trapezoidal sweep voltage waveform

EQUIPMENTS:

Pulse generator Power supply (0-20V DC) C.R.O. Circuit Board

COMPONENTS:

Transistor :BC 147 Resistors :100K, 4.7K, 100 ohm Capacitors :0.1 uF(No 2)

THEORY: Scanning of the electron beam requires saw tooth current passing through the deflection coils

To obtain saw tooth current in ckt. Shown below

i = kt

V = L di/dt + Ri

= Lk + Rkt

= k1 + k2t -> Trapezoidal Waveform

CIRCUIT DIAGRAM:

Component Value Selection:

Horizontal deflection frequency = 15625 Hz

The trace period = 52 uS

RC min = 3* trace period

= 3 * 25

= 156 uS

Let c=0.1 micro S => Rmin = 1.56 K

Here we have chosen R=4.7 K for better linearity.

The height of the pedestal = Vcc * R/(R+Rc)

= [100/ (100+4700)] * Vcc

= 0.02 Vcc

Thus we require trapezoidal voltage for the deflection coils as shown below.

As usual very linear voltage waveform is generated by charging a capacitor with the help of constant source. Here Vcc and RC provide current for charging and transistor provides low impedance path for discharging.

If charging and discharging times are 1/3 rd of the respective RC time constants then the distortion in linearity is negligible.

Here we have used the scheme in which transistor is off and capacitor charges through RC. At proper time transistor is made on. So, capacitor discharges through it. Current frequency is obtained as the transistor switching is controlled by external function generator generating rectangular waveforms with correct duty cycle.

INPUT AND OUTPUT WAVEFORMS:

DEFLECTION CURRENT WAVEFORMS:

Fig. (a) Illustrates the required nature of current in deflection coils. As shown in Fig, it has a linear rise in amplitude which will deflect the beam at uniform speed without squeezing of spreading the picture information. At the end of ramp the current amplitude drops sharply for a fast retrace or fly back. Zero amplitude on the saw tooth waveform corresponds to the beam at center of screen. The peak-to-peak amplitude of the saw tooth wave determines the amount of deflection from the center. The electron beam is at extreme left (or right) of the raster when the horizontal deflecting saw tooth wave has its positive (or negative) peak. Similarly the beam is at top and bottom for peak amplitude of vertical deflection saw tooth wave. The saw tooth waveforms can be positive or negative going, depending on the direction of windings on the yoke for deflecting the beam from left to right and top to bottom.

DRIVING VOLTAGE WAVEFORM

The current which flows into the horizontal and vertical deflecting coils must have a saw tooth waveform to obtain linear deflection of the beam during trace periods. However, because of inductive nature of the deflecting coils, a modified saw tooth voltage must be applied across the coils to achieve a saw tooth current through them.

The circuit of a deflecting coil consisting of a resistance R in series with a pure inductance L, where R includes the effect of driving source (internal) resistance. The voltage drops across R and L for a saw tooth current, when added together would give the voltage waveforms that must be applied across the coil. The voltage across R has the same saw tooth waveform as that of current flowing through it. The voltage across L depends on the rate of change of current and the magnitude of inductance. A faster change in current produces more self induced voltage. For the constant change in current, the voltage is constant.

As a result, VL in fig (b). Is at alternately low level during trace time, but because of fast drop in current across L during the retrace period, a sharp volt peak or spike appears across the coil. The polarity of fly back pulse is increasing. Therefore, a saw tooth current in ‘L’ produced a rectangular voltage. So, that to produce a saw tooth current in an inductor, a rectangular voltage should be applied across it. When voltage drops across R and L are added together, the result is a trapezoidal waveform. Thus to produce a saw tooth current in a circuit having R and L in series which in case under consideration represents a deflection coil a trapezoidal voltage must be applied across it.

For a negative going saw tooth current, the resulting trapezoidal will naturally have an inverted polarity.

For linear deflection, a trapezoidal voltage wave is necessary across the vertical deflecting coils. The resulting voltage waveform for the horizontal yoke will look closer to a rectangular wave shape, because voltage across the inductor over rides significantly the voltage across resistance on account of higher rate (15625) of rise and fall of coil current.

GENERATION OF DRIVING VOLTAGE WAVEFORMS:

Saw tooth voltage is usually obtained as the voltage output across a capacitor that is charged slowly employing a large time constant to generate the period and then quickly discharge through a short time constant circuit to obtain the retrace period. The initial exponential rise in voltage across the capacitor is linear and thus alternate charging and discharging of the capacitor at the rate of deflection frequency results in saw tooth output voltage across it. This is illustrated in fig. (c) Where Cs is allowed to charge through a large resistance Ri from dc (B+) source. The charging time is controlled by switch ‘S’ which is kept open during the trace period at the deflection frequency. At the end of trace, switch ‘S’ is close for a time equal to the retrace period, and the capacitor discharges quickly through a small resistance R2.

Actually switch ‘S’ represents a transistor that can be switched ‘ON’ or ‘OFF’ at the desired rate. When the transistor is in cut-off state, it corresponds to ‘OFF’ position of the switch. In the ON state during which the active device is allowed to go into saturation, the transistor conducts heavily allowing the capacitor to discharge through its very low internal resistance which corresponds to resistor R2 shown in the series with the switch. In this application the transistor is called a discharge device and capacitor C is often referred as ‘Saw tooth’ capacitor or ‘sweep’ capacitor.

As mentioned earlier, the trace voltage should rise linearly. For this, only linear part of the exponential volt-time characteristics is used. To achieve this, the time constant RC of the circuit should at least be thrice the trace period. Some results can be achieved by employing a higher B+ voltage. The wave shape in fig illustrates the effect of charging time constant RC and source voltage B+ on linearly and magnitude of saw tooth voltage.

TRAPEZOIDAL VOLTAGE GENERATION:-

As explained earlier, it is often necessary to modify the saw tooth voltage to some form of trapezoidal voltage before feeding it to the output stage for obtaining linear deflection. Fig. (d) Shows a basic circuit for generating such voltage.

It is the same circuit discussed earlier but employs a transistor as discharge switch and has a small resistance Rp (peaking resistance) in series with the saw tooth capacitor Cs. The transistor which is biased to cutoff by battery Vbb is driven into saturation by incoming, larger but narrow positive pulses. It just acts as a discharge switch to produce a fast retrace. During long intervals in between positive pulses, Cs charges towards B+ through a large resistance Ri to produce trace voltage. Since the value of Rp is small compared to Ri, voltage developed across it is quite small while Cs charges. However on arrival of positive pulses, Q1 goes into full conduction, thus providing a very low resistance path (small RC) for the capacitor to discharge. The high discharge current which also flows through Rp develops a large negative voltage pulse across it. This is illustrated by the waveform drawn along Rp in fig.(d) as shown by another waveform, the skipped voltage across Rp adds to saw tooth voltage across Cs to produce a

trapezoidal voltage Vo, between point ‘A’ and ground. Note that charge and discharge periods must be in accordance with the synchronized vertical and horizontal scanning rates. This function is assigned to the vertical and horizontal oscillator.

PROCEDURE:

1. Connect the circuit as shown in figure.2. Give square wave of 20 mV and 15625 Hz from pulse generator as an input.3. See output waveform on C.R.O.

CONCLUSION:

The waveforms are trapezoidal with correct frequency and corresponding timings. Non-linearity was not observed by this experiment.