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
Timebase
The voltage applied to the X plates (ie horizontal motion) is generated inside
the oscilloscope itself by what is usually referred to as the 'timebase'. The
speed of the timebase can be varied over a large range to accommodate signals of
widely differing frequencies. The timebase sweeps the spot across from left to
right of the screen (when triggered ), at whatever speed is set in the
timebase, 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
timebase is from 1s/cm
to 1s/cm.
Y amplifiers
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.
Trigger
To get a stationary display on the oscilloscope it is necessary to start the
timebase 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 timebase until it synchronises with the input waveform; such
oscilloscopes have a 'sync' control.
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