Q. What are the different types and uses of delay line in CRO?

OR

Why is a delay line used in the vertical section of the oscilloscope?

Sol. All electronic circuitry in the oscilloscope (attenuators, amplifiers, pulse shapers, generators and indeed the circuit wiring itself) causes a certain amount of time delay in the transmission of signal voltages to the deflection plates. Almost all of this delay is created in circuits that switch, shape or generate. Comparing the vertical and horizontal deflection circuits in the oscilloscope block diagram of. We observe that the horizontal signals (time base, or sweep voltage) is initiated, or triggered by a portion of the output signal applied to the vertical CRO plates. Signal processing in the horizontal channel consists of generating and shaping a trigger pulse (trigger pick off) that starts the sweep generator, whose output is fed to the horizontal amplifier and then to the horizontal deflection plates. This whole process takes time on the order of 80 ns. To allow the operator to observe the leading edge of the signal waveform, the signal drive for the vertical CRT plates must therefore be delayed by at least the same amount of time.

             This is the function of the vertical channel, so that the signal voltage to the CRT plates is delayed by 200 ns and the horizontal sweep is started prior to the vertical deflection as shown.

  Although the delay line can appear almost any where along the vertical signal path, the trigger pickoff must precede the delay line. There are basically two kinds of delay line the lumped-precede the delay line and the distributed-parameter delay line.

Lumped-parameter Delay Line: The lumped-parameter delay line consists of a number of cascaded symmetrical LLC networks, such as the so called T-section of fig.

         If the T-section is terminated in its characteristic impedance then, by definition, the impedance looking back into the input terminals is also This condition of termination gives the T-section the characteristics of a low-pass filter whose attenuation and phase shift are a function of frequency and whose passband is defined by the frequency range over which the attenuation is zero. The upper limit of the passband is called the cut off frequency of the filter, given by

   If the spectrum of input signal V, consists of frequency much less than the cutoff frequency, output signal will be a faithful reproduction of but delayed by a time.

A number of T sections, cascaded into a so called lumped-parameter delay line, increase the total delay time to.

     Where n is the number of cascaded T-sections. Because of the sharp cutoff frequency of the lumped-parameter delay line, amplitude and phase. Distortions become a problem when the frequency of the input signal increases.

  The application of a step-voltage input example, which contains high-frequency components (odd harmonics), causes an output voltage that suffers from transient response distortion in the form of overshoot and ringing, as shown in.

    The kind of response can be improved to more closely resemble the original step voltage input by modifying the design of the filter sections into, for example, m-derived sections. The m-derived section is a popular circuit that used mutual coupling between the two inductors of the T-section. It is important to match the delay line as closely as possible to its characteristic important to match the delay line as closely as possible to its characteristic impedance at both input and output ends. This requirement often leads to complex termination circuitry in an effort to optimize the balance between amplitude and phase distortion and to obtain better transient response.

       A practical delay line circuit in an oscilloscope is driven by a push-pull amplifier and then consists of a symmetrical arrangement of cascaded filter section as in fig.

          Optimum response of the delay line requires precise proportioning of the L and C components in each section; the variable capacitors must be carefully adjusted to be effective.

     Distributed Parameter Delay Line: The distributed parameter delay line consists of a specially manufactured coaxial cable with a high value of inductance per unit length. For this type of delay line, the straight center conductor of the normal coaxial cable is replace with a continuous coil of wire, wound in the form of a helix on a flexible inner core. To reduce eddy currents, the outer conductor is usually made of braided insulated wire, electrically connected at the ends of the cable. Construction details are shown schematically in fig.

            The inductance of the delay line is produced by the inner coil and it equals that of a solenoid with n turns per meter. The inductance can be increased by winding the helical inner conductor on a ferromagnetic core, which has the effect of increasing the delay time t and the characteristic impedance Z, The capacitance of the delay line is that of two coaxial cylinders separated by a polyethylene dielectric. The capacitance can be increased by using a inner dielectric spacing between the inner and outer conductors. Typical parameters for a helical, high-impedance delay line are Z=1,000and ns/m. The coaxial delay line is advantageous because it does not require the careful adjustment of a lumped-parameter line and it occupies much less space.