The cathode-ray oscilloscope is probably the most valuable tool used in electronics. It allows one to tap directly into the 'heartbeat' of the piece of equipment one is working on, and graphically display the continuous variations of voltage and current in a circuit.
The heart of the oscilloscope is the cathode-ray tube. In this device, electrons emitted from an indirectly heated cathode are focused into a beam of small diameter which, when it strikes the front of the tube or screen, causes a special coating on the internal surface of the screen to fluoresce, creating a spot of light which may be blue or green according to the coating material used. On its way to the screen, the electron beam passes between one pair of parallel plates and then another pair at right-angles to the first. These are called the 'X and Y deflection plates'. If a voltage is applied between the deflector plates, the beam is deflected one way or the other according to the polarity of the voltage applied, and so the spot on the screen moves. Thus, if a voltage proportional to time is applied to one pair of plates, the horizontal-deflector (X) plates, and an alternating voltage is applied to the vertical-deflector (Y) plates, the spot traces out the waveform of the alternating voltage.
A photo of a typical modern oscilloscope
The voltage applied to the X plates (ie horizontal motion) is generated inside the oscilloscope itself by what is usually referred to as the 'time-base'. The speed of the time-base can be varied over a large range to accommodate signals of widely differing frequencies. The time-base sweeps the spot across from left to right of the screen (when triggered ), at whatever speed is set in the time-base, the spot then returns back to the left hand at maximum speed. The return phase of the spot, is blanked out and therefore not usually seen.
For the less expensive oscilloscope as used by the amateur, the range of the time-base is from 1μs/cm to 1s/cm.
The signals to be observed are usually very small. If they are applied directly to the Y-plates, little, if any, movement in the vertical direction would be observed. It is necessary, therefore, to amplify the signals before applying them to the Y-plates. This amplifier is called the 'Y amplifier' and is built into the oscilloscope. Several ranges of amplification are available, and each is calibrated so that a known voltage applied to the input causes a pre-determined deflection in the vertical direction. For amateur equipment the range of the Y amplifiers is from 5mV/cm to 100V/cm.
It should also be noted that the Y-amplifiers have a limited bandwidth. Outside this bandwidth the calibration becomes less accurate.
To get a stationary display on the oscilloscope it is necessary to start the time-base at the same point on the waveform for every sweep. This is accomplished by the 'trigger' circuits, the 'trigger level' control performing this function. On some older oscilloscopes stabilisation is accomplished by altering the frequency of the time-base until it synchronises with the input waveform; such oscilloscopes have a 'sync' control.
