2 : AC signals, the signal generator and the Oscilloscope

Transcription

1 2 : AC signals, the signal generator and the Oscilloscope Expected outcomes After conducting this practical, the student should be able to do the following Set up a signal generator to provide a specific signal Display a steady signal on the oscilloscope Measure the amplitude, V pp and V rms, period and frequency of ac signals. Equipment needed Signal generator Oscilloscope Power supply Theory A signal generator and an oscilloscope will be used during this practical session. Below the two sets of instruments are defined. Figure 2.3 shows a sine wave, which, is a typical signal generated by the signal generator. The terms appearing in Figure 2.3, needed to describe the signal are also defined. Signal generator: An electronic device that generates repeating or non-repeating electronic signals in either the analogue or the digital domain. A signal generator such as shown in Figure 2.1 can be used to produce a sound/voltage/current signal and the frequency settings can be varied on the device. Figure 2.1 : A signal generator 6

2 Oscilloscope: A laboratory instrument commonly used to display and analyse the waveform of electronic signals such as the one shown in Figure 2.2. A typical sine wave is shown in figure 2.3. Figure 2.2: An Oscilloscope Figure 2.3: A typical sine wave Period (T): The time taken for one complete cycle of a repeating waveform. 7

3 Frequency ( f): The number of cycles completed per second. The measurement unit for frequency is the hertz, Hz. 1 Hz = 1 cycle per second. = (2.1) Conversely, the period is given by: = (2.2) Amplitude: In electronics, the amplitude, or height, of a sine wave is measured in three different ways. peak amplitude (V p ): is measured from the x-axis, 0 V, to the top of a peak, or to the bottom of a trough. peak-to-peak amplitude (V pp ): is measured between the maximum positive and negative root mean square (V rms ): is the DC voltage which will deliver the same average power as the AC signal. = Important specifications for oscilloscopes 1. Bandwidth (2.3) Bandwidth: is the highest frequency signal that you can reliably test with your oscilloscope, measured in megahertz (MHz). The average bandwidth of a low-cost bench scope falls in the MHz range. 2. Resolution The resolution of the scope has to do with its accuracy. The horizontal amplifier indicates the X-axis resolution. Most scopes generally have a resolution of 0.5 microseconds (millionths of a second) or faster. You can adjust the sweep time so that you can test signal events that occur over a longer time period, usually as long as a half a second to a second. Note that the screen can display signal events faster than 0.5 microseconds, but such a small signal may appear as a fleeting glitch or voltage spike. 3. Sensitivity The sensitivity of an oscilloscope indicates the Y-axis voltage per division. The lowvoltage sensitivity of most average-priced scopes is about 5 mv to 5 V per division. You turn a dial to set the sensitivity that you want. When you set the dial to 5 mv, each mark on the face of the scope tube represents a difference of 5 mv. Voltage levels lower than 5 8

4 mv may appear, but you cannot accurately measure them. Most scopes show very low voltage levels (microvolt range) as a slight ripple. Getting Started Although an oscilloscope can do some pretty cool things, you only have to perform a couple of steps to actually use one. Here is a quick rundown of the steps that you perform to measure the voltage of a DC signal with an oscilloscope: 1. Attach a test probe to the scope input. Note: Some scopes have several inputs, called channels; we assume you are dealing with just one input for now. 2. Adjust the Volts per Division control to set the amplitude or voltage range. For example, if the voltage you are testing is 0-5 volts, use the 1 volt per division range. With that setting, each volt corresponds to one tick mark on the screen of the scope. 3. Adjust the Sweep/Time per Division control to set the time slice of the signal. The time slice is the duration of the part of the signal that is shown on the scope. A shorter time slice shows only a brief portion of the signal, whereas a longer time slice shows you more of it. If you are testing a DC signal, you do not need this control because the signal does not change (much) over time. You can choose a medium range setting to ensure consistent readings, such as 1 millisecond per division (a millisecond is one one-thousandth of a second). 4. Select the signal type, either AC or DC, and the input channel. Note that you do not get an input channel selector if you have a single channel oscilloscope. 5. Most scopes have a trigger switch. If yours does, set it to Auto. 6. When you have set up the oscilloscope properly, connect the test probe to the signal that you want to test. 7. Connect the ground of the probe to the ground of the circuit. 8. Connect the probe itself to the circuit point that you want to test. 9. Read the waveform displayed on the screen. Unless your scope has a direct read-out function that displays voltages on the screen, you need to correlate what you are seeing with the settings. If you are testing a low-voltage AC or 9

5 pulsing digital signal, set the Sweep/Time per Division control so that you can adequately see each cycle of the signal. You can experiment with the Sweep/Time per Division control until the signal looks the way that you want it to. IMPORTANT: Do not test AC voltage coming from a wall outlet using an oscilloscope. Use your oscilloscope only to test low-voltage DC circuits, and low-voltage AC signals. If you connect your scope directly to 240 VAC from a wall outlet you can injure both you and your scope! Using an oscilloscope 1. Select the waveform type, amplitude and frequency of the signal from each output of the signal generator. 2. Initially select the 'Mode' switch to CH1 and work with only channel Amplitude Measurement: Vary the 'V/div' knob till your waveform expands vertically to the maximum extent without clipping. Measure the amplitude of the signal by counting the number of divisions the signal occupies peak-to-peak on Y axis and multiplying it by the sensitivity indicated by the 'V/div' knob. Verify the calculated amplitude with the amplitude indicated by the function generator. 4. Frequency Measurement: Vary the time base (ms/div) knob till your waveform expands horizontally to display at least one complete cycle of the waveform. Count the number of divisions one cycle of the waveform occupies along X axis. Multiply it by the sensitivity indicated by ms/div knob to get the period T of the waveform. 1/T gives the frequency of the waveform. Verify the calculated frequency with the frequency indicated by the function generator. 5. Change the 'Mode' switch to DUAL and the second input from the function generator will be displayed through channel 2 along with channel 1. You can adjust the type, amplitude and phase of the second signal. 10

6 Task 1-Operating the Signal generator 1.1.Turn on the signal generator. 1.2.Set it to provide a sine wave with a frequency of 1 khz. 1.3.Connect the voltmeter to the output of the signal generator and measure V rms. 1.4.Set the signal generator to have an output of 1 V pp. 1.5.Measure and record V rms using the voltmeter. V rms = V 1.6.Set the signal generator to have amplitude of 1 V p. Record the voltmeter reading V ****technician approval Task 2-Operating the Oscilloscope When operating an oscilloscope, the ground clips on the probe must be connected to the same point in the circuit as the ground clip from the signal generator. It is very important that during this practical session the student can alter the vertical and horizontal position, the vertical and horizontal scale as well as the triggering controls. Procedure 2.1.Connect channel 1 to the signal generator. Adjust the controls so that you get a good quality display of a sine wave from the signal generator 2.2.Set up a sine wave with V pp of 2 V and Frequency of 1 khz. 2.3.Measure and record the period of the signal 2.4.Calculate the frequency from the measured period 2.5.Record your results in Table Repeat the measurement of period for at least 7 other frequencies between 1 khz and 10 khz 2.7.Plot Plot your measured frequencies, f c (y-axis) against those marked on the oscillator dial, f o (x-axis). 2.8.Draw the line of best fit and determine the gradient of this line. 11

7 Frequency on Number of divisions Time base Period, T (s) Measured oscillator dial, one cycle of the knob Frequency of f o (Hz) waveform occupies indication, S T signal on CRO, f c along X axis, x (s/div) (Hz) 2.9. What is the percentage difference between the two frequencies? Decide whether you will use the frequency dial on the oscillator to determine frequencies for the remainder of the experiment or whether you will need to measure your frequencies from the CRO. Justify your choice. ****technician approval 12