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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 12

Timebase Principles

To make the spot sweep from P to Q, thereby producing a horizontal trace, we make the voltage on X2 rise evenly from a negative to a positive value relative to earth. The effect of this is shown in Fig 7.

When the sweep has ended we must cause the voltage on X2 to Jail rapidly from its final positive value to its original negative value so that as quickly as possible after the sweep ends the spot is again at point P, ready for the next sweep to commence. The shape of the flyback portion of the waveform is not usually important because the c.r.t. is blanked out for the duration of the flyback time.

The complete timebase waveform which we apply to X2 is as shown in Fig 8a. Because it resembles the teeth of a saw we call it a sawtooth waveform.

The same results would be obtained if we earthed X2 and applied a negative-going sawtooth waveform to Xi, as in Fig 8b. It does not matter, therefore, whether we consider positive-going or negative-going sawtooths because both produce the same result when applied to the appropriate X plate.

The slope of the sawtooth during the sweep time determines the speed with which the spot is deflected. This determines the timebase velocity, and the steeper the slope the greater is the velocity. The amplitude of the sawtooth determines the distance through which the spot is deflected, i.e. the trace length or timebase width.

It is usual to have a timebase which at all times occupies the full width of the c.r.t. screen so as to have as large a ‘picture’ as possible. This means that the sawtooth amplitude must always be constant.

Let us assume that the required length of trace on a radar display is four inches. We have seen earlier in this chapter that if we require to measure targets at a range of up to 20 miles

 


 

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