the movements of the spot across the screen to form the trace, and also the flyback, are repeated continuously and the repetition rate is made sufficiently high so that the eye sees the movement as a continuous trace on the screen.

Each sweep commences at the instant the transmitter fires, and to find the range of a target we really measure the time interval between the beginning of each sweep and the reception of the reflected pulse. By making the spot move over a definite path at a known velocity we say that we are providing a timebase against which the range of a target can be measured. The spot is made to move over the required path at the correct velocity by a specially-shaped waveform of voltage or current which we apply to the deflecting system of the c.r.t. The circuit which produces this waveform is called a timebase generator.

The actual shape of the timebase waveform depends upon whether an electrostatic or a magnetic c.r.t. is being used. For the moment we shall consider only the electrostatic type.


Timebase Deflection in Electrostatic CRT

The deflecting system in an electrostatic c.r.t. is shown in Fig. 6. As we already know, the electron beam is deflected from its central path by electric fields between the plates of either pair.

For certain practical reasons we usually apply the timebase waveform to the X plates and the signal to be examined to the Y plates. Let us now see what the shape of the timebase waveform must be if we wish to produce the type A display considered earlier.

The simplest way of obtaining the required timebase is to use single-ended deflection. This means that one of the X plates (say X1) is connected permanently to earth and the timebase waveform is applied to the other plate X2. In practice, in radar systems, balanced deflection is normally used. In this, the timebase voltage is split into two equal deflecting voltages of opposite polarity for application to both deflecting plates. However, single-ended deflection is easier to understand and, although it introduces a certain amount of distortion, it is the method we shall consider here. Balanced deflection is dealt with later in Chapter 16 (p219).


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