CONTENTS:CRO and Digital Oscilloscopes
- What is a CRO?
- Function of CRO
- CRO dual beam vs CRO dual trace
- Dual Trace CRO
- Function of Aquadag
- Digital Oscilloscope (DSO)
- Working principle digital oscilloscope
- Deflection Factor
What is a CRO?
“The cathode ray oscilloscope (CRO) is a type of electrical instrument which is used for showing the measurement and analysis of waveforms and others electronic and electrical phenomenon.”
Working principle of a CRO with Block Diagram:
The major block circuit of a general purpose CRO is as follows:
- Cathode ray tube (CRT)
- Horizontal Amplifier
- Perpendicular Amplifier
- Delay line
- Time Base circuit
- Power Supply circuit
- Trigger Circuit
Cathode Ray Tube | CRT
– CRT is actually a cathode ray tube that mainly emits electrons which hits the phosphor search internally and then it provides a visual display at signal.
The sawtooth voltage is amplified here at first place and then it is applied to the horizontal deflection plates.
– the sensitivity and bandwidth of an oscilloscope is determined by the vertical amplifiers. The smallest signal of a vertical amplifier is calculated from the gain of that particular amplifier. Hence, the oscilloscope can successfully produce images e on the CRT screen.
The sensitivity of oscilloscope is directly proportional to gain of the vertical amplifier.
– a delay line is used to delay any particular signal for a certain span of time in vertical sections. At what time the delay line is not in utilization, the portion of the signal will loss or distorted when delay not is operational. In case of the input signal the delay-line is not unswervingly applied to the vertical plate; instead it is delayed by a particular time while using a circuit. When the signal is delayed, the sweep generator output reaches to the horizontal plates as the time gets extended enough.
They are 2 types;
Distributed parameter delay line:
It is basically a transmission line constructed with a wound helical coil on a mandrel and extruded insulation between it.
Lamped parameter delay line:
The Lamped parameter delay line counts at no. of cascading symmetric L-C network.
– time base generates the sawtooth voltage required to reflect the beam to the horizontal section. Hence, the time is plotted in Y- axis, may be utilize to analyse time-varying signals.
– a voltage is required by CRT to generate and accelerate on electron beam and voltage required by the other circuits at the oscilloscope like horizontal amplifier, vertical amplifier etc. The power supply block provides that.
There are two sections at a power supply block. The high voltage section and low voltage section. The high voltages of the order of 1000 volt to 1500 volt are required by CRT. Such high negative voltages are used for CRT.
The -ve high voltage has advantages such as:
- Accelerating orders and the deflection plates are clean to ground potential. It’s good for operator safety from electrical incidents.
- The deflection voltages calculated in respect to ground hence blocking of coupling capacitors are not necessary.
- Insulation between controls and chains is less.
– to synchronize the input signal and the sweep frequency, trigger circuit is used. The incoming signals are changed into trigger pulse in this particular circuit.
Differences between Dual Time CRO and Dual Beam CRO:
|DUAL TRACE CRO||DUAL BEAM CRO|
|1. Single beam is utilized for producing two different wave forms. |
2. It cannot capture two fast transient events.
3. Signal loss in case of dual trace CRO about 50% of each signal.
4. Two operating modes under this CRO a) alternate & b) chop.
|1. Two separate electron beams are used for producing different wave forms.|
2. It can capture two fast transient events as it can display two signals simultaneously.
3. No loss occurs during dual-beam display.
4. Two operating modes under this CRO a) double gun tube & b) split beam
What is Digital Oscilloscope (DSO)?
Explain the working principle of a dual trace CRO:
To compare two or more voltages in electronics experiments CRO is an essential and accurate instrument. Sometimes multiple oscilloscopes might be utilized to trigger the sweep of each oscilloscope at an exact time.
To get rid from this problem a dual-trace oscilloscope is quite frequently employed as an economic but useful option. In this method, an electron beam is utilized to produce 2 traces signal, these are deflected from 2 perpendicular source.
The block diagram of dual trace oscilloscope is shown below:
For each of A and B signal, A separates preamp will pre-amplify and then it gets attenuated by an attenuator. The amplitudes of each i/p are precisely controlled. After preamplification, both of these signal feed to a switch and capability to pass single channel at a time thru a delay to the perpendicular amp. The time-based circuit utilizes the trigger select switch S2 and permits to be triggered by A or B individually by line freq. or by setting an exterior signal. The horizontal amp is feed via S1/ S3 /switch by the sweep generator.
Explain the function of Aquadag:
An aquadag is basically name used for trading as a water based colloidal graphite widely utilized in cathode ray tubes.
When electron beam strikes the phosphor screen, secondary electrons are emitted from the surface of the screen, and they get accumulated there unless they are removed. When their accumulation becomes quite large, they start repelling the electron beam away from the screen, which will create distorted images on the screen.
To prevent the above problem, in all modern CRO’s a conductive graphite coating called aquadag is deposited on the inner wall of the flared end of the CR tube. This coating is also kept at high positive potential as the accelerating anode. This will perform two functions;
- The aquadag coating is positive and so it attracts the secondary electrons and keeps removing them eventually.
- Since the aquadag is installed in front of the anode, it assist e- acceleration to the CRO screen.
What is Digital Storage Oscilloscope (DSO)?
Working principle digital oscilloscope:
In an oscilloscope, the i/p signal is applied to the amplifier and attenuator. The oscilloscope has an amplifier and attenuator circuitry as utilized same as conventional one. The attenuator signal is at that point go to the vertical amplifier part for further process.
A DSO has three modes of operation –
- Roll Mode – Here in this mode of operation very frequently changeable signals are displayed clearly in this mode.
- Store Mode – this is also termed as refresh mode. Here, input initiates trigger circuit.
- Hold or Save Mode – this is automatic refresh mode.
What is Deflection Factor or Deflection Sensitivity?
For CRO, it is represented as the deflection of the screen per unit deflection-voltage.
Therefore, deflection sensitivity (S),
Deflection factor of a CRT is expressed as the reciprocal of Deflection Sensitivity
Therefore, deflection factor (G)
Both of this parameter is used to characterize the CRT.
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