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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Square Waves applied to LR and LC circuits


Short, Medium and Long LR Circuits

With LR circuits we compare the time constant L/R seconds against the pulse duration T of the input square wave in the same way that we did with CR circuits. Thus, a short LR circuit is one where the time constant L/R is 0.1T (or less); a medium LR circuit is one where L/R is between 0.1T and l0T; and a long LR circuit is one where L/R is greater than l0T.

The waveforms of VL and VR, when a square wave of voltage is applied to short, medium and long LR circuits, are shown in Fig 3. These have the same shape as those of CR circuits but the corresponding waveforms appear across different components. In general the waveform of VR in a LR circuit is similar to that of VC in a CR circuit; VL in a LR circuit is similar to VR, in a CR circuit. For example, positive and negative-going pips appear across the resistor of a short CR and across the inductor of a short LR circuit. (Compare the short LR waveforms of Fig 3 opposite with those for the short CR circuit of Fig 7 on p 71).

Note that LR circuits may be used as differentiating and integrating circuits in much the same way that CR circuits are used. A short LR circuit in which the output is taken across the inductor acts as a differentiating circuit. A long LR circuit in which the output is taken across the resistor acts as an integrating circuit.

Effect of Applying a Square Wave to a Parallel LC Circuit

Many radar circuits contain conventional parallel tuned circuits of the type discussed in Part 1 of these notes. These tuned circuits may form the anode load of a valve or the collector load of a transistor and, since the input to the stage may be a pulse of voltage, it is important to discuss the effect of applying a square wave to such a circuit.

Fig 4a shows a parallel tuned circuit connected as the anode load of a valve which is being cut on and off by a switching square wave at the grid. When the valve is cut on, the supply current is steady at a value Ia determined by the circuit constants; when the value is cut off, the supply current is zero (see Fig 4b).


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