Exercise 1: IR Transmission and Reception

Transcription

1 Exercise 1: IR Transmission and Reception EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain how infrared light can be transmitted and received, describe typical infrared transducers and their uses, and explain the importance and effect of IR power levels. You will use an oscilloscope to make observations and measurements. DISCUSSION This is the radiation pattern of IR light as it is emitted from the IRED on your circuit board. In most IR applications, the physical positioning of the IR transducers is critical due to this typical pattern. The IR emitter on your circuit board is an IRED that is packaged in a clear, standard T LED style epoxy case. Its junction is made of gallium arsenide (GaAs) semiconductor material, which emits IR light at a peak wavelength of 940 nanometers when forward biased. FACET by Lab-Volt 321

2 Transducer Fundamentals f(pk) with the formula: I f(pk) = (V CC V f S Calculate and record I f(pk). I f(pk) = ma (Recall Value 1) The output power of an IR emitter is crucial for reliable operation of IR applications. With I f(pk) at 91 ma, the peak radiant power of the IRED on your circuit board is about 15 mw. radiant intensity of about 14 mw per 322 FACET by Lab-Volt

3 The IR detector on your circuit board consists of a three-pin epoxy package that contains a photodiode and all the support circuitry required to provide a digital output. The internal photodiode is exposed to IR light through a clear plastic lens on the case. The input sensitivity of an IR detector is also critical for proper operation of IR circuitry. The IR detector on your circuit board requires an irradiance of about one-half milliwatt per square 2 ) to trigger the output switch. This sensitivity is high enough to detect the IR light that is emitted from the IRED on your circuit board. waveform can become extremely distorted if the IR transducers and circuits cannot respond to the signal fast enough. FACET by Lab-Volt 323

4 Transducer Fundamentals Even useful IR transducers can cause the transferred pulses to be slightly rounded, as seen here. Therefore, the response time of IR transducers and their associated circuitry is also an important factor for reliable information transfers. Your IR detector can respond to signals up to 125 khz in frequency. when extremely low or excessively high power levels are detected. This is especially true of high-speed signals. 324 FACET by Lab-Volt

5 When irradiance at the IR detector is too low, the received pulse is narrower than the transmitted pulse. Excessively high power to the IR detector causes the received pulse to be much wider than the transmitted pulse. The design of IR circuitry should be optimized for a. speed. b. power. c. Both of the above PROCEDURE In this PROCEDURE, you will make observations and measurements on IR transmitted and received signals by using your oscilloscope. Insert a two-post connector in the BLOCK ENABLE position of the INFRARED CONTROLLER circuit block. FACET by Lab-Volt 325

6 Transducer Fundamentals Connect the CH 1 probe of your scope to the ENCODER OUT test point and the CH 2 probe to DECODER IN. Ground the scope probes to the GND terminal. X10 probes. Set the horizontal sweep for FACET by Lab-Volt

7 Place all DATA switches in the down position for a hex value of zero. To transmit the data (0H), press and hold the XMT pushbutton. FACET by Lab-Volt 327

8 Transducer Fundamentals Compare the transmitted and received waveforms on your oscilloscope. Are the two waveforms similar? a. yes b. no How do the received pulses differ from the original signals? a. They are delayed. b. They are stretched. c. They are shortened. d. They are delayed and stretched. 328 FACET by Lab-Volt

9 A propagation delay, due to the rounding of the received pulse, is caused by the response time of the detector and its associated circuitry. Set the horizontal sweep on your scope to 2 While pressing XMT, measure and record the propagation delay. propagation delay = s (Recall Value 1) The received pulses are stretched due to the relatively high power level of IR light that is incident to the IR detector. Measure and record the received pulse width. T = s (Recall Value 2) If the power level is low, the received pulses would be narrower than the transmitted pulses. Measure and record the transmitted pulse width. T = s (Recall Value 3) The received pulses are wider than the transmitted pulses by about s [(Step 10, Recall Value 2) (Step 11, Recall Value 3), which indicates that the power level for FACET by Lab-Volt 329

10 Transducer Fundamentals pressing XMT, observe the oscilloscope. What happens to the received waveform when the IR light path is completely blocked? a. Its pulse amplitude decreases to zero. b. Its pulse amplitude increases. c. Its pulse width increases. d. Nothing happens to it. only a portion of IR light is blocked from the IR detector. What happens to the received waveform when the IR light path is only partially blocked? a. Its pulse amplitude decreases. b. Its pulse period increases. c. Its pulse width decreases. d. Nothing happens to it. the radiant power and intensity of the IRED, and thus the irradiance at the IR detector. 330 FACET by Lab-Volt

11 While pressing XMT, observe the oscilloscope and toggle the CM switch off and on. What happens to the received waveform when the radiant power level is increased? a. Its amplitude increases. b. Its period decreases. c. Its pulse width increases. d. Nothing happens to it. Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION IR light can be emitted and detected through air. Special semiconductor PN junctions can be used as IR light sources or IR light sensors. An IRED is an output transducer that converts electrical energy to radiant energy in the form of IR light. An IR detector is an input transducer that converts radiant energy in the form of IR light to an electrically usable signal. The response time of IR transducers and circuitry is important to minimize signal distortion. IR power levels are crucial to ensure reliable communications. FACET by Lab-Volt 331

12 Transducer Fundamentals REVIEW QUESTIONS 1. When you activated the CM in the PROCEDURE, you reduced the value of R S to 103. What was the new peak forward current, I f(pk), of the IRED on your circuit board? I f(pk) = (V CC V f s a ma b. 91 ma c. 132 ma d. 1.5 A 2. What is(are) the advantage(s) of IR transducers with fast response times? a. minimum propagation delays b. minimum signal distortion c. maximum output power d. Both a. and b. 3. Why are the received pulses stretched at the DECODER IN test point on your circuit board, as compared to the pulses at ENCODER OUT? a. excessive irradiance at the IR detector b. fast response time of the IR detector c. inadequate irradiance at the IR detector d. slow response time of the IR emitter 4. On your circuit board, what could cause the received pulses from the IR detector to be narrower than the originally transmitted pulses? a. low irradiance b. low radiant intensity c. low radiant power d. All of the above 332 FACET by Lab-Volt

13 5. Why are the IR emitter and detector aligned with each other on your circuit board? a. to increase IR detector sensitivity b. to maximize coupling c. to decrease eye sensitivity d. to minimize transmission media length FACET by Lab-Volt 333