coalitionhihatElectronics - Devices

Oct 7, 2013 (3 years and 8 months ago)


By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed




It’s o.k. if we lose money on the product, we’ll just make it up in volume!

Harvard MBA Graduate

OBJECTIVES (Why am I doing this prelab?)

To learn about interfaces between the optical world and the electronic world.


At this point, you really should be working on your projects. Surprise! Again, no prelab
text to read!!!

While the materials in the lecture notes are sufficient, it would make sense to flip through
Horowitz and Hill to inves
tigate some of the many variants of optoelectronic circuits.

You will notice that the Prelab 5 Exercises are much more design oriented than the
previous ones, except for Prelab 4, which is similar.

By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed



Work With Your

SE 1:

Design an optical data transmitter and receiver. You can use LEDs or lasers as the output
devices, You will use a laser as the output device and a photodiode or phototransistor as
the input device.
Please keep in mind that
the drive voltage for the laser module must
not exceed 5 V

and for the LED array (more on this below) the voltage must not
exceed 13V (12 V nominal).

Your transmitter should frequency

an i
ncoming voltage signal (from a stereo,
a microphone, a function generator, etc...) and send out the modulated waveform through
the light source of choice.
We suggest using an AD654 (see datasheet/applicatio
n note on
the web and the additional material on the AD654 at the end of this section).

Modulate the light at a sufficiently high frequency that 60 and 120 Hz flicker from room
ts will not affect your receiver.

Your receiver will not be required to demodulate on its own. The signal analyzer (high
frequency) can perform FM demodulation if it is given a clean FM signal. You will have
to receive the signal, clean it up (remove
unwanted frequencies) and pass it with
sufficient energy to the demodulator.

Use a transresistance amplifier
(the schematic for this amplifier is in the lecture notes)
capture the light signal

you will only need to pass the AC signal, so you may want

design it so that the DC gain is minimal. The best way to do this is to use a low feedback
ance value (e.g., 10 kΩ), AC
couple using a series capacitor into a second amplifier
stage where more gain is provided. Please also review the Photodiode Amplifier notes
from National Semiconductor that were provided to you.

Consider using analog filters
to fight noise.

Flesh out the design, select components, and simulate what you can


Frequency modulation means that you take a carrier signal of frequency f

and change its frequency
according to the voltage of the signal you would like to transmit. Thi
nk of this operation as a Voltage to
Frequency conversion.


At the very least, you should have all of your transmit and receive circuitry designed (minus the
modulation circuitry.). This includes any amplifiers and filters that you'll need. You won't be
able to
simulate the transimpedance amp in SPICE. Just pretend you're simulating a regular inverting amp. Do not
worry about simulating the AD654. In addition, you cannot really predict how much current you will get
from the phototransistor under your spec
ific lab conditions until you try it. You will probably get much less
By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed


Comment on the design tradeoffs you considered

gain, bandwidth, power, etc. For
example, putting too much gain in the transresistance amplifier usually results in low

bandwidth and saturation by background light levels. Consider spreading the gain out
between stages. How would you couple the stages to block DC from being amplified?

Carry out a design review with your partner and, if you can, with another team. Prepa
some brief notes on the findings from your design review and how they affect your
design. Be sure to record these notes in your lab notebook with a heading that clearly
indicates that they are from a “Design Review.” This will be graded.


Frequency modulation is

accomplished with the AD654, found in your parts kit. The

basically outputs a square wave with a frequency proportional to the input voltage.
The 654 data sheet can be found on the class web page. For this l

we will be building

reference design shown on Fig. 1, pg. 3 of the datasheet

"V/F Connection for
Positive Input Voltages."

We suggest you become very familiar with pg. 1
3 of the
datasheet at least, and

read through the rest,

so you understand ho
w the AD654

You can browse through the app note, but we will not be using that for this lab as

You will probably want to use a full
scale voltage input of 10 V. The AD654 then maps
this voltage range into a frequency range centered at a cen
ter frequency set by a center
voltage level. It’s recommended you set this center voltage to FS/2 = 5 V, and have this
correspond to a center frequency of 50 kHz. Refer to the application notes and data sheet
for more details. The amplitude of your incom
ing voltage signal thus will set the
bandwidth (or frequency deviation from center) of your FM signal.

As a guideline, we can recommend that you use a total bandwidth of 40kHz. This means
you will be generating a signal with frequencies of 30
70kHz (hint
hint bandpass filter.)
You do not have to use these numbers
they are recommended if you intend to try
demodulating your received signal using the bench signal analyzer. The spectrum
analyzer can be used to demodulate the signal from your receiver as long
as the
frequency is higher than 9kHz. You are not required to do this, but it's fun.

Note that the AD654 is normally configured for positive input signals. However, our
signals of interest are centered around zero (see Figure 1. How do we achieve this
level shift?

than 1mA. Just make an assumption in order to complete your design, and then you can modify it as
necessary in lab. You do not need to spend time analyzing the laser and phototransistor.

Just treat the laser
like a normal LED which uses 20mA to turn on. Just assume that the phototransistor will give you a current
on the order of 10

By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed




Input signal (left), what the AD654 wants (right)

What we want to do is to block the DC component of our signal and set it to a new level.
What element is good at blocking DC? A capacitor in the sig
nal path with one end
biased to our desired voltage level by a resistor divider achieves this nicely (see Figure 2.
In the AC sense, the cap has an impedance of |Z
| = 1/2

fC. For AC signals, DC sources
are grounded, and we have a voltage divider attenuation of our AC component. Thus, we
should choose a cap at our frequencies of interest that has an impedance much less than
our bias chain. Also keep in mind that the AD65
4 also has its own input impedance,
which will matter if your bias chain resistors are on the same order of magnitude or



DC level shift circuit (left) and its AC equivalent (right)

The output of the AD654 is
an open NPN collector, meaning you need to provide it with
a load. You can tie this load to whatever voltage level you want. For TTL applications,
you would probably use 5 V, like on the data sheet. In our case, we want to drive a laser
module running a
t a fixed current and voltage drop. You can assume that the output of
the AD654 can swing all the way down to ground when calculating your values. If you
are using the IRLZ
34 MOSFET as a power driver, it requires a logic
level signal on its

By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed




Is this guy ever bright!

One LED talking about another.

NOTE: You will not be told exactly what to put in your write up. The idea is that you
present your data and what you learned from it. Typically, you will make plots and

analyses a part of the write
up. Write
ups must not be longer than ten pages. If you have
questions, please ask. We are here to help!!



When using the LED bars, remember not to stare directly at them while they are lit!
The same applies for the laser modules!

ALWAYS use 0.1 µF decoupling capacitors on each power supply rail, right next to
each op
amp. Use one capacitor
from the positive rail to ground and one from the
negative rail to ground.

If you use a signal generator, you may wish to put an input 50 Ω resistor from the signal
generator’s output to ground if you are using it to test your circuits. This is done so th
the amplitudes read out on the signal generator’s display are correct (they assume a 50 Ω
load). Please note that for many of the tests you will be asked to do, we will use the
“SYNC” (or logic
level) output of the function generator, not the normal out
put. You

want to use a 50Ω resistor on the “SYNC” output.

By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed



As described in lecture, the HP QPWR
C397 is a series chain of 20 super
bright red
LEDs arranged in four series groups of five LEDs in parallel. It is made u
p of a series
resistor (to limit LED current) followed by a parallel resistor used for sensing. For this
lab, you will be performing a number of measurements to characterize its performance.

1) Draw out a basic schematic for the entire LED array. How ar
e the two resistors
described above incorporated into the circuit?

2) Devise a method to measure the values of the series and parallel resistors so that you
can put the actual, measured numbers on your schematic. Describe your reasoning for
the measureme
nts (for example, under what assumptions do the diodes not matter when
measuring a resistance in parallel with them?).

3) Obtain an LED array and measure the resistances, adding the values to your schematic.

4) Connect your LED array to a digital power s
upply (Agilent E3648A) and take
measurements of the current it draws for a range of voltages; be thorough enough so you
can then take this data and plot it in Excel (or some other program of your choice) to
determine the device’s I
V characteristic. Remem
ber not to look directly at the LEDs.


4) Download and review the data sheet for the IRLZ
34 MOSFET from the EE122 class
website. Carefully determine which pins are for drain, gate and source, including a
sketch in your lab not
ebook. We will use the IRLZ
34 to characterize the array’s pulse

For this setup, first ground the source pin. Connect the drain to the black wire of the
LED array, and finally connect the gate pin to the TTL/SYNC (labeled “SYNC”) output

a standard signal generator; DO NOT CONNECT TO THE GENERATOR’S
STANDARD OUTPUT. The ground of the “SYNC” connector should go to the source
(ground) also. To power the LED array, connect a power supply’s positive output to the
red LED array wire, and the p
ower supply’s ground output to the common ground of the
resistor from the IRLZ
34’s source and of the SYNC output from the signal generator.

Set the power supply to 12 V and make sure it can deliver at least 1/2 amp of current.

Note the schematic and all

conditions of your setup in your lab notebook.

Please ask your TA if you have any doubts!

Connect a 10 Ohm or so resistor (capable of dissipating the power generated by 0.5A of
current passing through it

show this calculation in your lab boo
k) betw
een the IRLZ
34's source and ground, and then use a single
ended scope measure the voltage
across the resistor. Because you know the value of the resistor from part 2), you can then
By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed


divide the voltage waveform by resistance to obtain the value of

the current going
through the array at any given moment.

At some low frequency, say 10 Hz, verify that the LED array lights up and blinks,
indicating that it is not “always on.” Then, while monitoring the voltage across the
resistor to ground (tied b
etween the IRLZ
34’s source and the actual ground voltage),
determine the frequency at which the current can no longer reach its full value within the
time the SYNC pulse is high (i.e., when the IRLZ
34 is on). That we will take to be the
approximate maxim
um drive frequency for the LED array.

Note the approximate maximum drive frequency and capture the voltage waveform from
the oscilloscope onto a floppy. Transferring the date from the floppy into an Excel
spreadsheet, calculate and plot the actual current
, scaling for the value of the resistor you
used. Include the plot in your lab notebook. As a check, the current should be reasonable,
on the order of 0.2

0.5A at its peak.


This is your second effort to impleme
nt a complete system that you design. Build the
electronic FM transmitter and receiver circuit you designed in the prelab. You will
probably have to make some modifications to the circuit in the lab. Try building the
circuit with both a normal lase
r module and the LED arrays, noting any particular
tradeoffs in design considerations. As with last week, we provide you with a few
necessary, but not sufficient, suggestions for characterizing your circuit. Have fun!

Consider the IRLZ
34 circuit we use
d in the LED array characterization section as
a driver for your chosen output device.
Remember that the drive voltage for the
laser module must not exceed 5 V, yet the LED array needs a voltage of 12V.

How far can you transmit before you can no longer de
tect the signal?

Can you estimate the signal
noise ratio?

Compare the results when blocking ambient light and not blocking it.

Can you estimate the channel capacity? (You will need Shannon’s Channel Capacity
equation from the Lecture Notes). For the

LED array, you can estimate the channel
bandwidth (assuming a faster receiver circuit) based on the measurements you did. If
your receiver is slower, it, not the LED array, will determine the bandwidth.


To Demodulate the FM signal at the receiver, use the HP8591E Signal Analyzer, press the “Aux Ctrl”
key, select “demod” and then set the appropriate settings.

By Professor Gregory Kovacs

Edited and Updated by Rizwan Ahmed


Describe any design changes you made to your circui
t in the lab. Why weren’t they
obvious from the simulations you did? Be sure to note down your observations in your
laboratory notebook.

In addition to your normal lab write
up (which adequately describes the circuit you have
built, tests you have run on

it, and an evaluation of its performance), please summarize in
a few sentences the key points you learned from this exercise, focusing primarily on the
design and planning aspects. Remember that the all of the information you will draw
from for your write
up should be in your lab notebook, and remember that we will grade
the notebooks.