Time base generator

It has been mentioned earlier that the time domain oscilloscopes require a sweep generator that is linear with time for the x-axis display. The motion of spot on the screen from extreme left to extreme right is called sweep.

The generator which generates a waveform which is responsible for the movement of spot on screen horizontally is called time base generator or sweep generator. The sweep circuit along with the display gating functions is called time bases.

The linear sweep moves the spot from left to right while the movement of spot from right to left is not visible. This portion of waveform generated by time base is called flyback or retrace. During this time, the cathode ray tube is blanked.

The time base generator also controls the rate at which the spot moves, across the screen. This rate is to be adjusted from front panel control.

Why sweep generator is called time base generator?

All the time dependent waveforms need x-axis to be calibrated as time axis. The sweep generator produces the movement of spot on screen such that it acts as a time axis or time base for the waveforms to be displayed. Hence the sweep generator is also called time base generator.

The two front panel controls which are used to control rate and duration of time base waveform are time/division and time variable control.

Note: the trigger circuit ensures that the horizontal sweep starts at the same point of the vertical input signal.

Requirements of time base:

The time base requirements are:

1. Sweep time variations from 10nsec to 5 sec per division.
2. Time accuracy is better than 3%.
3. Linearity better is better than 1% across the cathode ray tube.
4. Ten times expansion in the horizontal amplifier which allows 1 nsec per division displays for very high speed transients.
5. The speed of the spot should be constant across the entire screen.
6. The spot should be invisible while tracing from right to left and should be visible only from left to right.

Basic principle of time base generator

The basic sweep generator uses the charging characteristics of a capacitor to generate linear risetime voltages. Linearly increasing ramp which becomes zero with very short duration of time ensures that the spot is visible from left to right and invisible from right to left.

The image below shows a capacitor charged from a constant current source.

When switch S1 is closed, S2 is open and capacitor charges to produce linear ramp at the output. The sweep rate can be controlled by changing the value of capacitor or charging current.

Reaching to the maximum value of ramp voltage, the switch S2 is closed and S1 is open. Thus capacitor gets discharged through the resistance R. this is called flyback or retrace. The time t1 is called sweep time. The circuit is a sort of relaxation oscillator which generates saw tooth waveform. But this circuit has less accuracy. The bootstrap techniques allow much greater linearity but he techniques are much more costly.

During the sweep time, the spot moves from left to right. During retrace, the screen is blanked and spot comes back to its starting level but its movement from right to left is invisible.
Practically a trigger circuit is associated with the time base generator. This circuit generates a trigger pulse which activates the time base generator to produce a ramp.

Now when one cycle, sweep and retrace is completed then time base generator takes certain time to start the next cycle. This time is divided into two types as hold off time and waiting time.

Hold off time:

Through the tigger circuit applies the pluse immediately after completion of cycle, the time base generator takes some time to start the ramp. This time is required to stabilize the flyback circuitry. This time is called hold off time.

Waiting time:

Now when trigger pulse is generated b trigger circuit, the pulse has to cross certain reference level so as to activate the time base generator. This reference level is called trigger threshold.

Now after the end of hold off period, though circuit is ready, due to trigger threshold crossing, the pulse takes some time to activate time base generator. So time required by the triggering pulses to cross the trigger threshold is called waiting time.

Fluorescent Screen of CRT

The light produced by the screen does not disappear immediately when bombardment by electron ceases, i.e., when the signal becomes zero. The time period for which the trace remains on the screen after the signal becomes zero is known as persistence. The persistence may be as short as a few microseconds, or as long as tens of seconds or even minutes.

Medium persistence traces are mostly used for general purpose applications.

Long persistence traces are used in the study of transients. Long persistence helps in the study of transients since the trace is still seen on the screen after the transient has disappeared.

Short persistence is needed for extremely high speed phenomena.

The screen is coated with fluorescent material called phosphor which emits light when bombarded by electrons. There are various phosphors available which differ in color, persistence and efficiency.

One of the common phosphor is willemite, which is zinc, orthosilicate, ZnO+SiO2, with traces of manganese. This produces the familiar greenish trace. Other useful screen materials include compounds of zinc, cadmium, magnesium and silicon.

The kinetic energy of the electron beam is converted into both light and heat energy when it hits the screen. The heat so produced gives rise in phosphor burn which is damaging and sometimes destructive. This degrades the light output of phosphor and sometimes may cause complete phosphor destruction. Thus the phosphor must have high burn resistance to avoid accidental damage.

Similarly the phosphor screen is provided with an aluminum layer called aluminizing the cathode ray tube. This is shown in image below.

These Aluminizing layers serve three functions:

1. To avoid buildup of charges on the phosphor which tend to slow down the electrons and limits the brightness.
2. It serves as a light scatter. When the beam strikes the phosphor with aluminized layer, the light emitted back into the tube is reflected back towards the viewer which increases the brightness.
3. The aluminum layer acts as a heat sink for the phosphor and thus reduces the chances of the phosphor burning.

Phosphor screen characteristics

1. Many phosphor materials having different excitation times and colors as well as different phosphorescence times are available.
2. The type P1, P2, P11 or P31 are the short persistence phosphors and are used for the general purpose oscilloscopes.
3. Medical oscilloscopes require longer phosphor decay and hence phosphors like P7 and P39 are preferred for such applications
4. Very slow displays like radar require long persistence phosphor to maintain sufficient flicker free picture. Such phosphors are P19, P26 and P33.
5. The phosphors P19, P26, P33 have low burn resistance. The phosphors P1, P2, P4, P7, P11 have medium burn resistance while P15,P31 have high burn resistance.

Why P31 is commonly used?

Out of these varieties, the materials P31 is used commonly for general purpose oscilloscopes due to following characteristics:

1. It gives color to which human eye response is maximum.
2. It gives short persistence required to avoid multiple image display.
3. It has high burn resistance to avoid the accidental damage.
4. Its illumination level is high.
5. It provides high writing speed.

Note: the light output of a fluorescent screen is proportional to the number of bombarding electrons, i.e., to the beam current.

Cathode Ray Tube - Deflection system

This Post Deflection system of CRT is a continuation of my previous post Electron Gun of CRT. This post completely covers the Deflection system of the CRT. When the electron beam is accelerated it passes through the deflection system, with which beam can be positioned anywhere on the screen.

The deflection system of the cathode-ray-tube consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. One of the plates in each set is connected to ground (0 V). to the other plate of each set, the external deflection voltage is applied through an internal adjustable gain amplifier stage. To apply the deflection voltage externally, an external terminal, called the y input or the x input, is available.

As shown in the image below, the electron beam passes through these plates. A positive voltage applied to the y input terminal (V y) causes the beam to deflect vertically upward due to the attraction forces, while a negative voltage applied to the y-input terminal will cause the electron beam to deflect vertically downward, due to the repulsion forces.

Similarly, a positive voltage applied to X-input terminal(V x) will cause the electron beam to deflect horizontally towards the right; while a negative voltage applied to the X-input terminal will cause the electron beam to deflect horizontally towards the left of the screen. The amount of vertical or horizontal deflection is directly proportional to the corresponding applied voltage.

When the voltages are applied simultaneously to vertical and horizontal deflecting plates, the electron beam is deflected due to the resultant of these two voltages.

The face of the screen can be considered as an X-Y plane. The (X,Y) position of the beam spot is thus directly influenced by the horizontal and the vertical voltages applied to the deflection plates Vx and Vy respectively.

The horizontal deflection (X) produced will be proportional to the horizontal deflecting voltage, Vx, applied to X-input.

X = KxVx

Where, Kx is constant of proportionality.

The deflection produced is usually measured in cm or as number of division , on the scale, in the horizontal direction.

Then Kx = x/Vx where Kx expressed as cm/volt or division/volt, is called horizontal sensitivity of the oscilloscope.

Similarly, the vertical deflection (y) produced will be proportional to the vertical deflecting voltage, Vy, applied to the y-input.
Y= KyVy

Ky=y/Vy and Ky, the vertical sensitivity, will be expressed as cm/volt, or division/volt.

The schematic arrangement of the vertical and the horizontal plates controlling the position of the spot on the screen is shown in the figure.

The values of vertical and horizontal sensitivities are selectable and adjustable through multi positional switches on the front panel that controls the gain of the corresponding internal amplifier stage. The bright spot of the electron beam can thus trace (or plot) the X-Y relationship between the two voltages, Vx and Vy.

Cathode Ray Tube (CRT)

The cathode ray Tube (CRT) is the heart of the C.R.O. the CRT generates the electron beam, accelerates the beam, deflects the beam and also has a screen where beam becomes visible as a spot. The main parts of the CRT are:

1. Electron Gun
2. Deflection system
3. Fluorescent screen
4. Glass tube or envelope
5. Base

A schematic diagram of CRT, showing its structure and main components is shown in the figure below.

Since I want to explain each and every parts of the CRT in detail, I will spilt this topic into 3 parts.