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CATHODE RAY OSCILLOSCOPE
5.1ADVANTAGES OF ELECTRONIC INSTRUMENTS:
The following are the advantages of electronic instruments:
1. High accuracy.
2. Smaller in size.
3. Low power consumption.
4. High frequency range.
5. Detection of low level signals.
6. Resolution is good.
5.2 NEED OF CATHODE RAY OSCILLOSCOPE AS A LABORATORY INSTRUMENT:
A CRO which uses a cathode ray tube (CRT), is very useful and versatile laboratory instrument for measurement, display and analysis of waveforms and other phenomenon in electrical and electronic circuits. The CRO allows the amplitude of electrical signals such as voltage, current, power etc.., to be displayed primarily as a function of time. It produces a two - dimensional graph with the voltage presented at an input terminal plotted on the vertical axis and time plotted on the vertical axis and time plotted on the horizontal axis.
5.3 BLOCK DIAGRAM OF C.R.O:
1. Cathode Ray Tube:
This is the heart of CRO.
• CRT consists of electron gun, which emits electrons to strike the phosphor screen
through deflecting plates. The deflecting plates are two types, horizontal deflecting plates and vertical deflecting plates.
• The vertical deflecting plates move the beam up and down vertically.
• The horizontal deflecting plate’s moves the beam from left to right or right to left horizontally.
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2. Vertical Amplifier:
• This is a wide band amplifier used to amplify signals in the vertical section.
3. Delay Line:
• It used to delay the signal time in the vertical sections.
4. Trigger Circuit:
• The trigger circuit controls the instants at which the sweep voltage is to be applied to the horizontal deflecting plates.
• This is used to convert the incoming signal into trigger pulses so that the input signal and the sweep frequency can be synchronized.
5. Time Base Generator:
• The time base generator is used to generate the saw tooth voltage required to deflect the beam in the horizontal section.
6. Horizontal Amplifier:
• This is used to amplify the saw tooth voltage before it is applied to horizontal deflection plates.
7. Power Supply:
There are two sections of a power supply block.
• High voltage supply (HVS): it is applied to CRT of the order of 1000V to 1500V.
• Low voltage supply (LVS): it is required for heater of electron gun and remaining circuits except CRT only.
8. Fluorescent Screen:
• The screen is inside face of CRT and coated with a fluorescent material called phosphor.
• Different types of phosphor of oscilloscope are designated as follows.
P1- green medium P2-Blue-green medium P5- blue very short P11-blue short
5.4 NECESSITY OF TIME BASE VOLTAGE:
• If the two deflection voltages were held constant, the electron beam would strike a fixed point on the phosphor screen and a stationary point would be visible on the screen.
• When an ac voltage is applied to a CRO, without applying the time-base signal , a vertical line is only displayed indicating peak to peak voltage of input signal.
• In the above case, the input signal time period or frequency details will not be displayed on the CRO screen.
• To obtain such a display the signal voltage is applied to the vertical plates directly and it moves through horizontal plates by a sweep voltage applied to the horizontal plates.
• The horizontal sweep voltage produces the time base by moving the spot horizontally with the time, while the signal moves the spot vertically in proportional to the voltage at a particular instant of time.
Vertical line
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5.5 FUNCTION AND USE OF VARIOUS CONTROLS AND INPUT TERMINALS OF CRO:
The various input terminals provided on CRO are:
1. Horizontal amplifier input: ( X-input)
This is used for connecting the external horizontal signal as an input for horizontal amplifier.
2. Vertical amplifier input: ( Y-input)
This signal under testing is connected to the vertical amplifier input.
3. Basic controls:
a) Intensity: This control is for correct brightness of the trace on the screen.
b) Focus: This control is for sharp trace on the screen.
c) Astigmatism: This is another focus control. With the help of focus control and
astigmatism control, a very sharp spot can be obtained both in the centre and also at the edges of the screen.
4. X-shift or horizontal position control: This control is for moving the pattern left to right and right to left on the screen.
5. Y- Shift or vertical position control: This control is used for moving the pattern up and down on the screen.
6. Time/Div. control: This control adjusts the horizontal gain that is the width of the pattern.
7. Volts/Div. control: This control adjusts the vertical gain that is the height of the pattern.
8. Variable speed: This control adjusts the internal sweep frequency into sub multiples of vertical input signal.
9. EXT: In EXT position, the horizontal deflection system is fed from an external input.
10. Coupling (AC-DC-GND): It decides the mode of input coupling i.e.., whether the input to the vertical amplifier is to be a.c coupled, d.c, coupled or GND.
5.6 APPLICATIONS OF CRO:
1. Waveform analysis
2. Amplitude (voltage) measurement 3. Time period measurement
4. Frequency measurement
5. Phase angle and time delay measurement 6. Current measurement
7. Medical instrumentation
8. Observation of ICU medical units.
9. Patient monitoring system.
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5.7 VOLTMETER SENSITIVITY:
• Sensitivity of a voltmeter is defined as amount of resistance offered by a voltmeter per one volt of measurement.
• It is the ratio of sum of meter resistance (Rm) and series resistance (Rs) to full scale deflection (v) in volts.
• A low sensitivity meter may give correct readings when measuring voltages in low resistance circuits.
• It is produce very unreliable readings in high resistance circuits.
• Voltmeter connected across two points in a highly resistive circuits acts as a shunt that reduces the equivalent resistance. The meter will give a lower indication than actual value. This effect is called Loading effect.
• In general, when the load resistance across which a voltmeter is connected is in comparison with voltmeter internal resistance, loading effect is experienced.
• In other words, the internal resistance of the voltmeter should be as high as possible to avoid the loading effect.
5.8NEED FOR ANALOG TO DIGITAL CONVERTERS:
• Most of the signals available in nature are analog form; e.g., voice, music sound temperature, pressure, velocity etc.
• Therefore the analog signals first converted into digital form before they processed in digitally by using A/D converter.
5.9 NEED FOR DIGITAL TO ANALOG CONVERTER:
• In digital systems, after a CPU has processed data, it is often necessary to convert digital data into an analog voltage or current to communicate with outside world.
• Hence we need digital to analog conversion.
5.10 BASIC PRINCIPLE OF D/A CONVERSION:
The process of converting a digital signal into analog signal is known as ‘ Digital to Analog conversion’ to convert a digital signal to analog, it is necessary to treat each bit in weighted value current or voltage.
The input digital data is applied to the switches. These switches are connected with reference voltage source. The corresponding switches closes for binary 1 input and opens for binary 0 input.
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When the switches closes, the reference voltage applied to resistive network.
Therefore the switches feed a resistive network which converts each bit in to its weighted current value and sums them for a total current. This total value is then fed to the amplifier and gives the proper analog voltage value for the equivalent digital data.
5.11DEFINITIONS RELATED TO D/A CONVERTER:
Resolution:
• It is defined as the reciprocal of the number of discrete steps in the output of DAC.
% Resolution = x 100 Accuracy:
• Accuracy is a comparison of the actual output of a D/A converter with expected output.
Monotonicity:
• A device is said to be monotonic, if the output of DAC increases for every raise in digital input
Settling time:
• It is defined as the time which takes a DAC to settle within ± ½ LSB of its final value, when a change occurs in the input code.
• The settling time determines the speed of a DAC.
5.12 D/A CONVERSION USING R-2R LADDER NETWORK:
It uses a ladder network containing series – parallel combination of two resistors of values
‘R’ and ‘2R’. Hence this type of network is called as “R-2R resistor network.”
Working:
The operation of the circuit can be simplified by using. Thevenin’s theorem. Let us assume that the digital input is S3S2S1S0 = 0001 (S0=1 S1= 0 S2=0 S3= 0) by the applications of thevenin’s theorem tot the resistive network of the fig. Thus the LSB has been assumed as 1 and the equivalent voltage is VR / 24.
Similarly for the digital input 0010, 0100, and 1000 the equivalent voltages the VR / 23, VR / 22 and VR / 21respectively, Note that the value of equivalent resistance is 3R in each
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case. From the circuit of the fig. The output analog voltage can be determined. Then the V0 is given by:
Hence by using the above expression; the analog output voltage is determine for various switch positions of the digital input.
Advantages:
• Only two values of resistors R & 2R are required.
• It is easy to match temperature coefficients of resistors.
Disadvantages:
• Lower conversion speed than binary weighted DAC
• Propagation delay is more.
5.13 SUCCESSIVE APPROXIMATION A/D CONVERTER:
• The successive-approximation type uses a comparator, a successive-approximation register, output latches, and a D/A converter
• The heart of the circuit is an 8bit successive-approximation register (SAR), whose output is applied to an 8-bit D/A converter.
• The analog output (VA) of the D/A converter is then compared to an analog input signal Vin by the comparator.
• The output of the comparator is a serial data input to the SAR.
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• The SAR then adjusts its digital output (8-bits) until it is equivalent to analog input Vin.
• The 8-bit latch at the end of conversion holds onto the resultant digital data output.
Working:
• At the start of a conversion cycle, the SAR is reset by holding the start (S) signal high.
• On the first clock pulse LOW-to-HIGH transition, the most significant output bit Q7
of the SAR is set.
• The D/A converter then generates an analog equivalent to the Q7 bit, which is compared with analog input Vin.
• If the comparator output is low, the D/A output > Vin and the SAR will clear its MSB Q7.
• On the other hand , if the comparator output is high, the D/A output< Vin and SAR will keep the MSB Q7 set
• During the next clock pulse LOW-to-HIGH transition, the SAR will set the next MSB Q6.
• Depending on the output of the comparator, the SAR will then either keep or reset the bit Q6. This process is continued until the SAR tries all the bits.
• As soon as the LSB Q0 is tried, the SAR forces the conversion complete (CC) signal HIGH to indicate that the parallel output lines contain valid data.
• The CC signal in turn enables the latch, and digital data appear at the output of the latch.
• Digital data are also available serially as the SAR determines each bit.
• To cycle the converter continuously, the CC signal may be connected to the start conversion input.
Advantage:
• High speed
• Excellent resolution.
5.14 RAMP-TYPE DVM:
• Operating Principle: The operating principle in the Ramp type DVM is based on the measurement of the time it takes for a linear ramp voltage to, rise from 0V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicator.
• The block diagram of a Ramp type DVM is as shown in Fig 1 .
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• • FIG 1: Block Diagram of Ramp Type DVM
• Description: The Ramp type DVM consists of a ranging & attenuator which is used to select the required range of measurement. A ramp generator is used to generate the ramp voltage. An input comparator continuously compares the dc input voltage with the ramp voltage. The ground comparator compares the ramp voltage with the ground voltage i.e., 0 V. A counter is used to count the number of pulses and the read out gives the digital display.
• Working :
• At the start of the measurement cycle a ramp voltage is initiated; this voltage can be positive going or negative going. This -ve going ramp is illustrated in Fig. 2.
• • FIG 2: Voltage to Time Conversion
• This negative going ramp is continuously compared with the unknown input voltage. At the instant that the ramp voltage equals to unknown voltage, the comparator generates the pulse which opens the gate. The ramp voltage continues to decrease with time until it finally reaches 0 V and a second comparator generates an output pulse which closes the gate.
• An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units which totalize the number of pulses passed through
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the gate. The decimal number, displayed by the indicator tubes is a measure of the magnitude of the input voltage. The sampled rate multivibrator determines the rate at which the measurement cycles are initiated. The sample rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage .At the same time, a reset pulse is generated which resets the counter to the zero state.
5.16 DIGITAL FREQUENCY METER:
Principle of Operation:
The signal waveform is converted to trigger pulses and applied continuously to an AND gate, as shown in Fig 14. A pulse of 1s is applied to the other terminal, and the number of pulses counted during this period indicates the frequency.
FIG 14 : Principle Of Digital Frequency Measurement
The signal whose frequency is to be measured is converted into a train of pulses, one pulse for each cycle of the signal. The number of pulses occurring in a definite interval of time is then counted by an electronic counter. Since each pulse represents the cycle of the unknown signal, the number of counts is a direct indication of the frequency of the signal.
Since electronic counters have a high speed of operation, high frequency signals can be measured.
Basic Circuit of a Digital Frequency Meter :
The block diagram of a basic circuit of a digital frequency meter is shown in Fig 15.
FIG 15 : Basic Circuit of a Digital Frequency Meter
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The signal may be amplified before being applied to the Schmitt trigger. The Schmitt trigger converts the input signal into a square wave with fast rise and fall times, which is then differentiated and clipped. As a result, the output from the Schmitt trigger is a train of pulses one pulse for each cycle of the signal.
The output pulses from the Schmitt trigger are fed to a START/STOP gate. When this gate is enabled, the input pulses pass through this gate and are fed directly to the electronic counter, which counts the number of pulses.
When this gate is disabled, the counter stops counting the incoming pulses. The counter displays the number of pulses that have passed through it in the time interval between start and stop. If this interval is known, the unknown frequency can be measured.
The block diagram of a digital frequency meter is shown in fig 16.
FIG 16: Diagram of a Frequency Meter
The input signal is amplified and converted to a square wave by a Schmitt trigger circuit.
In this diagram, the square wave is differentiated and clipped to produce a train of pulses, each pulses separated by the period of the input signal. The time base selector output is obtained from an oscillator and is similarly converted into positive pulses.
The first pulse activates the control Flip-Flop, this gate control F/F provides an enable signal to the AND gate. The trigger pulses of the input signals are allowed to pass through the gate for a selected time period and counted. The second pulse from the decade frequency divider changes the state of control F/F and removes the enable signal from the AND gate, thereby closing it. The decimal counter and display unit output corresponds to the number of input. Pulses received during a precise time interval; hence the counter display corresponds to the frequency.
The frequency of the input signal‘s computed as F=N / t
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Where,
F= Frequency of the input signal N = Number of pulses counted t = Duration of gate pulses
In some applications it is desirable to measure the timeperiod of the signal,the timeperiod is computed as
T = N / f Where,
T = Time period of input signal.
N = Number of pulses countyed, f = Clock frequency.
5.17 FUNCTION GENERATOR:
A function generator is a versatile instrument that produces a choice of different waveforms whose frequencies are adjustable over a wide range. The common output waveforms are the sine, square, triangular and saw tooth waves. The frequency may be adjusted from a fraction of a Hertz to several hundred KHz.
The block diagram of a function generator is illustrated in the figure. Usually the frequency is controlled by varying the oscillator capacitance or resistance.
• But in function generator frequency is controlled by varying the magnitude of the current produced by the current sources which drives integrator.
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• The frequency controlled voltage is used to regulate two current sources namely upper current source and lower current source.
• The upper current source supplies constant current to an integrator.
• The output voltage of integrator then increases linearly with time.
• If the current, charging the capacitor increases or decreases, the slope of output voltage increases or decreases respectively. Hence this controls frequency.
• The voltage comparator multivibrator circuit changes the state when the output voltage of integrator equals the maximum predetermined upper level. Because of this change in state, the upper current source is removed and the lower current source is switched ON.
• This lower current source supplies opposite current to the integrator circuit. The output of integrator decreases linearly with time.
• When this output voltage equals maximum predetermined upper level on negative side, the voltage comparator multivibrator again changes the condition of the network by switching OFF the lower current source and switching ON the upper current
source.
• The output voltage of the integrator has triangular waveform.
• To get square wave, the output of the integrator is passed through comparator.
• The triangular wave is synthesised into sine wave using diode resistance network.
