.. _free:
**************************************
Free Space Optical Communication Link
**************************************
Objectives
==================================================
#. Experiment with IR LED
#. Experiment with photo transistor
#. Experiment free space communication using
an infrared optical source and a
photo transistor detector.
#. Experiment with amplitude modulation (AM) IR link
#. Experiment with frequency modulation (FM) IR link
#. Explore using arbitrary waveform generator (ARB)
Required Soft Front Panels (SFPs)
====================================================
#. 2-wire I-V analyzer (2-wire)
#. Variable power supply + (VPS +)
#. Function Generator (FGEN) with AM and FM Inputs
#. Scope
#. Arbitrary Waveform Generator (ARB)
Required Components
=============================================
#. 510 :math:`\Omega` resistor
#. 4.7 :math:`k\Omega` resistor
#. SFH4110 Infrared LED (IR LED), center wavelength 950nm. You can download or view the data sheet :download:`here <./datasheet/irlink/sfh4110_ir_led.pdf>`.
#. SDP8406-003 silicon photo transistor. You can download or view the data sheet :download:`here <./datasheet/irlink/sdp8406_photo_transistor.pdf>`.
#. OPB804 slotted optical switch. You can download or view the data sheet :download:`here <./datasheet/irlink/opb804_opto_switch.pdf>`. Read page 4 in particular for pin identification. This can replace a pair of IR LED and photo transistor.
Overview
============================
We all likely have watched TVs and used a remote control to change
channel or volume. Have you ever wondered how these remote controls work?
Most remote controls today use a infrared (IR) LED to emit light, which
is then detected by a silicon photo transistor inside the television.
It is essentially a free space optical communication link using infrared light.
Light as Information Carrier
--------------------------------------------------------------------------------
We have experimented with LEDs in the previous labs.
The LED we will use in this lab is a
little bit different in the sense that
the infrared light emitted is invisible to our eyes.
It has a longer wave length than visible light,
ranging from the nominal edge of red light at 0.7 um to
about 300 um. About half of our planet Earth's heating is
due to infrared light from the Sun.
By varying the amount of current
we pass through an LED, we can control the intensity of
the light emitted. That is, we can modulate light with an electrical signal.
If we pass a sine wave current through the LED, the light intensity will vary as a sine wave as well,
thereby carrying information.
In the context of a communication link,
this LED acts as a **transmitter**.
Light Detection with Photo transistor
-------------------------------------------------------------
Light propagates in free space.
Light arriving at the receiver can be
detected by a silicon photo transistor,
as shown below in
:num:`figure #fig-circuit-configuration`.
A photo transistor is essentially
a transistor like the 2N3904 we measured
in previous labs,
but without an external base terminal.
Optical energy from
light produces electron-hole pairs
in the collector-base junction.
Holes will drift towards the base, producing
an internal base current
that is amplified by the transistor action.
The amount of current
produced is proportional to
the intensity of received light.
We can pass this light induced current through
a resistor to produce a voltage, which contains
the transmitted information.
See :ref:`phototx-label`
for experimental data.
.. _fig-circuit-configuration:
.. figure:: images/irlink/circuit-configuration.png
:scale: 100 %
:alt: circuit-configuration.png
:align: center
circuit schematic of the free space IR communication link
.. topic:: Saturating the photo transistor
If you look at the receiver circuit carefully,
and compare it with
the bipolar transistor inverter
we experimented in the transistor lab, you find
great similarities. There, if we give
high enough base current, the collector
current will saturate, and
the collector to emitter voltage will also saturate to a small value around 0.2V or so.
Here, if we give strong enough light to
the photo transistor, voltage and current saturation will occur as well.
You can force saturation by passing a
large current to the LED to produce
high intensity light input to the
photo transistor. Similarly, you can
saturate the photo transistor more easily
when placing it very close to the IR LED.
A photo of the above circuit on the Elvis board is shown below in :num:`figure #fig-breadboard-irlink`.
Note the dots on the LED and photo transistor packages
are windows for light to go through.
The LED and photo transistor should always be inserted to the breadboard with the dots facing each other.
.. _fig-breadboard-irlink:
.. figure:: images/irlink/breadboard_irlink.png
:scale: 100 %
:alt: breadboard-irlink
:align: center
breadboard photo of the free space IR communication link
That is pretty much the secret behind a remote control,
and free space optical communication links in general.
There are more details of course.
We will experiment a few in this lab.
.. topic:: Distance
As you might intuitively expect,
the strength of received signal
will decrease with increasing
distance between light emitter and light detector.
The LED we use here is fairly low power, so you
have limited distance to work with.
You can, however,
still experiment with distance effect as I did in the video at
http://www.eng.auburn.edu/~niuguof/2210labdev/videos/irlink/distance.html
Voltage to Light Conversion Linearity and Dynamic Range
--------------------------------------------------------------------
Consider that our original information is a voltage waveform, e.g. a sine wave.
We want to first convert this voltage to current, as light emission is
proportional to the amount of current passing through the LED.
Often we need this conversion to be linear to recover a sine wave
at the receiver output. However, diode current increases
with voltage exponentially, making the light emitted and ultimately
light detector output a strongly nonlinear function of the input voltage.
That is why we add a resistor in series with the LED in
:num:`figure #fig-circuit-configuration`.
We learned earlier that this resistor will help limiting current for
the same voltage. That remains true. More importantly, it
produces a negative feedback that helps
linearize the voltage to light conversion
process, and increases the dynamic range as well.
An ideal resistor alone has a perfectly linear I-V curve.
With a resistor in series with the LED,
at higher current, the diode resistance will become
smaller than the resistor's resistance, making
the total resistance look more like that of the resistor,
that is, constant.
As a result, the voltage to current conversion
becomes more linear.
We will see this effect experimentally
in
:ref:`linearize-label`.
This idea of using a resistor or inductor to
increase linearity of
voltage to
current conversion is widely used in
modern radio-frequency integrated
circuits (RFICs) found in our cell phones, GPS,
tablets, and laptops.
As voltage has to drop across the resistor,
the voltage range has to be increased for the same
amount of LED current change. The allows the handling of
a much larger input voltage. The upper limit of dynamic range
is thus increased.
There is, of course, trade-off. For the same voltage change, we
get less current change.
AM and FM Modulation Schemes
--------------------------------------------
In communication, we need to modulate a bit stream
of digital data or continuous valued analog data onto a
higher frequency sine wave for transmission.
There are many modulation schemes, the choice of which
affects performance such as bit error rate and bandwidth.
We experiment here two modulation schemes,
amplitude modulation (AM) and
frequency modulation (FM).
AM and FM can be used for both analog and digital signals.
An example of analog application is
AM and FM broadcasting radios.
Consider a digital bit stream that needs to be modulated.
We can simply use two amplitudes to
represent "0" and "1".
The amplitude of the carrier wave changes
when the data changes between "0" and "1".
This is amplitude modulation, or simply AM.
We can also vary the carrier frequency to represent "0" and "1".
We can use one frequency, e.g. 1 kHz, for "0", and 1.1 kHz, for "1".
This is frequency
modulation, or FM.
Analog signals are continuous in time and value.
To AM modulate an analog signal,
we simply make the carrier amplitude at a give time
proportional to the instantaneous value of the analog signal.
FM modulation of an analog signal is similar.
The carrier frequency is made to
vary proportionally to the instantaneous value
of the analog signal, e.g. sound.
In this lab, we will experiment with AM modulation of
an alternating "0" and "1" digital bit stream,
and FM modulation of a sinusoidal analog signal.
If you have extra time left, feel free to experiment
with AM modulation of
a digital bit stream and FM modulation
of a sinusoidal analog signal.
You can in fact use the arbitrary waveform generator (ARB) to
generate arbitrary waveforms.
How Exactly is AM and FM Modulation Done inside ELVIS?
---------------------------------------------------------------
We can get a concrete feel of amplitude modulation using
the function generator on ELVIS.
Consider that our original signal is a
2V amplitude square wave,
with a period of 0.1 second, as shown below in
:num:`figure #fig-am-illustration`:
.. _fig-am-illustration:
.. figure:: images/irlink/am-illustration.png
:scale: 100 %
:alt: am-illustration
:align: center
explanation of amplitude modulation in ELVIS function generator
We can produce this signal with the arbitrary waveform generator (ARB).
We can set a gain of 2 in ARB settings (not visible on the illustration),
meaning
the amplitude will become 4V at the output of the ARB,
which is then connected to the AM input of the FGEN.
This AM input is called Vin(t) to the function generator in ELVIS.
We then generate a sine wave with a carrier frequency of 1 kHz,
that is much higher than the 10 Hz frequency of our to be modulated signal.
The FGEN then outputs a sine wave with an amplitude A(t)
dependent on Vin(t),
using the equation shown at the bottom
of
:num:`figure #fig-am-illustration`.
As you can see from the math example,
the amplitude modulation measured on the scope
is consistent
with your calculation.
Now you know precisely how the FGEN generates an AM signal for a given input!
You are in a position to AM modulate an arbitrary waveform you want using FGEN and ARB.
At the receiving end,
we will need to take the received modulated waves and
recover the original digital bit stream, e.g. Wifi data, or
analog data, e.g. voices in AM and FM broadcasting radios.
A demodulator does exactly this.
We will not
experiment with demodulators in this lab.
As communication is often bidirectional, we need both
a modulator and a demodulator, the combination of which is called a MODEM.
You likely have used some sorts of MODEM in the past for Internet connection.
Prelab
==================================
#. Download the data sheets for all required components. Find out how to tell the positive and negative terminals of
the IR LED, and the collector and emitter terminals of the photo transistor.
You need to do this for both
the SFH4110 Infrared LED, SDP8406-003 silicon photo transistor,
and the OPB804 slotted optical switch,
which is basically a combo of a IR LED and a photo transistor packaged
together with a slot that separates them.
In the lab, we will use the SFH4110 Infrared LED and SDP8406-003 silicon photo transistor
first to make transmitter and receiver.
After that we will simply replace them with the combo OPB804. Both scenarios are useful in practice.
#. Print out a blank ELVIS design sheet from within Multisim. Draw a pin-level wiring diagram
for all lab exercises. Use a new sheet for each step. All pin connections including ground, supply, FGEN, and scope inputs
must be shown.
#. Look up the manual of ELVIS II, which can be downloaded
:download:`here <./manuals/elvismanual.pdf>`,
find out how to generate AM and FM waveforms using the AM and FM inputs.
Specifically, pay attention to the AM and FM inputs to the FGEN, and
the use of ARB, AO1 and AO2.
Write down the equations used by ELVIS II to determine amplitude at any
given time t for AM. You will need to use this equation
and compare calculated and measured
amplitudes versus t in your lab report.
Write down the equations used by ELVIS II to determine frequency at any
given time t for FM.
Print out screen shots of the manual pages where you found the equations.
#. The IR LED and photo transistor as shown in the
breadboard photo of the IR link are hard to see,
because the packages are clear or transparent,
unlike the 2N3904 transistor we used earlier.
Can you think of any reasons for such difference in packages?
Lab Exercises
=========================================
We will begin with characterizing
voltage to current (light) conversion of the LED,
and then use the LED to build the transmitter.
We will then
measure the photo transistor using the transmitter to provide
light excitation.
We will then build the receiver
using the photo transistor, adjust the
transmitter and receiver settings to achieve faithful transmission of
signals without distortion.
Once the link is built and optimized to work, we can have more fun with
AM and FM modulated digital bit streams and analog signals.
In general, have the GTA check off each part before proceeding to the next part.
If the GTA is busy with checking off another student, you can also proceed with
other steps, or build the circuit in the next steps in another area of the board.
V to I Conversion Using an LED and Linearity of Conversion
-----------------------------------------------------------
Let us first measure the voltage to current conversion
characteristics of the IR LED. Current is an indicator of the light
intensity.
IR LED V to I Conversion
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#. Insert the SFH4110 IR LED on the ELVIS II board.
Connect the IR LED as follows:
* Positive node or anode of IR LED (long lead) :math:`\rightarrow` ``DUT+``.
* Negative node or cathode of IR LED (short lead) :math:`\rightarrow` ``DUT-``.
#. Power on the board.
#. Start the Instrument Launcher and select the
2-wire analyzer SFP.
Measure I-V
characteristics of the IR LED for
a forward voltage sweep from 0.5 to 1.4V in step of 0.02 V.
A sample plot is given below in :num:`figure #fig-irled-iv`:
.. _fig-irled-iv:
.. figure:: images/irlink/irled-iv.png
:scale: 100 %
:alt: irled-iv
:align: center
measured voltage to current conversion (I-V)
characteristics of the IR LED
Save a screenshot.
Using the ``Log`` icon, save results in a text file.
From the saved text file,
look for and record the diode voltage needed to
produce approximately 15 mA current.
The I-V characteristics will vary due to
inevitable fluctuation in manufacturing. Adjust your voltage sweep range if necessary.
The 2-wire analyzer will limit the current to 40 mA.
.. _linearize-label:
Transmitter V to I Conversion
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#. Power off the board.
#. Add a 510 :math:`\Omega` resistor in series with
the IR LED.
#. Power on the board.
#. Repeat the I-V measurement on
the IR LED and resistor series combo as follows.
This is essentially our transmitter.
* From the 15 mA diode voltage recorded in the previous step,
calculate your sweep voltage upper limit
such that your measurement will stop when current is around 15 mA.
* Set your voltage increment such that approximately 20 data
points are measured. See hints below if you have trouble
working this out. Run.
A sample setting is provided below
in :num:`figure #fig-transmitter-iv`
for your reference. However, to obtain full credit,
you should determine the settings yourself.
.. _fig-transmitter-iv:
.. figure:: images/irlink/transmitter-iv.png
:scale: 100 %
:alt: transmitter-iv
:align: center
measured voltage to current conversion (I-V)
characteristics of the transmitter
#. Save a screenshot.
Using the ``Log`` icon, save data in a text file.
From the text file, find
the applied voltages needed to
produce approximately 5 mA and 10 mA currents.
These numbers will be used for photo transistor measurement below.
.. topic:: Estimating Sweep Voltage Upper Limit and Increment
Voltage drop across a resistor is I x R.
Diode voltage drop for 15 mA was recorded earlier.
0 V is a safe choice for voltage sweep start point.
Increment can then be calculated using the
voltage range size and the number of points required.
This also illustrates how logical thinking and planning
can help make
good measurements.
.. topic:: What to do in lab report
Show screen shots of I-V with
and without the resistor, observe differences.
Detail your procedures of
determining the voltage sweep setting parameters for the
IR LED + series resistor combo. The calculations must match your own screen shots
for full credit.
If you simply used the settings provided, state so.
Find the applied voltage necessary for 5mA and 10mA of current through the IR LED
+ resistor combination.
Explain
all roles of this resistor you can think of. See the lab overview for ideas.
.. _phototx-label:
photo transistor Characteristics
---------------------------------------------------------
A photo transistor has two terminals, and looks just like
a diode. So how can we measure its transistor behavior?
We can actually measure its transistor characteristics using the
2-wire analyzer that was designed for diode measurement. Here is how:
#. Place the IR LED + resistor
series combo
near the photo transistor.
Use the VPS
to control the LED current and intensity of
light emission.
Think about how your connections should be made to achieve this.
Set VPS output voltage to the value
recorded in previous part for approximately 5 mA LED current.
#. Measure the I-V of the photo transistor with the 2-wire analyzer.
Collector :math:`\rightarrow` ``DUT+``,
Emitter :math:`\rightarrow` ``DUT-``.
Repeat the measurement with the other voltage recorded earlier for
approximately 10 mA IR LED current.
Take screen shots of the measured
photo transistor I-V for both LED currents.
A sample plot of the measured photo transistor I-V is shown in
:num:`figure #fig-photo-tx-iv`.
.. _fig-photo-tx-iv:
.. figure:: images/irlink/photo-tx-iv.png
:scale: 100 %
:alt: photo-tx-iv
:align: center
measured I-V of the photo transistor with a LED nearby emitting light,
the absorption of which produces an internal
base current
.. topic:: What to do in lab report
Show screen shots of all I-V measurements,
discuss the similarity of the measured photo transistor I-V with
the 2N3904 I-V from previous lab experiments.
Calculate the ratio of forward mode collector current (at a higher VCE, e.g. 1 V)
to the LED current for both measurements.
As the incoming light is proportional to LED current, this ratio
is an indicator of the photo transistor's "photo beta" - a measure of
how much current is produced for a given amount of light input.
Free Space IR Optical Link
---------------------------------------------------
#. Power off the board.
#. Construct circuit as shown in
:num:`figure #fig-circuit-configuration`
based on the circuit you built in previous part.
Use the SFH4110 for the IR LED, or light emitter.
Use the SDP8406 for the photo transistor, or detector.
The dots of SFH4110 IR LED and
the SDP8406 photo transistor should face each other
to create a direct light path.
Space the light emitter, or transmitter and
the detector, or the receiver 2 or 3 holes apart.
R1=510 :math:`\Omega`.
R2=4.5 :math:`k\Omega`.
VCC=5V.
.. note::
These values do not need to be exact.
R1 affects linearity and dynamic range of
voltage to light conversion in the
transmitter.
R2 affects the output voltage and
the speed of light to voltage conversion in
the receiver.
#. Connect the function generator output to
the input of the circuit.
The analog signal from the function generator
controls the current and hence
the light emission of the IR LED.
#. Connect the photo transistor output to ``AI 0+``,
the function generator output to ``AI 1+``.
Connect ``AI 0-`` and ``AI 1-`` to ground.
The photo transistor serves as the
detector. Light from the IR LED transmitter
produces electron-hole pairs in the collector-base
junction of the photo transistor, which
then produces an internal base current
to turn on the photo transistor.
The amount of current and hence the voltage
at the emitter is proportional to the optical
power received.
#. Power on the board.
#. Run function generator with the settings
shown below in :num:`figure #fig-func-setting-irled-input`.
.. _fig-func-setting-irled-input:
.. figure:: images/irlink/func-setting-irled-input.png
:scale: 100 %
:alt: fgen setting
:align: center
FGEN settings for a sine output to drive the IR LED
#. Observe transmitted and received signals on the scope channels.
Sample plots are shown below in
:num:`figure #fig-photo-tx-out-no-block`.
.. _fig-photo-tx-out-no-block:
.. figure:: images/irlink/photo-tx-out-no-block.png
:scale: 100 %
:alt: photo-tx-out-no-block.png
:align: center
transmitted and received signals using the free space optical link
#. Play with the dc offset, find the best offset that gives
minimum distortion.
#. If your output hits a ceiling when the input is near peaks,
photo transistor is saturated, because too much light goes into the base.
Decrease input amplitude and/or dc offset, or increase separation between LED and
photo transistor.
#. If your output is ``clipped`` at zero when the input is near valleys, it means too little or
no light is received. Try increasing dc offset or adjust amplitude in FGEN
so that the lowest output voltage of FGEN is
above turn-on voltage of the IR LED, to allow significant light emission.
#. The IR optical link is then ready to
send data faithfully. A `bad` setting that results
in distortion is shown below in :num:`figure #fig-photo-tx-out-distorted`.
.. _fig-photo-tx-out-distorted:
.. figure:: images/irlink/photo-tx-out-distorted.png
:scale: 100 %
:alt: photo-tx-out-distorted.png
:align: center
transmitted and distorted received signals
when the dc offset is too low such that
the IR LED cuts off during the negative half cycle
#. Block light by placing a piece of paper between the IR LED emitter and
the photo transistor IR detector, check output.
The received signal should become very weak, as shown below in
:num:`figure #fig-photo-tx-out-block`.
.. _fig-photo-tx-out-block:
.. figure:: images/irlink/photo-tx-out-block.png
:scale: 100 %
:alt: photo-tx-out-block.png
:align: center
transmitted and received signals using the free space optical link
when a piece of paper blocks the infrared light
#. Change the spacing between the IR LED and the
photo transistor, see how the received signals respond.
#. Power off the board. Replace the IR LED and photo transistor pair
with the OPB804 optocoupler. Power on the board, observe your
transmitted and received signals.
#. Demonstrate working IR link to the GTA.
#. Take representative screen shots for lab report.
Anything that helps your understanding is fine.
It is necessary to change your scope settings for both x and y
axes so that the input voltage shows up clearly as a sine wave.
.. topic:: What to do in lab report
Show all screen shots. Good and bad.
Over clipping or under clipping are both good to have.
Briefly explain what settings you adjusted, how you adjusted them to
achieve an output waveform that does not show distortion, and why
such adjustment worked.
Amplitude Modulation (AM) IR Link
-----------------------------------------
#. Power off the board.
#. Remove the OP804 optocoupler, put back the
IR LED and the photo transistor.
#. Connect ``AO 0`` to ``AM`` input of FGEN.
#. Select ``AM`` from the modulation type drop down menu in the FGEN setting.
#. Power on the board.
#. Open Arbitrary Waveform Generator (ARB), check the enabled box for ``AO 0``.
Use waveform generator to generate
a waveform, or simply load in one of the existing waveforms from the default
waveform directory, as shown below in
:num:`figure #fig-am-modulation`.
You can change the gain setting of the ARB to make modulation
stronger and more obvious to see, as was discussed in the overview section.
.. _fig-am-modulation:
.. figure:: images/irlink/am-modulation.png
:scale: 100 %
:alt: am-modulation.png
:align: center
arbitrary waveform generator screen
#. Run ARB and FGEN, adjust scope setting, e.g. timebase and voltage scales,
observe transmitted and received AM signals.
Samples of
transmitted and received AM signals are given below in
:num:`figure #fig-irled-input-am-modulated` and
:num:`figure #fig-tx-out-am-modulated`, respectively.
.. _fig-irled-input-am-modulated:
.. figure:: images/irlink/photo-tx-out-am-modulated.png
:scale: 100 %
:alt: irled-input-am-modulated.png
:align: center
transmitted AM signal
.. _fig-tx-out-am-modulated:
.. figure:: images/irlink/irled-input-am-modulated.png
:scale: 100 %
:alt: photo-tx-out-am-modulated.png
:align: center
received AM signal
#. Demonstrate working AM IR link to the GTA.
#. Take screen shots for lab report:
* FGEN settings
* ARB settings
* transmitted and received AM signals
.. topic:: What to do in lab report
* Show all screen shots.
* Discuss if the measured amplitudes on the
transmitted AM signal is consistent with
hand calculations using the equations you obtained from
the manual in pre lab.
Frequency Modulation (FM) IR Link
--------------------------------------
#. Power off the board.
#. Connect ``AO1`` to ``FM`` input of FGEN.
#. Power on the board.
#. Select *FM* instead of *AM* modulation in FGEN.
#. Check the enabled box for ``AO 1`` in ARB. Specify a sine waveform.
The examples in the waveform directory
work just fine. Experiment with `gain` setting in ARB, as found in
:num:`figure #fig-sine-fm` below:
.. _fig-sine-fm:
.. figure:: images/irlink/sine-fm.png
:scale: 100 %
:alt: sine-fm.png
:align: center
Settings for frequency modulation of a sine wave, an analog signal
#. Observe FGEN output. In prelab, you should have
looked up Elvis II manual for
equations used to calculate
the instant frequency inside ELVIS II.
#. Observe received signal. Adjust your FM settings so that you can
clearly see frequency variation over time on the scope.
A higher gain in the ARB produces
a stronger frequency modulation.
See :num:`figure #fig-sine-fm` for a
sample of settings for frequency modulation of an analog signal, a sine wave.
#. Demonstrate working FM IR link to the GTA.
#. Take screen shots for lab report, including:
* ARB settings
* FGEN settings
* Scope display of the transmitted and received FM signals
.. topic:: What to do in lab report
* Show all screen shots.
Clean up
------------------------------------------
Please put the components you used
back to the drawers.
GTAs: please include this in your final clean up check off for your sections.
Thank you for keeping our lab clean and organized.
.. topic:: Feedback
You are encouraged to
write down your experience with this lab and any feedback, suggestion on
how to improve this lab.
You can also document mistakes or missteps that occurred, e.g.
* my receiver ground was not connected to GROUND of the board, or
* my transmitter ground was not connected to board GROUND
* my LED was placed backward
* my photo transistor was placed backward
* my LED and photo transistor were not facing each other, so light was not received at all
Such information will be used to improve this lab
and your experience will help future students.