Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
1 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Willkommen, Hermann Freiberger (Das sind nicht Sie?)
Dokumententyp: Tutorium
Von NI unterstützt: Ja
Veröffentlichungsdatum: 18.11.2011
Lesson 8 - Free Space Optical Communications
Übersicht
As part of the National Instruments Introduction to NI ELVIS II, NI Multisim, and NI LabVIEW courseware, the labs introduces students to the
capabilities of the NI ELVIS II educational design and prototyping platform. Students can explore how NI ELVIS II allows for an easy
transition from design, simulation to prototype as it interfaces with both NI Multisim and LabVIEW software.
The courseware includes 11 lab experiments starting with the an introduction to the NI ELVIS environment and steps you through AC circuits
to communications. The labs are designed as a starting point for your own curriculum design, demonstrations in the classroom, and method
to inspire students to be imaginative and creative in their design projects.
View all of the labs for the Introduction to NI ELVIS II, NI Multisim and NI LabVIEW courseware.
Goal
This lab uses an infrared optical source to communicate information over free space to a
phototransistor detector. Explore several modulation schemes including amplitude
modulation and nonreturn-to-zero (NRZ) digital modulation. For the Multisim challenge at the
end of this lab, simulate the free space optical link built on the NI ELVIS II protoboard.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
8.
Downloads and Required Components
Exercise 8.1: A Phototransistor Detector
Exercise 8.2: Infrared Red Optical Source and Test Circuit
Exercise 8.3: Free Space IR Optical Link (Analog)
Exercise 8.4: Amplitude and Frequency Modulation (Analog)
Exercise 8.5: Free Space IR Optical Link (Digital)
Multisim Challenge: Design a High-Speed Optical NRZ Data Link
Related Links
Downloads and Required Components
Filenames:
lab8_lv2011.zip
lab8_ms11.zip
lesson8-optical_comm.pdf
Download the files above to access the courseware pertaining to this lesson that illustrate
how the NI ELVIS II can be used with NI Multisim and NI LabVIEW covering a range of tools
from introductory level exercises to design challenges.
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
2 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Figure 8.0. Free Space Infrared Optical Digital Communications Link
Many homes today have several remote controllers lying around the house such as those
controlling televisions, stereos, and DVD players. Do you know how these controllers work?
The secret is an infrared optical data link, which is a type of free space optical
communication link.
Required Soft Front Panels (SFPs)
Two-wire current-voltage analyzer (2-Wire)
Three-wire current-voltage analyzer (3-Wire)
Function generator (FGEN)
Oscilloscope (Scope)
Digital writer (DigOut)
Required Components
220 W resistor (red, red, brown)
470 W resistor (yellow, violet, brown)
1 kW resistor (brown, black, red)
22 kW resistor (red, red, orange)
0.01 mF capacitor
IR emitter (LED)
IR detector (phototransistor)
1
2N3904 NPN transistor
555 timer chip
1
RS276-142 IR emitter and detector pair are available at www.radioshack.com.
Exercise 8.1: A Phototransistor Detector
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
3 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Understanding how a phototransistor works starts with understanding transistor
characteristic curves. A transistor is basically a current-controlled amplifier. A small base
current controls the current flowing through the transistor from the collector to the emitter.
Complete the following steps to determine the characteristic curve of a transistor.
1. Insert a 2N904 transistor on an NI ELVIS II protoboard.
2. Connect the emitter, base, and collector leads to pin sockets DUT-, DUT+, and BASE, as
shown in Figure 8.1.
Figure 8.1. Connections for 3-Wire Transistor Curve Tracer Measurements
NOTE: Base → Base, DUT- → Emitter, and DUT+ → Collector Leads
3. Launch the NI ELVIS II Instrument Launcher strip and select the Three-Wire CurrentVoltage Analyzer (3-Wire).
4. Power on the protoboard.
5. Set the Base Current and Collector Voltage as shown in Figure 8.2 and click on Run.
Figure 8.2. Typical characteristic curves for a 2N3904 Transistor
The graph displays the collector current versus collector voltage for different values of the
base current. You can set many parameters for the collector voltage and the base current
ranges. When running, this SFP first outputs a base current, then outputs the collector
voltage, and finally measures the collector current. Data points (I,V) are plotted as sequential
points with the same base current connected with a line. You can see the curves develop as
the program proceeds, resulting in a family of [IV] curves with different base currents.
Observe that for a given collector voltage, the collector current increases with an increase in
base current.
A phototransistor has no base lead. Instead, light falling on the transistor generates a base
current proportional to the light intensity. For example, with no light, the phototransistor might
follow the bottom (yellow) curve. For a low-light level, the middle (red) curve might be
generated, and, at a higher-light intensity, the upper (green) curve might be found. For
collector voltages greater than 0.4 V, the collector current follows the light intensity falling on
the base region in an almost linear fashion. To build an optical detector, all that is needed is
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
4 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
a power supply, a current limiting resistor, and a phototransistor, as shown in Figure 8.3.
Figure 8.3. Phototransistor Detector Circuit
6. Close any SFPs
End of Exercise 8.1
Exercise 8.2: Infrared Red Optical Source and Test Circuit
The optical transmitter is made up of just two components, an infrared (IR) LED (forwardbiased) and a current limiting resistor.
Complete the following steps to test and analyze an IR LED and build a simple optical link:
1. Connect the IR LED to the DMM/Impedance Analyzer pin sockets (DUT+) and (DUT-).
Make sure the LED anode (short lead) is connected to (DUT-).
2. From the NI ELVIS II Instrument Launcher strip, select the Two-Wire Current-Voltage
Analyzer (2-Wire). Set voltage sweep parameters to
Start
0.0 V
Stop
+2.0 V
Increment
0.05 V
3. Click Run.
The [IV] curve for the infrared diode is developed and displayed.
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
5 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Figure 8.4. IR LED I-V Characteristic Curve
For voltages greater than about 0.9 V, the IR LED emits light in the forward-bias region. Light
is emitted at a wavelength of 950 nm, outside the spectral range of human eyesight and in
the near infrared region. The LED specs tell you that the maximum allowed current is more
than 100 mA, making IR LEDs about 10 times brighter than normal visible LEDs. The
intensity of the IR LED gives the remote controllers a great amount of range. Connecting the
LED in series with a 220 W resistor and a +5 V power supply produces a current of about 11
mA, yielding about 10 mW of invisible optical power. It takes a special detector like your
phototransistor to see it.
4. Build the LED transmitter circuit and the phototransistor detector circuit on the
protoboard, as shown in Figure 8.5.
Figure 8.5. Free Space IR Transmitter-Detector Circuit
5. Connect the output of the function generator pin socket (FGEN) to the IR LED anode pin.
6. Connect the output of the phototransistor to the pin socket (AI 0+) and Ground to (AI 0-).
Taken together, these circuits form a simple optical data link. Figure 8.6 shows this circuit on
the NI ELVIS II protoboard.
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
6 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Figure 8.6. Free Space IR Communications Link on NI ELVIS Protoboard
7. Close any SFPs.
End of Exercise 8.2
Exercise 8.3: Free Space IR Optical Link (Analog)
Complete the following steps to test your free space optical link:
1. From the NI ELVIS II Instrument Launcher strip, select Function Generator (FGEN) and
Oscilloscope (Scope). The function generator provides the analog signal to be transmitted.
The oscilloscope monitors the input signal on CH 0 and the output signal on CH 1.
To transmit an analog signal on the LED, you need to bias the LED into the forward-biased
region with a bias voltage greater than the turn-on voltage (~1 V).
2. On the FGEN SFP, set the Offset voltage to +1.5 V.
3. Set the additional parameters on the FGEN SFP
Amplitude:
0.5 V
Waveform:
Sine
Frequency:
1000 Hz
4. Run the function generator and oscilloscope, and observe the transmitted and received
signals.
5. Change the offset voltage and amplitude levels. When the received sine wave starts to
distort, the transmitter becomes nonlinear.
6. Find the best offset voltage value where the signal amplitude shows minimum distortion.
The IR optical link is now ready to send data.
7. Leave the function generator and oscilloscope SFPs open.
End of Exercise 8.3
Exercise 8.4: Amplitude and Frequency Modulation (Analog)
Complete the following steps to test a free space optical link:
1. On an NI ELVIS II protoboard, wire the analog output pin sockets (AO 0) and (AO 1) to
the function generator pin sockets amplitude modulation (AM) and frequency modulation
(FM), respectively.
2. Launch the LabVIEW program Modulation.vi.
NOTE: This LabVIEW program is configured to connect to“Dev1” for your NI ELVIS
workstation. If your device is configured to another device name, you need to rename your
NI ELVIS workstation to “Dev1,” in Measurement & Automation Explorer (MAX) or modify
the LabVIEW programs to your current device name.
3. The function generator can only utilize one type of modulation at a time. Be sure to
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
7 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
select which one you want to use in the waveform settings box on the function generator
SFP.
4. Run both the function generator and the Modulation VI.
This program sends a DC signal from the NI ELVIS II analog output to the modulation input of
the function generator to produce an amplitude or frequency modulated signal. The
modulated signal is converted to an intensity modulate light signal that is sent across your
free space optical link. The phototransistor detects it and converts it back into an electrical
signal. You have just built an elementary free space optical communication link for analog
signals.
5. Close any SFPs and LabVIEW.
End of Exercise 8.4
Exercise 8.5: Free Space IR Optical Link (Digital)
IR remote controllers use a special encoding scheme called NRZ. A HI level is signaled by a
tone burst of 40 kHz square waves while a LO level is signaled by the absence of any
signal. A tone burst is generated using the 555 timer circuit shown in the Figure 8.7 circuit. A
digital switch is connected to pin 4 [RESET], so that when the switch is HI, a tone burst is
generated. When the switch is LO no oscillations occur.
To demonstrate the modulation scheme, use a 1.0 kHz tone burst so it is easy to see on the
oscilloscope.
Figure 8.7. Tone Burst Oscillator connected to Optical Link
Complete the following steps to build the gated digital oscillator.
Build a gated oscillator using a 555 timer chip and the following components:
RA: 1.0 kW
RB: 1.0 kW
C: 0.5 mF
Connect pin 4 on the 555 timer chip to the digital line (DI 0) on the NI ELVIS II protoboard.
Connect the oscillator output pin 3 to the IR LED anode pin.
Connect the output of the detector circuit to the oscilloscope (CH 0) BNC socket.
7. Connect pin 1 of the 555 timer chip to Ground.
8. From the NI ELVIS Instrument Launcher strip, select Oscilloscope (Scope) and Digital
Writer (DigOut).
7. For the oscilloscope, select Scope CH 0 as Source, Trigger on Edge, CH 0 Source,
Level 1.0 V.
In operation, every time you set Bit 0 (DO 0) of the Digital Writer to HI, a 1 kHz signal
appears on the oscilloscope. No signal is presented when Bit 0 is LO.
29.01.2012 23:20
Lesson 8 - Free Space Optical Communications - Developer Zone - Nat...
8 von 8
http://zone.ni.com/devzone/cda/tut/p/id/13557#h32
Try some of the other digital patterns like Walking 1s or Ramp, and view the modulation
scheme on the oscilloscope panel.
In remote controllers, the encoding scheme is slightly more complicated. If you are interested
in building a computer-controlled IR remote transmitter, refer to Sensors, Transducers and
LabVIEW by Barry E. Paton for details.
Figure 8.8. Tone Burst Oscillator Controlled from a Digital Line (red lead)
End of Exercise 8.5
Multisim Challenge: Design a High-Speed Optical NRZ Data Link
Code the digital IR optical data link circuit of Exercise 8.5 with Multisim. Use the NI ELVIS II
schematic format for the virtual circuit. Determine the bit rate that you can send using this
circuit.
How can you improve the bit rate performance? Change the components in the design to
generate a high-speed digital optical link.
Related Links
1.
2.
3.
4.
5.
Go back to Lesson 7 - LEDs to the Rescue!, part of the Introduction to NI ELVIS II, NI Multisim, and NI LabVIEW Courseware
Go on to Lesson 9 - RF Wireless Communications, part of the Introduction to NI ELVIS II, NI Multisim, and NI LabVIEW Courseware
Back to the Introduction to NI ELVIS II, NI Multisim, and NI LabVIEW courseware
Watch this introductory NI ELVIS II video to learn more about the education design and prototyping platform
Find more courseware on NI ELVIS II, NI LabVIEW, and NI Multisim
AGB
Dieses Tutorium ("Tutorium") wurde von National Instruments ("NI") entwickelt. Auch wenn National Instruments dieses Tutorium technisch
unterstützt, ist es jedoch möglich, dass dieses Tutorium nicht umfassend getestet und überprüft wurde. NI übernimmt weder Garantien
bezüglich der Qualität des Tutoriums noch bezüglich der weiteren technischen Unterstützung neuer Versionen ähnlicher Produkte und
Treiber. DIESES TUTORIUM WIRD IM "IST-ZUSTAND" ZUR VERFÜGUNG GESTELLT UND NI ÜBERNIMMT KEINERLEI GARANTIEN.
AUSFÜHRLICHERE ERLÄUTERUNGEN ZU ANDEREN EINSCHRÄNKUNGEN ENTNEHMEN SIE BITTE DEN
NUTZUNGSBEDINGUNGEN AUF NI.COM (http://ni.com/legal/termsofuse/unitedstates/us/).
29.01.2012 23:20