Laboratory experiments and reports

Transcription

1 LABORATORY INSTRUCTION MANUAL Page 1 of 8 Laboratory experiments and reports Summary This document describes how to carry out experimental exercises, and how to prepare the lab reports for the Electronic System course. Goals and procedures for specific exercises are in the respective instruction manuals. Goals Lab exercises are an important part of various courses in Electronics. Their main goals are: Become familiar with the standard electronic instruments; verify the correspondence of actual circuits with design specifications; train in the preparation of lab and project reports; train in teamworking. The circuit behaviour verification has positive and negative aspects. From the measurements it is possible to check if the actual circuit matches the expected behaviour, and verify the limits of the mathematical models described in the lessons and used for the design. Every experiment includes the simulation of the circuit, the setup of the measurement bench, using measurement instruments (power supply, signal generator, scope, digital multimeter) and a pre-wired circuit, which can be configured for specific measurements. The lab work includes preparation of a report, which must must describe simulation results (when required), measurement procedure and results. The report must be prepared during the lab session, and must be left in the lab. Forms for la reports are provided at the end on the instruction booklets; the first one requires only insertion of experimental results, while the following ones require increasing completion work (the last one is fully open). Within lab schedule constraints, it is possible to repeat an experience, and prepare a new report. The design, simulations, circuit setup, and the measurements are carried out by a working team. To complete the requested task in the available time the simulations must be carried out before the lab session, and all the work must be carefully organized. Each group member should have well identified tasks and roles, which must be described in the lab report. These notes are organized in the following sections: 1. Goal analysis 2. Simulation 3. Measurement setup and wiring 4. Measurements 5. Malfunction analysis 6. Lab reports 7. Demo experiment The information provided with this document must be combined with the instructions for each experience and with the general rules for the LADISPE, available from the LADISPE website:

2 LABORATORY INSTRUCTION MANUAL Page 2 of 8 Goal analysis For each experiment, the instruction guide specifies a set of goals. The first step (before starting the bench setup and the measurements) is to verify the understanding of these goals. The working team must be able to answer the following questions: - Which is the theory subject related with the experiment? - Which is the specific behaviour to be verified or measured? - Which are the steps to be carried out for the measurements? - Which are the expected results? - How could these results change because of faults, malfunction, errors? - How can we check if the global goal of the experiment has been achieved? Each experiment must be developed according to the following sequence: 1. Read the guide and verify understanding of goals; 2. Perform Spice simulations and/or other analysis as required. The time available in the lab is limited, and must be used mostly for measurements. The analysis must be completely carried out before the lab session. Each group must come to the lab with the analysis results, and a draft work plan for the measurements. 3. Verify instrumentation and other material required, as specified in the lab guide. 4. Set the measurement bench, as specified in the lab guide. 5. Verify the measurement setup. Check cable position and connection; Check correct operation of instruments; Set voltage and current limit in power supplies; Turn on instruments (to allow for temperature and other parameters stabilization) Verify correct compensation of scope probes. 6. Carry out measurements, taking notes of instruments setup and results. 7. Analyse results (compare with expected ones, find reason of differences); 8. Complete the lab report; 9. Verify, sign and deliver the report. In many cases the same tests can be carried out with different measurement set-up; procedures different from the one indicated in the lab guides are acceptable, as long as they allow to achieve the planned goals, and are described in the lab report. The sequence of tests in the lab guides can cover a time longer than the one assigned for the experiments. Some parts may be carried out as demo by the lab assistant. The mandatory part will be specified in the lesson before the lab.

3 LABORATORY INSTRUCTION MANUAL Page 3 of 8 Simulation The behaviour of the circuits used for the lab experiments must be verified before the measurements, using simulators like SPICE. This step allows verifying correctness and compliance with specs, within the limits of available models. The demo version of PSPICE is available from the lab website (page bottom): and from the Cadence website: The first lab exercise is actually a Spice learning session; it is available also as video lesson (follow the links in the website). Prepare and run simulations at home or in the LAIB, to reserve the lab session for measurements. Measurement bench setup and wiring At the beginning of lab sessions all groups must check the instruments and other equipment on the workbench, for compliance with the displayed list. Verify if some additional instrument or part is required. To connect instruments and the measurement circuit use only the cables and wires available on the work bench. Do not move instruments, cable, tools, any material from one bench to another. The measurement setup must be disassembled at the end of the experience or before the end of the lab session (whichever comes first). Instruments, devices, boards must be placed in the same position as at the beginning of the experiment. If some additional passive component has been used, it must be returned at the correct position in the lab storage. Other notes for correct wiring, circuit assembly, and equipment use are in the document Use of solderless breadboard.

4 LABORATORY INSTRUCTION MANUAL Page 4 of 8 Measurements Avoid damages The measurements are required to verify the correct circuit operation and compliance with specs. In some cases also the effect of small design variations can be evaluated. Follow the measurement sequence described in lab guides. To avoid damage to the circuit or other equipment, NEVER apply on any node voltages outside the power supply range (ground +5 V for logic circuits, V for analog circuits with Op Amp.). Voltage levels less than the minimum negative supply voltage (or 0V) or higher than the maximum positive supply voltage cause the pn junctions between input nodes and supplies, causing latch-up and high currents which may destroy the circuit. Therefore: do not apply any external signal to circuits with power off, turn off signal generators (or disconnect the inputs) BEFORE turning off or removing the power supply. When the circuit requires a variable voltage (e.g. to create a variable reference), get it with a voltage divider (potentiometer) connected between ground and the circuit power supply. In this way no dangerous voltage (higher than supply) can reach the circuit. For trimming of the voltage, use two potentiometers, connected respectively as coarse and fine regulation. To avoid damage, apply external voltages through high-value resistors, to limit the current in case of fault or mistakes. In circuits with dual voltage supply, remember to connect the intermediate node of the power supply to the circuit reference node (GND). Direct cable or Probe? Oscilloscopes and other instruments can use direct cables or probes. Direct cables put on the node under measurement a load corresponding to input impedance (1 MΩ for scopes), and cable capacitance (about 100 pf/m). Usual probe impedance is 10 MΩ with pf in parallel. Use probes instead of direct cables to reduce the load on the circuit under measurement Probes are mandatory for HF, RF, or fast digital signals. The probes usually attenuate the signal by a factor 10. In some cases the scope can sense the attenuating probe and compensates automatically the display scale; in more simple instruments the attenuation must be entered with hand commands.

5 LABORATORY INSTRUCTION MANUAL Page 5 of 8 Grounding Scope input have a ground pin connected to the instrument metal body which, for electrical safety, is connected to the earth through the mains cord. The same can occur for other instruments, and you must assume that Therefore the ground pins of various instruments may be connected each other through the earth. The ground pin (usually an alligator clip) of probes must always be connected to the circuit reference node (Ground). Do not try differential measurements using the ground and the signal pins; the ground would short the node where it is connected. Laboratory power supplies usually have all outputs floating with respect to ground. Any measurement is affected by errors; therefore results must be given with a number of digits related with this error. Instruments with digital readout usually have an error corresponding to some units of less significant digit. Unless otherwise indicated, the error in scope measurements is about 5% (of full scale). The same figure applies to cursors with digital position reading, due to misalignment with signal traces. Error evaluation is an integral part of the measure process; the results must be included in the lab report. But in few special cases (e.g. multiple measurements in the proper conditions), mathematical processing does not improve precision. For instance, from scope measurements is not possible to get results with 6 correct digits. The scope allows to keep an eye on circuit behavior, for a quick estimation of quantities under measurement, and to verify the results of other instruments. Keep always the scope connected to the circuit at nodes which allow to verify correct operation, even if the actual measurements are performed using other instruments. Malfunction analysis The goal of lab exercises is to verify circuit operation, and understand the reasons of deviations with respect to circuit analysis. During preliminary circuit analysis and simulation, you should try to figure out expected results of measurements (not the exact value; waveforms, order of magnitude, ). If measurement results are not the expected ones, put in the report actual experimental results, and analyze the reasons of disagreements. Do not modify the circuit to get desired performance; only in the case of total malfunction or strong disagreement with simulation results, execute the following fault analysis procedure.

6 LABORATORY INSTRUCTION MANUAL Page 6 of 8 1. Verify the power supply and connection with measurement instruments. 2. Check instrument setup and operation (supply voltage, scope scale, level and frequency of signals, ). 3. Verify matching between design, schematic, wiring. 4. Verify (in the case by measurements) the value of components used. 5. Check for false contacts by measuring signals directly on the device and IC pins. 6. Divide the circuit in simple parts which can be individually verified, and repeat the sequence above on each of them. Lab reports For each lab experience the working group must write a report, which should include: 1. the title of the exercise, date, participants; 2. a short description of the circuit (when pre-assembled) or the design specs. If some design has been carried out, outline the procedure, the approximation, and the results. 3. the electric diagram of the circuit, with standard functional symbols; 4. a complete part list, (the test circuit on PCB must be considered a single part ), with all information regarding type and values of additional components; 5. a plan of measurements to verify circuit operation; 6. instruments used to carry out measurements; 7. measurement procedures and results, with error evaluation; use table, graphs, histograms, and anything else can be useful to point out unusual or important results. The measurement result table must be filled during the measurements; data can be processed or translated in graphic form later. 8. a comparison of measurement results vs expected results (computed in the design phase) taking into account tolerances 9. a discussion of differences design/simulation/measurements. Do not include topographic schematics of the ICs or of the wiring. As for any report on experimental work, the report must include all information required to repeat the experience and verify results. As for any document, the report must include information such as date, filename, author, version, file location.

7 LABORATORY INSTRUCTION MANUAL Page 7 of 8 How to verify correct operation of probes Connect the probe to a squarewave generator (usually available inside the scope, with output on the front panel), and check the shape of edges (figure A). A response with exponential parts indicates wrong probe compensation, which means poles and zeroes in the frequency response (figure B). In such conditions the measurements carried out using the probe are not correct. To tune the probe ask assistance from lab staff (tuning requires special tools). Vu Vu t t Fig. A Proble response with correct-compensation Fig. B Probe response with wrong compensation (blue and red signals) Further details on probes seen as a voltage divider are in the instruction manual for lab 2. Demo experiment Some experiments include a final part, without measurements, to outline some specific behavior of the circuit. In most cases it requires special instruments, non available on alla student lab benches. These experiments are carried out by the assistant, when the other tasks are completed.

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