Lab 1 - Analogue and Digital Signals

Transcription

1 Lab - Analogue and Digital Signals Objective. To reintroduce the equipment used in the lab. 2. To get practical experience assembling and analyzing circuits. 3. To examine physical analogue and digital signals and devices. Introduction In most practical applications the information and signals we are interested in are in an analogue form. These analogue signals can vary continuously in time and can take on an infinite number of values. For digital electronics to process theses 'real world' signals, the analogue signals must be first converted into a digital form. Likewise, for the processed results to be useful to a human, the digital information often, but not always, needs to be converted back into an analogue form. We can build circuits to analyze these types of signals using a breadboard and some simple electronic devices. Recall, from the prerequisite course, the general concept of the breadboard. It is a device which allows us to connect circuit components without needing a circuit board or solder. No connection between points No connection between points Electrically connected If we want to connect two devices using the breadboard, for example the resistors R and R2: R R2 A B C We can use the internal connections within the breadboard to act as the nodes 'A', 'B', and 'C'. B. YOSHIDA, P.ENG 206 V2.2.0 LAB -

2 2 e d c b a In this case we can use columns, 4, and 7 for the nodes A, B, and C, and insert the resistors into the breadboard as follows: e d c R2 b a R In this scenario the connection between R and R2, at node B, is made by the internal connections in column 4 which exists in the breadboard. It is important to keep in mind the internal connections in the breadboard and not connect the resistors as follows: e d c R2 b a R If the parts are inserted into the breadboard as shown, both ends of R2 are connected to each other. In other words nodes B and C are effectively connected together, and R2 is shorted out of the circuit. B. YOSHIDA, P.ENG 206 V2.2.0 LAB -2

3 Part. - Analogue Signals To 'see' an actual analogue signal, we can build the following circuit which uses a microphone to convert the sound waves into an electrical signal. Pin +5V + 2.2kΩ 00nF Microphone (Bottom view) microphone + + Output (monitor with oscilloscope) To view the output signal we will need to configure the oscilloscope appropriately. The connection to the oscilloscope involves connecting the tip of the probe (or the alligator clip connected to the plastic insulated wire) to the + and the ground clip (or the alligator clip connected to the braided wire) to the. Remember that the oscilloscope measures voltage which is the potential difference between two points, therefore we need to connect the oscilloscope to both the + signal, and reference terminals of the circuit. We also need to adjust the resolution/scaling of the display. Vertical Horizontal Volts/Div Sec/Div CH 20.0mV CH2 V M2.50ms CH.5V Using the Volts/Div control, set the vertical scale to 20.0mV/division. Using the Sec/Div control, set the horizontal scale to 2.50ms/division. The current setting are shown at the bottom of the display. At this point you should be able to speak into the microphone and see a signal on the oscilloscope screen. Rather than try to examine a constantly changing display we can configure the oscilloscope to capture a single event using the oscilloscope's single trigger mode. (The following instructions apply to the Tektronics TDS 002 oscilloscope, for other models the procedure is the same but the controls may be in different positions or have different labels.) B. YOSHIDA, P.ENG 206 V2.2.0 LAB -3

4 Auto Set Run/Stop Single Seq R Ready Horizontal Position Trigger Level CH 20.0mV CH2 V M2.50ms CH.5V Press the Single Seq button on the right side of the oscilloscope. This will set the oscilloscope in a standby mode where it will wait for an input signal. To set the parameters of the input signal which will be captured: adjust the trigger level (as indicated by the arrow on the right side of the display) using the Trigger Level control knob such that it is above the reference level (as indicated by the arrow on the left side of the display - in this case channel is the input); adjust the start position of the capture (as indicated by the arrow at the top of the display) using the Horizontal Position knob such that the arrow is on the left side of the display and with a vertical orientation. At this point the word Ready will appear at the top of the screen. You can now speak into the microphone and a single stationary waveform representing whatever you said will be captured on the oscilloscope. At this point the word Acq Complete will appear at the top of the screen. To reset the oscilloscope press the Single Seq button again or the Run/ Stop button located at the top corner of the oscilloscope. Provide an input sound for the microphone (speak, clap, etc.) Sketch the waveform you captured: Does this waveform correspond to what you would expect? Since the circuit is translating a sound pressure wave into an electrical signal, does the duration match the sound you provided to the microphone? Does the general shape of the amplitude match the sound you provided as the input? B. YOSHIDA, P.ENG 206 V2.2.0 LAB -4

5 Part.2 - Sample and Hold In order to process a signal using digital electronics the signal has to be maintained (held constant) while it is converted. A sample and hold circuit can perform this holding function. Build the following circuit: Push Button Switch Hz sinusoid Signal Generator (monitor with oscilloscope) 00µF + + Output (monitor with oscilloscope) Construction notes: When mounting the push button, have the switches (as shown in the above figure) on either side of the channel in the breadboard (i.e. one switch above the channel and one switch below the channel.) Look at the labelling on the capacitor and make sure that it is the negative terminal of the capacitor which is connected to ground. The connection to the Signal Generator involves connecting the signal wire (or the alligator clip connected to the plastic insulated wire) to one side of the switch, and the ground clip (or the alligator clip connected to the braided wire) to the shared ground. Adjust the output of the signal generator to provide a 5v Hz sinusoid (5sin(2πt)V). For the Stanford Research Systems DS340 Function generator: to set the frequency to Hz, press the FREQ button, followed by, finally the units of Hz (Vrms/Hz) button; to set the amplitude to 5 V, press the AMPL button, followed by 5, finally the voltage peakto-peak (Vpp/kHz) button. Adjust the vertical scale on the oscilloscope to 5V/div, and the horizontal scale to 250ms/div. The connection to the oscilloscope involves connecting the tip of the probe (or the alligator clip connected to the plastic insulated wire) to the + and the ground clip (or the alligator clip connected to the braided wire) to the. Try to press and release the switch (sample the input signal) so that the output is at the peak (5V). Try to sample the input so that the output is 0V. Determine if it the closing of the switch, or the opening of the switch which determines the output voltage (the value of the sample). Explain why. B. YOSHIDA, P.ENG 206 V2.2.0 LAB -5

6 Sample the input signal at a slow rate (i.e. infrequently close the switch compared to frequency of input signal). Sketch the input and output waveforms. Sample the input signal rapidly (i.e. close the switch at a frequency higher than the frequency of the input signal). Sketch the input and output waveforms. Compare the resulting output waveforms (as compared to the input waveform) from the slow sampling rate and the rapid sampling rate. What can you conclude? Is one preferable over the other? Which is better? Part.3 - Analogue to Digital Conversion Devices We can use prebuilt, commercial-off-the-shelf devices to perform analogue to digital conversions. B. YOSHIDA, P.ENG 206 V2.2.0 LAB -6

7 Build the following circuit using an ADC0804 A/D converter: 5V To built in LEDs on green circuit board. D8 D7 D6 D5 D4 D3 D2 D Connection to LEDs: Construction Notes: D8 D7 D6 D5... SD SC SB SA 0µF 0k pF 0. µf 5V 0.µF make momentary connection to ground to start 0k 23 0kΩ Potentiometer: b a a c Potentiometer (Bottom View) b 9 0 c Determine the full range of motion for the potentiometer. Adjust the setting of the potentiometer and determine the what positions are required to turn on each of the LEDs. i.e. If the full range of motion is from 0º to 350, estimate the angle to turn on each successive LED. Alternatively if the range of motion is from o'clock to o'clock determine when each successive LED turns on. D D2 D3 D4 D5 D6 D7 D8 Is the scale linear? Part 2 - Points to Ponder B. YOSHIDA, P.ENG 206 V2.2.0 LAB -7

8 . Analogue Signals.. Why is the output of this circuit considered to be analogue (i.e. what are the characteristics of an analogue signal)?.2. What would the output of the circuit look like if we were able to whistle a pure tone into the microphone?.3. Why did we need to use the single trigger mode to analyze the signal..4. What would happen if we used the output of the microphone as the input to the D/A converter? What would the output of the D/A converter be? 2. Sample and Hold 2.. Is the output of the sample and hold an analogue signal or a digital signal? 2.2. What would the output look like if the switch was continuously held closed? 2.3. What is the result of not sampling fast enough? Is there an upper limit to the sampling rate? 2.4. What is the purpose of the capacitor? 2.5. What would be the effect of increasing the size of the capacitor be on the output of the circuit? 2.6. Why does the input waveform get distorted when the switch is closed? (You can think about this in terms of a mechanical system where signal source is replaced by an engine) 2.7. Based on your previous answer, what effect would changing the size capacitor have (i.e. is the response/performance of the circuit better with a smaller capacitor or a larger capacitor)? 3. A/D Converter 3.. What is the actual analogue input for circuit which is being converted? 3.2. Is there a mathematical relationship between the input voltage and the LEDs? Is yes what is the relationship? Linear? Exponential? Something else? 3.3. What is it about the output of the A/D converter that makes it a digital value? 3.4. How many different (unique) outputs can the specific A/D converter used in the lab produce? 3.5. What, if any, limitations are there in using the specific A/D converter used in the lab? 3.6. Why did we not need a sample and hold at the input of the A/D converter? Under what conditions might we need a sample and hold at the input? B. YOSHIDA, P.ENG 206 V2.2.0 LAB -8