The Oscilloscope. No Prelab

Transcription

1 The No Prelab The oscilloscope is one of the most versatile tools in the laboratory. The oscilloscope can be used as a D.C. voltmeter, A.C. voltmeter, a timing instrument, and a frequency meter. The oscilloscope allows us to view signals that are cyclic or transient. The electrical signals of the heat or light intensity of a distant quasar can be viewed with an oscilloscope. The heart of an oscilloscope is the cathode ray tube. The principle component of which is shown in Figure 1. Fig. 1 The beam of electrons, which comprise the cathode ray in the cathode ray tube, is formed by an electron gun. The gun consists of a filament, which becomes hot when electrical current flows through it. Electrons become so energetic that they can escape the surface of the filament. The electrons are then accelerated through a series of plates that focus the electrons into a small beam. When the beam hits the fluorescent screen, a glowing spot is formed on the screen. Between the electron gun and the screen are two pairs of parallel plates between which the beam passes. One pair of plates is horizontal and consequently will defect the beam vertically. The other pair is vertical and will deflect the beam horizontally. If the tube is long in comparison to the deflection, then the amount of deflection is proportional to the voltage applied to the plate. The electrical signal to be studied is usually applied through an amplifier. The vertical amplifier can be a D.C. or A.C. amplifier and is usually calibrated such that the voltage input can be read directly from the screen. The horizontal deflection plates are frequently connected to an internal signal. The internal signal is a saw tooth wave generator. In the saw tooth wave (Figure 2), the voltage increases linearly with time. This means that the horizontal deflection becomes proportional to time. The horizontal amplifier is calibrated such that time of the sweep can be read directly from the screen. The combination of the vertical deflection being proportional to the input voltage and the horizontal deflection being proportional to time causes the electron beam to trace out a voltage versus time graph. If the input voltage is repetitive, then a stationary pattern will be produced. 53

2 Fig. 2 To accommodate variation in signals, a triggering circuit is added to the horizontal time circuit. The triggering circuit causes the time sweep to wait until a positive voltage (or negative voltage) occurs before the horizontal sweep starts. This eliminates the drifting of the pattern across the screen. The time base enables one to select for the horizontal sweep (1) any one of several frequency ranges from the internal sweep generator, (2) to calibrate the horizontal sweep such that a horizontal distance on the screen can be read directly in time units, and (3) to select an external input (x External) to the horizontal deflection. Vertical input (y input) enables one to control the gain of the vertical input such that (1) the vertical distance on the screen can be calibrated to read directly in volts, and (2) and the input signal can be read as DC or AC voltages. The triggering level is used to lock into the vertical input signal such that a stationary trace can be seen on the screen. The triggering can be done by (1) the input internally or (2) by the input of an external signal. Apparatus: Frequency Generator AC Power Supply Multimeter Coil of wire Bar Magnet Dry cell Procedure: 1. Turn on the oscilloscope and let it warm up for about 30 seconds. Adjust the horizontal and vertical position such that the trace is in the center of the screen. The triggering level should be internal and the mode set to Auto. 2. Connect the signal (or frequency) generator to the vertical input of channel 1. (The CH 1 button should be in). The signal generated is an A. C. voltage whose frequency and magnitude can be changed. Observe the trace produced when the frequency is changed or the time base on the oscilloscope is changed. Observe both the sine wave and a square wave. Become familiar with the controls on the 54

3 oscilloscope and the frequency generator. 3. To measure voltages on the oscilloscope, turn the vertical amplifier to calibrate. (Rotate the dial in the middle clockwise until it locks.) To measure D.C. voltages, set the input for ground (or connect the terminals of the input together) and set the trace on the centerline on the screen. Set the switch to D.C. and set the amplifier to 1 V. Now measure the voltages of a dry cell. (It should be a straight line 1.5 V from the centerline). Compare with the measurement of a D.C. Voltmeter. Record the value in the data section. Repeat for the three other voltage outputs of Power Supplies 4. To measure A.C. voltages, connect a sine wave from the frequency generator to the oscilloscope. (Be sure to connect the ground output of the frequency generator to the ground input of the oscilloscope.) Record the voltages from the bottom of the valley to the top of the peak voltage or the peak-to-peak voltage (V pp ). Now measure the same voltage using the an A.C. voltmeter. Repeat the measurements for three different voltages. The A.C. voltmeter reads the rms voltages (V rms ) and not the peak-to-peak voltages. The relationship is V rms = (0.707)(V pp /2). 5. To measure the frequency of an A.C. signal, the period is first measured and then the frequency is found by taking the inverse of the period. The period is the time for one complete cycle and can be read directly off the screen of the oscilloscope. Set the time base to calibrate by rotating a dial clockwise until it locks. 6. The oscilloscope is very useful in analyzing A.C. circuits. Connect a square wave generator in series to a resistor box and capacitor box. Connect the input of the oscilloscope across the terminals of the capacitor box as shown in Fig. 3. (Be sure the ground of the frequency generator is connected to the ground of the oscilloscope.) Set f = 1000 Hz, R = 1000 Ω, and C = 0.1 µf. Sketch the waveform in the data section. The time constant is the time for the voltage of the capacitor to decrease to 1/e th (or 0.37) of is original value. This time can be read off the screen of the oscilloscope as shown in Fig

4 Fig. 3 Fig The oscilloscopes are used to study transient signals. Electromagnetic induction can be studied with the oscilloscope. Hook up a coil of wire to the vertical amplifier. Set the vertical amplifier to 0.1 V/div. and the time base to 0.5 sec/div. Observe the form of the voltage as the signal goes across the screen when the N pole of the magnet is inserted into the coil and then draw the magnet out. Sketch the curve produced in the data sheet. Repeat with the south pole of the magnet 56

5 Data: 1. D.C. Voltage Measurements D.C. Voltmeter Dry Cell Power Supply V 1 V 2 V 3 +5 V +12 V - 15 V 2. A.C. Voltage Measurements Peak to Peak Voltages Calculated rms Voltage A. C. Voltmeter V 1 V 2 V V 2.6 V 3.3 V 3. Frequency Measurement Period on the Calculated Frequency Frequency Meter T 1 T Hz 2100 Hz 17,000 Hz T 3 4. RC Circuit (frequency = 1000 Hz, C = 0.1 μf) Resistance Capacitance Time Constant (RC) Ω Ω Ω Time constant from the 57

6 Sketch of the RC circuit. 5. Sketch of the north pole of magnet inserted into the coil and then pulled out. 6. Sketch of the south pole of magnet inserted into the coil and then pulled out. 58