For procedures, see your electrical laboratory manual.

Title: The Oscilloscope

Aim

To introduce the oscilloscope as a measuring instrument to the student; In particular, we want to
1. Find the deflection sensitivity on an oscilloscope.
2. Measure DC and AC voltages
3. Measure the frequency of the mains supply and of an oscillator

Apparatus

1. Digital oscilloscope
2. DC power supply unit
3. AC variac
4. AC voltmeter
5. Signal generator (oscillator)

Theory


An oscilloscope is a type of electronic test instrument that graphically displays varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time.

An oscillator is an electronic circuit that produces a periodic oscillating electronic signal often a dime wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal.

The heart of the oscilloscope is a cathode ray tube or CRT, of the sort you have already studied. Looking at the face of the instrument, you are viewing the screen that the electron beam strikes. Electronic circuits in the scope apply voltages to one set of deflection plates to sweep the beam across the screen from left to right at a constant rate, thereby providing the time axis. Other circuits amplify or attenuate the input signal as needed, and apply voltages to the other set of deflection plates to move the beam vertically, providing the voltage axis.

The oscilloscope is useful because it allows electrical signals, especially time-varying ones, to be observed. The signals may be slow-moving or rapid. The oscilloscope has amplification and delay features that allow part or all of the signal to be observed. Other features allow the signals to be physically moved about the screen. All of this makes it easier for the signal to be measured.

Oscilloscopes are used to view the signals coming directly from devices such as sound cards, allowing the real-time display of waves. They are used as electrocardiograms, to test circuits and to troubleshoot electronic devices such as televisions. Oscilloscopes with storage features allow signals to be captured, retrieved, and analyzed for later use.

Answers to questions

1. Sources of error apart from human error include
— Fluctuating Voltage supply from the mains
— Calibration error
— Timebase center frequency accuracy,
— Timing instability,
— Vertical noise,
— Bandwidth effects,
— Interpolation (aliasing) error.

2. Expressions for Voltage.
a. When the voltage is DC,
V=SD (where S=Deflection and D=Deflection sensitivity). See Table 205.1 in your manual.

b. When Voltage is AC,
V=DL/2 (where L=length of the trace) See Table 205.2 in your manual.