Delay Line in CRO

Delay Line in CRO : 


In the circuit of the CRO we have several stages like the attenuator, amplifier, pulse shaper, sweep generator etc. When an external signal is applied to the vertical input terminals, it reaches the vertical deflecting plates and starts deflecting the beam in the vertical direction (up and down). As explained earlier, if the time base voltage also appears at the characteristic instant, of commencement, of the cycle of the applied voltage, on the horizontal deflecting plates, the wave form can be viewed completely.

The horizontal deflection section of the CRO has the sweep generator, the trigger circuit, the horizontal amplifier stages in which the sweep signal is generated, shaped, amplified and presented to the horizontal deflecting plates. A delay is caused by this circuity in transmitting the sweep signal to the horizontal deflecting plates. The delay that is caused by the horizontal deflecting circuits is about 80 ns. In order to present the leading edge of the signal under observation, the signal to the vertical deflecting plates must be delayed by the same time atleast. The delay line provides the required delay to the vertical deflecting voltage.

Delay line can theoretically be inserted at any place in the vertical deflecting system. One point to be noted here is that the trigger pick-off must precede the delay line. (the trigger circuit is explained in horizontal section)

There are two types of delay lines used with CROs. The lumped parameter delay line and the distributed parameter delay line, are the two types.

A block diagram showing the position of the delay line is given below. Fig. 3.16.
Position of Delay Line
In the above block diagram the delay line is placed after the main amplifier. The delay line adds a time delay of 200 ns to the vertical signal. Trigger pick-off is obtained from the output of the main amplifier. The delay produced by the horizontal section is 80 ns. Hence the time base signal precede the vertical signal in appearing at the horizontal deflecting plates than the vertical signal appearing at the vertical deflecting plates thus serving the purpose of delaying the vertical signal to the vertical deflecting plates. 

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