CRO (Cathode-Ray Oscilloscope) Notes

CRO (Cathode-Ray Oscilloscope)

CRO is a display instrument used for observation, measurement, and analysis of waveforms. It plots amplitude (Y-axis) against time (X-axis). It functions as an X-Y plotter capable of displaying multiple signals via different channels for parameters like amplitude, frequency, phase shift, temperature, and pressure. The CRO may convert signals for display compatibility, showing a moving spot that represents the signal when the device is on.

Application: used to study waveforms and time varying signals from low to high frequencies.

Working Principle:

  • The working of the CRO follows the movement of the electron ray due to electrostatic force

  • When an electron ray strikes the phosphor surface, then a bright spot in created over it.

  • CRO controls the electron ray by applying electrostatic energy from 2 ways.

  • This leads to movements of the spot over the phosphor monitor and applied electrostatic forces are manually perpendicular. This helps in creating the waveform of the input signal.

Block Diagram and Components:

The CRO comprises the following key components:

  • CRT (Cathode-Ray Tube)

  • Vertical Amplifier

  • Delay Line

  • Horizontal Amplifier

  • Time-Base Generator

  • Triggering Circuit

  • Power Supply

Cathode-Ray Tube (CRT):

  • The CRT's vacuum tube converts electrical signals into visual representations.

  • An electron gun generates a focused electron beam accelerated to high frequency.

  • Vertical deflection plates control up-down motion, while horizontal plates manage left-right movement independently, allowing spot positioning anywhere on the screen.

Electron Gun:

  • Located within the CRT, the electron gun emits electrons, shaping them into a focused beam directed at the fluorescent screen.

  • Components include a heater, cathode, grid, and anodes (accelerating, pre-accelerating, and focusing).

  • Strontium and barium layers on the cathode enhance electron emission at moderate temperatures.

  • A control grid with a fine hole regulates electron flow, controlling the spot's brightness on the CRO screen via negative voltage adjustments.

Deflection Systems:

  • Two sets of deflecting plates which manipulate the path of the electron beam after it exits the electron gun.

  • Vertical plates generate Y-axis deflections, and horizontal plates produce X-axis deflections based on plate potential variations.

  • Frequencies above 16 Hz on either plate result in vertical or horizontal lines.

  • Simultaneous potential application creates oblique lines, with deflection amount proportional to applied voltage.

Fluorescent Screen:

  • The CRT's end wall, the screen, typically measures about 100mm×100mm100mm \times 100mm.

  • Its inner surface is coated with phosphor crystals that convert electrical energy into light through fluorescence when struck by an electron beam.

  • Light intensity depends on:

    • Electron energy

    • Number of electrons striking

    • Strike duration on the phosphor area

    • Phosphor's physical properties

Glass Body:

  • A conical evacuated glass housing protects the assembly.

  • The inner faces of the CRT, including the neck and screen, are coated with a conducting material called aquadag, which acts as a high-voltage electrode.

  • This coating is electrically connected to the accelerating anode to center the electron beam and prevent negative charge buildup, maintaining screen equilibrium.

CRO Controls

Time-Base Generators:

  • Essential for accurate waveform reproduction on the CRO screen.

  • Ensures a constant horizontal beam velocity, dependent on deflecting voltage that increases linearly with time.

  • Utilizes a sawtooth waveform, where:

    • Flyback or retrace time is the time to return to the initial value.

    • Sweep/trace time is the duration of incremented voltage.

  • Controls the sweeping speed of the spot across the screen

The calibration is done in:

  • s/cm

  • ms/cm

  • $\mu$s/cm

Types of Sweep:

  • Free Running or Recurrent Sweep: Starts a new sawtooth wave immediately after the previous one terminates, without requiring an external signal.

  • Triggered Sweep: Used for periodic waveforms with parts of interest shorter than the waveform period. The spot sweeps once across the screen in response to a trigger signal, which can be the signal itself or an external source. Useful for photographing waveforms.

  • Driven Sweep: A recurrent sweep triggered by the signal itself.

  • Non-Sawtooth Sweep: Used for specific applications like frequency comparison or phase shift determination between two voltages.

Sweep frequencies vary with the CRO type.

Vertical Amplifier:

  • Determines the oscilloscope's sensitivity and bandwidth, with sensitivity expressed in V/cm of vertical deflection at mid-band frequency.

  • Gain dictates the smallest measurable signal by reproducing it on the CRT screen.

  • Sensitivity is directly proportional to the gain; higher gain increases sensitivity.

  • Vertical sensitivity measures the smallest selectable deflection factor which can be selected with the rotary switch.

Horizontal Amplifier:

  • Amplifies the sweep generator output before applying it to horizontal plates.

  • Voltage can be applied via an internal (INT) or external (EXT) source.

  • In EXT mode, plates disconnect from INT sources, resulting in a stationary beam at the screen's center.

  • In INT mode, the waveform appears on the screen.

Other Controls:

  • Position Control: Regulates DC voltage to horizontal and vertical deflection plates using knobs for position adjustments.

  • Intensity Control: Adjusts the control grid's potential relative to the cathode using a potentiometer to control spot brightness, influencing the number of electrons leaving the cathode.

  • Focus Control: Adjusts the potential of the middle anode in the CRT's electron gun to act as an electrostatic lens, focusing the electron beam. Increasing positive potential narrows the beam, creating a pinpoint spot.

  • Astigmatism: Corrects focus deviations between the screen's center and edges by adjusting deflection plate and accelerating anode potentials.

  • Blanking Circuit: Applies sawtooth sweep voltage to horizontal deflection plates, moving the spot horizontally during the sweep period. Fast movements result in a thin, dim, or invisible line, while slower movements create a solid line if exceeding the persistence of vision threshold.

  • Delay Circuit: Delays vertical signals to synchronize their arrival with horizontal signals for accurate waveform reproduction, typically with a delay time around 200200 nsec.

Measurements Using the Cathode-Ray Oscilloscope:

  1. Measurement of Frequency:

    • Time-base Measurement: Determines the frequency of a time-varying signal displayed on the CRT screen.

    T=txT = \frac{t}{x}, where t is the time interval and x is the number of complete cycles. Therefore, frequency can be determined as:

    f=xtf = \frac{x}{t}

    • Measurement Using Lissajous Figures: Applying sinusoidal waves to deflection plates produces patterns (Lissajous figures) based on relative amplitudes, frequencies, and phases of waveforms.

  2. Measurement of Phase:

    • The phase difference between two waveforms on the CRT screen can be found from the time axis.

    where t1 and t2 are times when two sinusoidal signals of time period TT are in the same phase.

    • Measurement of Phase Using Lissajous Figures: Lissajous figures are used to measure the phase difference between two sinusoidal voltages of the same amplitude and frequency.

    The signals are applied simultaneously to the horizontal and vertical deflection plates. The values of the deflection voltages are given by:

    vx=Asin⁡(ωt+ϕ)v_x = A \sin(\omega t + \phi)

    vy=Asin⁡(ωt)v_y = A \sin(\omega t)

    Here A is the amplitude, ω\omega is the angular frequency and ϕ\phi is the phase angle by which vx leads vy.

    The Lissajous figure is thus, an ellipse represented

Measurement of Phase Using Lissajous Figures: Cases

Special Purpose Oscilloscopes:

Dual Beam Oscilloscope:

  • Employs a CRT with two sets of vertical deflection plates and one set of horizontal deflection plates.

  • Each channel includes a pre-amplifier, attenuator, delay line, vertical amplifier, and a set of vertical deflection plates.

  • The horizontal deflection is common for both input signals, producing two simultaneous input signals on the CRT screen.

Dual Trace Oscilloscope:

  • Uses a CRT with one set of vertical and one set of horizontal deflection plates.

  • Each channel has a pre-amplifier, attenuator, delay line, and vertical amplifier, but the last two blocks are common to both channels which is the trigger circuit and sweep generator.

  • An electronic switch connects the delay line output of a channel to the vertical amplifier.

  • It uses the same electron beam for deflecting input signals A & B in the vertical direction using an electronic switch, producing two traces. The blocks that deflect the beam horizontally are common for both input signals.

Digital Storage Oscilloscope:

  • Adds digital data storage capabilities to a basic oscilloscope.

  • Additional blocks for digital data storage are added between the pre-amplifier/attenuator and vertical amplifier.

  • These include a sample and hold circuit, Analog-to-Digital Converter (ADC), memory, and Digital-to-Analog Converter (DAC).

  • Control logic manages these blocks.

  • DSO stores data digitally before displaying the waveform, a feature absent in basic oscilloscopes.

Other CROs Classification:

  • CROs are categorized as either digital or analog.

    1. Analog CRO: Measures amplitude, phase, and frequency directly from the displayed waveform through manual reading.

    2. Digital CRO: Provides digital readouts of signal information (time, voltage, or frequency) along with the signal display, using an electronic counter.

    3. Storage CRO: Retains the display for a significant time after the initial trace, which is especially useful for displaying low-frequency waveforms.