Cathode Ray Oscilloscope | CRO

What is a Cathode Ray Oscilloscope?

A Cathode Ray Oscilloscope (CRO) is an analogue instrument that displays and measures voltage waveforms in electrical circuits. Its cathode-ray tube acts as a fast X-Y plotter. In normal operation it plots input voltage against time; in X-Y mode it plots one input signal against another.

The electron beam produces a luminous spot when it strikes the phosphor-coated screen. Deflection voltages move that spot in response to the input signal and the horizontal time base.

The beam has very low inertia, so electrostatic fields can deflect it quickly enough to follow rapid changes in voltages. The phosphor converts the beam’s path into a visible trace.

A CRO measures voltage at its input. Suitable probes, shunts and transducers can convert quantities such as current, pressure or acceleration into a safe voltage signal. The cathode ray oscilloscope can then display that signal within its bandwidth and input limits.

Construction of Cathode Ray Oscilloscope

The cathode-ray tube is the display and beam-deflection assembly at the centre of a traditional CRO.

To understand the construction of cathode ray oscilloscope, start with these five main parts of its cathode-ray tube:

1. Electron gun
2. Deflection plate system
3. Fluorescent screen
4. Glass envelope
5. Base

The linked DIY oscilloscope is a separate low-voltage electronic project, not a complete CRT CRO. A real CRO also needs vertical, horizontal, trigger and high-voltage circuits. CRT equipment can retain lethal voltage and contains an evacuated glass envelope, so only trained people should open or service it.

Electron Gun:
The electron gun emits, controls, accelerates and focuses the beam. A typical gun includes a heater, cathode, control grid, pre-accelerating anode, focusing anode and accelerating anode. Heating an oxide-coated cathode releases electrons. A negatively biased control grid regulates beam current and therefore trace intensity. Positive anodes accelerate the electrons towards the screen. The required potentials depend on the tube design; 1500 V is only an illustrative value and is hazardous.

The focusing anode shapes the electron stream into a small spot on the screen. Its adjustable potential depends on the tube; 500 V is an example rather than a universal setting. Electron beams can be focused by two general methods:

1. Electrostatic focusing.
2. Electromagnetic focusing.

CRO tubes commonly use the electrostatic focusing method described below.

Electrostatic Focusing
The electric force is F = qE. For an electron, q = -1.602 × 10^-19 C, so the force acts opposite to the electric field. Shaped electrodes create curved equipotential surfaces that act as an electrostatic lens and bend electron paths towards a focus. The next two cases illustrate this effect.

Case One
Two parallel plates A and B create an approximately uniform field in the region shown.

If A is at +V and B is at -V, the field points from A to B, normal to the plate surfaces. Equipotential surfaces lie perpendicular to the electric field. Because electrons carry negative charge, the beam bends opposite to the field direction. Changing the voltage between the plates changes the field and deflection angle.

Case Second
Two concentric electrodes with a potential difference create curved equipotential surfaces, as the figure shows.

An electron crossing a curved equipotential surface accelerates in the direction normal to that surface. Its tangential velocity component remains continuous across the idealised boundary. For the notation shown, V₁sin(A) = V₂sin(B), where V₁ is the initial speed and V₂ is the speed after crossing. Therefore sin(A)/sin(B) = V₂/V₁.
The change in speed and direction bends different electron paths towards a common region, which is why the electrode field acts as a focusing lens.

Electrostatic Deflection
The following idealised geometry gives the deflection of an electron beam between parallel plates.


Plates A and B form the deflection system. A voltage between them creates a transverse field, while an electron beam enters along the x-axis. The electron accelerates opposite to the electric field; the sign of the plate voltage sets the direction. First use conservation of energy between the cathode and accelerating anode.

Here e is the magnitude of electron charge.
E is the accelerating potential difference used in the displayed equations.
m is the electron mass.
v is the electron speed along the tube axis.
The energy relation is eE = 1/2 mv².
Solving it gives v = (2eE/m)^1/2.
If the deflection voltage in the notation is E and plate spacing is d, the electric field magnitude is E/d and the transverse force magnitude is eE/d.
The resulting transverse acceleration is eE/(dm). Starting with zero transverse velocity, the displacement during the time between the plates follows the next equation.

The axial speed is treated as constant, so the distance travelled along x gives the transit time.

Here u denotes the electron speed along the x-axis.
Combining the time and displacement relations gives the beam trajectory.

Differentiating that trajectory gives its slope at the plate exit.

Here l is the effective plate length.
The plate displacement and exit slope determine the final spot position on the screen.

L is the distance defined in the geometry. With consistent voltage symbols, the final deflection expression is:

Deflection sensitivity is screen displacement per unit deflection voltage.

Graticule: The graticule is the calibrated grid used to read time and amplitude on a cathode ray oscilloscope. This page shows two graticule types:

1. Internal Graticule:
   The fixed scale is deposited on the inside of the CRT faceplate. Its close alignment with the phosphor reduces parallax error.
2. External graticule: The scale is mounted in front of the screen. It can be replaced, but viewing it at an angle can introduce parallax error.

The following block diagram shows the principal systems of a cathode-ray oscilloscope.

Basic Circuit Diagram of Cathode Ray Oscilloscope

A cathode ray oscilloscope combines vertical, horizontal, trigger and CRT display systems.

1. Vertical Deflection System:
   The input attenuator and vertical amplifier scale the measured signal before applying it to the vertical deflection plates. The volts-per-division control sets the displayed amplitude scale within the instrument’s bandwidth and input limits.
2. Horizontal Deflection System:
   The horizontal amplifier drives the horizontal deflection plates. In normal time-base operation, a ramp generator moves the beam from left to right at a rate set by the seconds-per-division control, then returns it rapidly. The trigger system starts each ramp from a repeatable event. The following terms describe common operating arrangements:
   1. Free Running or Recurrent Sweep
      The time base starts a new ramp after each previous sweep, even when no valid trigger is present. The trace may drift if it is not synchronised to the signal.
   2. Triggered Sweep
      The time base waits for a selected signal condition before starting. This produces a stable repetitive display and can also show a single event when the instrument supports single-sweep operation.
   3. Driven Sweep
      An external or derived waveform drives the horizontal system directly. Its shape, rather than the internal ramp generator, controls horizontal beam motion.
   4. X-Y Operation
      A second input signal drives the horizontal axis instead of the time-base ramp. Lissajous patterns can compare the phase or frequency of two periodic voltages.
3. Synchronization:
   A stable trace requires each sweep to start at a repeatable point on the waveform. The first three entries are trigger sources. The remaining entries are CRT display and calibration controls retained in this list’s existing layout.
   1. Internal
      The trigger signal comes from the measured channel through the vertical system.
   2. External
      A separate signal at the external trigger input starts the sweep.
   3. Line
      The trigger is derived from the mains-frequency power-supply signal for measurements synchronous with the line.
   4. Intensity Modulation
      A signal applied to the CRT intensity or Z-axis circuit changes beam current and therefore trace brightness. The allowed input and polarity depend on the instrument.
   5. Positioning Controls
      Small internal DC voltage sources offset the deflection signals. A front-panel potentiometer can act as a voltage divider to move the trace vertically or horizontally.
   6. Focus Control
      Changing the focusing-anode potential changes the electrostatic lens and adjusts the spot size.
   7. Intensity Control
      Changing the control-grid potential relative to the cathode varies beam current and trace brightness. Excess intensity can damage the phosphor.
   8. Calibration Circuit
      An internal square-wave source of known amplitude and frequency lets the user check probe compensation and scale response.
   9. Astigmatism
      An astigmatism adjustment balances horizontal and vertical spot focusing. It complements the focus control rather than duplicating it.