Lab 1 - Analogue and Digital Signals

Transcription

1 Lab 1 - Analogue and Digital Signals Objective 1. 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 R1 and R2: R1 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 2013 V

2 e d c b a In this case we can use columns 1, 4, and 7 for the nodes A, B, and C, and insert the resistors into the breadboard as follows: 1 2 e d c R2 b a R1 In this scenario the connection between R1 and R2, at node B, is made by the internal connection 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: 1 2 e d c R2 b a R1 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 2013 V

3 Part1.1 - 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 + Microphone (Bottom view) +5V 2.2k microphone To view the output signal we will need to configure the oscilloscope appropriately. This involves adjusting the resolution/scaling of the display nF Output (monitor with oscilloscope) Vertical Horizontal 1 Volts/Div Sec/Div CH1 20.0mV CH2 1V M2.50ms CH1 1.5V 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. 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. 1 CH1 2V CH2 1V M2.5ms CH1 1.5V Edge Slope Source Ch1 Mode Single Coupling Trigger Level Trigger Menu Press the Trigger Menu button on the right side of the oscilloscope. Using the onscreen controls: B. YOSHIDA, P.ENG 2013 V

4 Set the Source to Ch 1. Set the Mode to Single. Using the Level adjustment knob, adjust the arrow on the right side of the display to be higher than the 0 V level which is indicated by the left arrow on the screen. 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 Stop will appear at the top of the screen. To reset the oscilloscope press the Run/Stop button located at the top corner of the oscilloscope. 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? Part 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: B. YOSHIDA, P.ENG 2013 V

5 Push Button Switch 1Hz sinusoid Signal Generator (monitor with oscilloscope) 100µF + + Output (monitor with oscilloscope) Construction notes: When mounting the push button, have the switches on either side of the channel in the breadboard. Look at the labelling on the capacitor and make sure that it is the negative terminal which is connected to ground. Adjust the output of the signal generator to provide a 5v 1Hz sinusoid (5sin(2πt)V). Adjust the vertical scale on the oscilloscope to 5V/div, and the horizontal scale to 250ms/div. 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. Sample the input signal at a slow rate (i.e. infrequently compared to frequency of input signal). Sketch the input and output waveforms. B. YOSHIDA, P.ENG 2013 V

6 Sample the input signal rapidly (i.e. at a frequency higher than the frequency of the input signal). Sketch the input and output waveforms. Part Analogue to Digital Conversion Devices We can use prebuilt, commercial-off-the-shelf devices to perform analogue to digital conversions. 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 D1 10µF 10k Construction notes: 150pF make momentary connection to ground to start b b a c a c Potentiometer (Bottom View) 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 10º to 350, estimate the angle to turn on each successive 0.1 µf 5V 10k 0.1µF B. YOSHIDA, P.ENG 2013 V

7 LED. Alternatively if the range of motion is from 1 o'clock to 11 o'clock determine when each successive LED turns on. D0 D1 D2 D3 D4 D5 D6 D7 Is the scale linear? Part 2 - Points to Ponder 1. Analogue Signals 1.1. Why is the output of this circuit considered to be analogue? 1.2. What would the output of the circuit look like if we were able to whistle a pure tone into the microphone? 1.3. Why did we need to use the single trigger mode to analyze the signal What is the effect of the sampling rate on generating an accurate set of numbers which can represent the continuous signal? What is the result of not sampling fast enough? Is there an upper limit to the sampling rate? 1.5. 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 2.1. Is the output of the sample and hold an analogue or a digital value? 2.2. What would the output look like if the switch were held closed? 2.3. What would the be the effect of increasing the size of the capacitor be on the output of the circuit? 2.4. 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) 2.5. Based on your previous answer, what effect would changing the capacitor have? 3. A/D Converter B. YOSHIDA, P.ENG 2013 V

8 3.1. What is the actual analogue input 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. Why did we not need a sample and hold at the input of the A/D converter? 3.5. How many different (unique) outputs can the specific A/D converter used in the lab produce? 3.6. What, if any, limitations are there in using the specific A/D converter used in the lab? B. YOSHIDA, P.ENG 2013 V