Figure 1: Basic Block Diagram of a Cathode Ray Oscilloscope
The above figure shows the basic block diagram of a cathode ray oscilloscope. This block diagram is divided into three distinctively unique systems:
The Vertical Deflection System: It basically consists of an input attenuator, vertical amplifier and the delay line and is generally responsible for the formation of the vertical component of the signal.
The Horizontal Deflection System: It consists of the trigger circuit, the internal time base generator and the horizontal amplifier and is responsible for the formation of the horizontal component of the signal.
The Cathode Ray Tube: Which basically is a device that basically displays the signal in terms of the variation of the magnitude in terms of a constant ti me period? The input to the CRO is given across the ground to the input attenuator. The input attenuator basically consists of a resistor ladder network that is used to attenuate the signals before being applied to the vertical amplifier. The vertical amplifier takes the output of the attenuator circuit and amplifies it to the levels that create a considerable potential difference across the vertical defle ction plates (Y-Y plates).
The vertical amplifier consists of cascaded sections of unity gain and push pull amplifiers and basically provides the high impedance that isolates the amplifier form the attenuator. The phase inverter in the above circuit provides two anti-phase signals that are required to operate the push pull amplifier. The push pull output amplifier delivers equal signal voltages of opposite polarity to the vertical deflection plates of the CRT. Pre Amplifier
Main Amplifier
Phase Inverter
From I/P
A
Attenuator Driver
Output
Amplifier
Amplifier
To Y-Y Plates
Figure 2: Block Diagram of a Vertical Amplifier
As it can be seen from figure 1 a part of the signal of the vertical amplifier is given to the horizontal deflection system at the input of the trigger circuit. The trigger circuit basically consists of a Schmitt trigger and a RC differentiator that ensure that the horizontal deflection system is triggered into action along with the output of the vertical amplifier. The output of the trigger circuit basically forces the internal time base generator to start its next cycle. The time base generator in the horizontal deflection system is basically a UJT based sweep signal generator which basically generates a voltage signal that is a function of a time period. A simple schematic circuit of the internal time base g enerator is shown below.
VBB
VBB Sink Pulse Input
R2 VP
R1
CT
V0 Saw tooth Output VE (min)
Figure 3: Continuous Sweep Time base Generator
Figure 4: Sawtooth output wave form
When the power is first applied, the UJT is off and C T charges exponentially through RT. The UJT emitter voltage VR rises towards VBB and when VE reaches the peak voltage VP as shown in the figure 4 the UJT triggers ON. This provides a low resistance discharge path and the capacitor discharges rapidly. The emitter voltage VE reaches the minimum value rapidly and the UJT goes off. The capacitor recharges and the cycle repeats. The trigger pulse enables the frequency to be exactly equal to the input signal frequency so that the pattern does not drift on the screen. The output of the internal time base generator is given to the horizontal amplifier which in construction is similar to that of the vertical amplifier, and its utility is to amplify the output of the internal time base generator to the levels that can create a considerable potential difference across the horizontal deflection plates or the X-X Plates.
NOTE: The potential difference across the vertical deflection plates is a function of the magnitude of the input signal where as the potential difference across the horizontal deflection plates is a function of the time period. As it can be seen from figure 1 the output of the vertical amplifier is ready to be applied to the vertical deflection plates, but since the horizontal deflection system, consists of wave shaping and wave modifying circuits which require some time to give their output, the potential difference across the vertical deflection plates appears before the potential difference across the horizontal deflection plates as the output of the vertical amplifier triggers the horizontal deflection system. This results in the loss of the leading edge of the signal, which in case of non-periodic signals would lead to loss of information. In order to avoid the loss of the leading edge of the signal a delay line circuit is introduced in the vertical deflection system. The delay line delays the signal to applied to the vertical deflection plates for a small amount of time (typically between 90 to 120 ns) so that the signals across the vertical and horizontal deflection plates appear across the respective plates at the same time.
NOTE: The utility of the delay line circuit in a CRO is to see that the outputs across the vertical and horizontal deflection plates appear at the same time. The Heart of the CRO is the Cathode Ray Tube. It basically is a vacuum evaporated glass tube which is maintained at a pressure of 1 torr. It consists of a indirectly heated cathode which emits electrons. The cathode is generally coated with oxides of barium and strontium in order to obtain high emission of electrons. The electrons emitted by the cathode come out of the control grid in the form of a slightly divergent beam. The intensity of the beam of electrons coming out of the heated cathode depends upon the emission of the electrons by the cathode. The control grid is basically a nickel cup which is maintained at a potential. The potential across the control grid is controlled by the INTENSITY control on the front panel of the cathode ray oscilloscope. The electron beam coming out of the control grid passes through a set of three electrodes namely the
Pre Accelerating Anode, the Focusing Anode and the Accelerating Anode .
These set of three anodes form the Electrostatic Focusing Mechanism of the cathode ray oscilloscope. The pre accelerating anode, the focusing anode and the accelerating anode are coaxial with respect to each other. The pre accelerating anode and the accelerating anode are placed at the same potential while the focusing anode is placed at a lower potential. Due to the non uniform field present at the two ends of the focusing anode, the beam of electrons is focused towards the center of the tubes axis. The potential difference across the set of three anodes is controlled by the FOCUSING control on the front panel of the CRO All the components from the heated cathode to the accelerating anode put together are known as the
Electron Gun Assembly of the cathode ray oscilloscope whose utility is to produce a focused beam of electrons that are travelling towards the screen wi th a high velocity. This beam of electrons first pass through the vertical deflection plates and are deflected I the vertical plane due to the potential difference e xisting across them. The deflection of the beam of ele ctrons being proportional to the potential difference across the YY plates which in turn is a function of the magnitude of the input signal. This deflected beam of electrons then passes through the horizontal deflection plates and is spread in the horizontal plane due to the potential difference across the XX plates. This spreading being proportional to the potential difference across the XX plates which in turn is a function of a time peri od. When this distorted beam of electrons goes and hits the internal surface of the CRT which is coated with a material that exhibits the property of flurocess, a light pattern is formed on the other surface depicting the variation of the magnitude of the input signal with respect to a time period. The electrons bombarding the internal surface of the CRT screen produce a cloud of secondary electrons and these electrons are collected by an aques solution of graphite (AQUADAG) which is coated o n the internal surface of the CRT walls there by making the CRT electrically stable and free from noise.
NOTE: An Electrostatic deflection method is used in a cathode ray tube in a CRO in contrast to the electromagnetic deflection technique used in television picture tubes as in a CRT the area of sweep is small, the image is generally formed at the center of the screen and the sensitivity does assume a large significance.
