Electronic Instrumentation
Experiment 8: Diodes (continued)
Project 4: Optical Communications Link
Agenda

Brief Review: Diodes
 Zener Diodes
 Project 4: Optical Communication Link
• Why optics?
• Understanding Modulation
• Initial Design of optical link
• Transmitter
• Receiver
• PSpice Model
• Your final design
What you will know






Half wave and full wave rectifier circuit
Voltage limiter circuit
What a Zener diode is used for
How a signal is modulated to carry
information
How what you’ve learned to this point in
this course can be used for the optical link
What is expected in Project 4
Introduction to Diodes
D1
ANODE
CATHODE
DIODE

A diode can be considered to be an
electrical one-way valve.
 They are made from a large variety of
materials including silicon, germanium,
gallium arsenide, silicon carbide …
Introduction to Diodes

In effect, diodes act like a flapper valve
• Note: this is the simplest possible model of a
diode
Introduction to Diodes
D1
VAMPL = 10V
V
V1
D1N4002
V
R1
FREQ = 1k
1k
Only positive
current flows
0
10V
5V
0V
-5V
-10V
0s
V(D1:1)
0.5ms
V(D1:2)
1.0ms
1.5ms
Time
2.0ms
2.5ms
3.0ms
Diode i-v Characteristic Curves
• What is a i-v characteristic curve?
• i-v curve of an ideal diode
• i-v curve of a real diode
i-v characteristic of a real diode

Real diode is close to ideal
Ideal Diode
Real diode
When a voltage
source is less than
0.7V it looks like an
open circuit
When a voltage
source is greater than
0.7V it looks like a
0.7V dc voltage source
Diode Circuits
• Rectifiers
• Voltage Limiters (Clippers)
A Half Wave Rectifier
Since the diode only allows current in
one direction, only the positive half of
the voltage is preserved.
Smoothing Capacitors

Filtering can be performed by adding a
capacitor across the load resistor
D1
D1N4148
V2
R1
1k
C1
47uF
0

This RC combination is a low pass filter
 It smoothes out the output to make it more
like DC
A Full Wave Rectifier
The rectifier we have just seen is called a halfwave rectifier since it only uses half of the
sinusoidal voltage. A full wave rectifier uses both
the negative and positive voltages.
A Full Wave Rectifier
R3
50
V
D1N4148
D5
D6
R4
V2
VOFF = 0
VAMPL = 10
FREQ = 1k
10k
V-
D7
V+
D8
D1N4148
0


Note the path of current when source is positive.
What diodes does the current pass through when
the source voltage is negative? In what direction
does the current travel through the load resistor?
A Full Wave Rectifier
1.4V (2 diodes)
10V
5V
0V
-5V
-10V
110.0ms
V(D5:2)
110.5ms
V(R4:2,D7:1)
111.0ms
111.5ms
112.0ms
112.5ms
113.0ms
Time
Note: Since a small voltage drop (around 0.7V)
now occurs over two diodes in each direction, the
voltage drop from a full wave rectifier is 1.4V.
Full Wave Rectifier With Smoothing
R1
50
D1N4148
D1
D3
R2
V1
VOFF = 0
VAMPL = 10
FREQ = 1k
10k
C1
D4
0.1uF
D1N4148
D2
0
Capacitor holds charge
10V
5V
0V
-5V
-10V
110.0ms
V(R1:2)
110.5ms
V(R2:2,D1:1)
111.0ms
V(R4:2,D7:1)
111.5ms
Time
112.0ms
112.5ms
113.0ms
Voltage Limitation

In many applications, we need to protect
our circuits so that large voltages are not
applied to their inputs
 We can keep voltages below 0.7V by
placing two diodes across the load
R1
A
B
1k
V1
D1
D1N4148
0
D2
D1N4148
Voltage Limitation
R1
1k
V
V3
V
VOFF = 0
VAMPL = 10
FREQ = 1k
D1
D1N4148
D2
D1N4148
0
10V
5V
(1.2420m,718.277m)
0V
-5V
-10V
0s
V(R1:1)
0.5ms
V(R1:2)
1.0ms
1.5ms
Time
2.0ms
2.5ms
3.0ms
In Class Problem:
Diode Limiter Circuits



Redraw what the circuit looks like for and give the value of Vout:
+2V
-2V
0V
Replace the diodes that are on with voltage sources and diodes that
are off with open circuits. Indicate the voltage value of Vout.
REMEMBER note direction of diodes!
Part II: Diode Limiter Circuits

Vin= +2V


Vin= 0V
Vin= -2V
Part II: Diode Limiter Circuits

Sketch the output at Vout on the graph of
Vin
0.7V
-1.4V
Zener Diodes
• Introduction
• i-v curve for a Zener diode
• Zener diode voltage regulation
Zener Diodes

Up to this point, we have not taken full
advantage of the reverse biased part of the
diode characteristic.
Ideal Zener Diode
I
-VZ
V
Zener Diodes


For the 1N4148 diode, the breakdown voltage is
very large. If we can build a different type of
diode with this voltage in a useful range (a few
volts to a few hundred volts), we can use such
devices to regulate voltages. This type of diode is
called a Zener diode because of how the device is
made.
Zener diodes are rated according to where they
break down. A diode with a Zener voltage (VZ) of
5V, will have a breakdown voltage of -5V.
i-v characteristic of Zener diodes
Knee
Current


For a real Zener diode, a finite current (called
the knee current) is required to get into the
region of voltage regulation
Just like regular diodes, Zener diodes have a
small reverse saturation current in the reverse
bias region and a forward bias threshold voltage
of about 0.7V
R1
A
Zener Diodes Circuits
B
1k
V1
D1
1V
D1N7
0


Although Zener diodes break down at negative
voltages, Zener voltages are given as positive and
Zener diodes are typically placed in circuits
pointing away from ground.
The voltage in this circuit at point B will
• hold at VZ when the Zener diode is in the breakdown
region.
• hold at -0.7 when the Zener diode is forward biased
• be equal to the source voltage when the Zener diode is
off (in the reverse bias region).

Zener Diodes
R1
1k
V
V
D1
V1
VOFF = 0
VAMPL = 10
FREQ = 1k
D1N750
Note the voltage
limitation for both
positive and
negative source
voltages
0
10V
5V
0V
-5V
-10V
0s
V(R1:1)
0.5ms
V(D1:2)
1.0ms
1.5ms
Time
2.0ms
2.5ms
3.0ms
Wall Warts
Transformer Rectifier
R1
V1
TX1
5
V
VOFF = 0
VAMPL = 120
FREQ = 60
D1
D1N4148
D4
D1N4148
0

D3
D1N4148
D2
V
C2
33uF
R2
1k
D1N4148
0
Adding a full wave rectifier to the
transformer makes a low voltage DC power
supply, like the wall warts used on most of
the electronics we buy these days.
(In reality, VAC is 120Vrms => 170Vpeak)
Transformer Rectifier
120V
80V
Filtered
40V
-0V
-40V
Unfiltered
-80V
-120V
10.000s
10.005s
10.010s
10.015s
V(R1:2)
V(R3:2)
V(D2:2)
V(R4:1)
10.020s
10.025s
Time
10.030s
10.035s
10.040s
10.045s 10.050s
Diodes and Light
• Light Emitting Diodes (LEDs)
• Photodiodes and Phototransistors
Light Emitting Diodes
• The Light-Emitting Diode
(LED) is a semiconductor
pn junction diode that
emits visible light or nearinfrared radiation when
forward biased.
• Visible LEDs emit
relatively narrow bands of
green, yellow, orange, or
red light. Infrared LEDs
emit in one of several
bands just beyond red
light.
Photodiodes and Phototransistors
• Photodiodes are designed to detect photons and
can be used in circuits to sense light.
• Phototransistors are photodiodes with some
internal amplification.
Note:
Reverse current flows through the
photodiode when it is sensing light.
If photons excite carriers in a reversebiased pn junction, a very small
current proportional to the light
intensity flows.
The sensitivity depends on the
wavelength of light.
Phototransistor Light Sensitivity
The current
through a
phototransistor
is directly
proportional to
the intensity of
the incident
light.
Electronic Instrumentation
Project 4: Optical Communication Link
1. Optical Communications
2. Initial Design
3. PSpice Model
4. Final Design
5. Project Report
Why use optics?
Advantages of optical communication
(over Radio Frequency)






Wider bandwidth
http://www.andor.com/image_lib/lores/introduct
ion/introduction%20(light)/intlight%201%20sm
Larger capacity
all.jpg
Lower power consumption
http://spie.org/x8857.xml
More compact equipment
Greater security against eavesdropping
Immunity from interference
1. Optical Communications
“Lighting the way to a revolution”
http://news.bbc.co.uk/1/hi/sci/tech/4671788.stm



The exponential increase of sharing
information is largely due to optical
communication technology
A few revolutionary technologies
based on or effected by optical
communication
• Internet (ex. Ethernet LAN based
on Infrared Technology)
1966 Dr. Kao and
• Cell phones
George Hockham:
• Satellite communication
fiber optics to carry
Others?
information with light
Transmitting an audio signal using light
In free space (air)
Transmitter Circuit
Receiver Circuit
Modulation

Modulation is a way to encode an
electromagnetic signal so that it can be
transmitted and received.
 A carrier signal (constant) is changed by
the transmitter in some way based on the
information to be sent.
 The receiver then recreates the signal by
looking at how the carrier was changed.
Modulation
8.0V
7.0V
Modulating
Input signal
6.0V
5.0V
4.0V
3.0V
2.0V
1.0V
Carrier
signal
0V
-1.0V
-2.0V
-3.0V
-4.0V
0s
4.0ms
V(R2:1)
V(R1:1)
Time
Output (modulated carrier)
depends on the type of modulation used
Modulation Types
 General
• Frequency Modulation
• Amplitude Modulation
 Pulse
• Pulse Width Modulation
• Pulse Position Modulation
• Pulse Frequency Modulation
Amplitude Modulation
Frequency of
carrier remains
constant.
Input signal
alters
amplitude of
carrier.
Higher input
voltage means
higher carrier
amplitude.
http://cnyack.homestead.com/files/modulation/modam.htm
Frequency Modulation
Amplitude of
carrier remains
constant.
Input signal
alters
frequency of
carrier.
Higher input
voltage means
higher carrier
frequency.
http://cnyack.homestead.com/files/modulation/modfm.htm
Pulse Modulation

Remember duty cycle definition and equation
Ton
Toff
T  Ton  Toff

Ton
Duty _ Cycle 
T
Pulse _ width
Duty _ Cycle 
Period
Carrier has a constant variable
• Pulse Width Modulation - Period is constant
• Pulse Position Modulation - Pulse width is constant
• Pulse Frequency Modulation - Duty cycle is constant

Input modulates carrier and effects other two
variables
Pulse Width Modulation
Period of carrier
remains
constant.
Input signal
alters duty
cycle and pulse
width of carrier.
Higher input
voltage means
pulses with
longer pulse
widths and
higher duty
cycles.
http://cnyack.homestead.com/files/modulation/modpwm.htm
Pulse Position Modulation
Pulse width of
carrier remains
constant.
Input signal
alters period
and duty cycle
of carrier.
Higher input
voltage means
pulses with
longer periods
and lower duty
cycles.
http://cnyack.homestead.com/files/modulation/modppm.htm
Pulse Frequency Modulation
Duty cycle of
carrier remains
constant.
Input signal
alters pulse
width and
period of
carrier.
Higher input
voltage means
pulses with
longer pulse
widths and
longer periods.
2. Initial Design
transmitter


receiver
The initial design for this project is a circuit consisting
of a transmitter and a receiver.
The circuit is divided into functional blocks.
• Transmitter: Block A-B and Block B-C
• Transmission: Block C-D
• Receiver: Block D-E, Block E-F, Block F-G, and Block G-H

You will need to examine each block of the circuit.
555 Timer
Transmitter Circuit
Rpot
R2
V1
2
4
5
6
7
27k
1k
5V
C8
4.7uF
C3
330u
X1
TRIGGER
RESET OUTPUT
CONTROL
THRESHOLD
DISCHARGE
3
GND
R3
VCC
8
100k
1
RRC
with
variable
resistor:
Changes
sampling
frequency
(of
carrier
signal)
555D
R19
.001uF
100ohms
C2
Function_Gen_1
LED
D1
0
Similar to
astable
multivibrator
configuration:
Pin five input
alters frequency
of pulses
Transmitter Circuit:
Input and Modulated Output
Rpot
V1
2
4
5
6
7
27k
1k
5V
C8
4.7uF
C3
330u
X1
TRIGGER
RESET OUTPUT
CONTROL
THRESHOLD
DISCHARGE
3
GND
R2
1
R3
VCC
8
100k
555D
R19
.001uF
100ohms
C2
Function_Gen_1
LED
D1
Input signal:
function
generator or
audio
0
Output signal:
Light modulation
from LED
Special Capacitors
Rpot
V1
2
4
5
6
7
27k
1k
5V
C8
4.7uF
C3
330u
X1
TRIGGER
RESET OUTPUT
CONTROL
THRESHOLD
DISCHARGE
3
GND
R2
1
R3
VCC
8
100k
555D
R19
.001uF
100ohms
C2
Function_Gen_1
LED
D1
Bypass Capacitor
(Low Pass Filter)
0
DC Blocking
Capacitor
(High Pass Filter)
Keeps DC offset
from 555 Timer
from interfering
with input
Sample Input and Output


When input is higher, pulses are longer
When input is lower, pulses are shorter
Your signal is what?
The type of modulation this circuit creates is most closely
categorized as pulse frequency modulation.
But the pulse width is also modulated and we will use that
feature.
Sampling Frequency


The pot (used as a variable resistor) controls your
sampling frequency
Input frequency in audible range
• max range (20 - 20kHz)
• representative range (500 - 4kHz)


Sampling frequency should be between 8kHz and
48kHz to reconstruct sound
Input amplitude should not exceed 2Vp-p
• Function generator can provide 1.2Vp-p
Receiver Circuit
56k
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
Receive Light Signal
56k
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
Inverting Amplifier (Pre-Amp)
56k
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
Audio Amplifier
56k
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
Audio Amplifier Details
increases
gain 10X
(not needed)
386 audio
amplifier
high pass
filter
volume
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
low pass filter
Special Capacitors
56k
DC Blocking
Bypass
Capacitor Capacitor
Not needed
Add a 100 Ohm resistor in series with the
speaker to avoid failures.
3. PSpice Model

You will compare the performance of your
circuit to a PSpice model.
 The PSpice for the initial design will be
given to you.
 You will use the PSpice to help you make
decisions about how to create your final
design.
Comparing Output of Blocks




Take pictures of the signal on each side of the circuit
block.
• A on channel 1 and B on channel 2
• B on channel 1 and C on channel 2
Take all measurements relative to ground
Does the block behave as expected?
How does it compare to the PSpice output?
Comparing Output of Blocks
10V
“wide-angle” view
 Shows overall
shape and size of
input and output
5V
0V
-5V
8.0ms
V(R1:1)
8.4ms
V(L1:2)
8.8ms
9.2ms
9.6ms
10.0ms
Time
1.0V
“close-up” view
 Output divided
by 10
 Shows sampling
frequency
 Shows shape of
samples
0V
-1.0V
8.301ms
V(R1:1)
8.400ms
V(L1:2)/10
8.500ms
8.600ms
Time
8.700ms
8.799ms
4. Final Design



The signal is reconstructed well enough by the
initial design that it will be audible.
In order to improve the quality of the signal, you
will add an integrator, which will more exactly
reconstruct it.
Types of integrators
• passive integrator (low pass filter)
• active integrator (op amp integrator circuit)

You will then improve the signal further with a
smoothing capacitor.
Passive Integration
R1
Vin
Vout
500mV
E
C1
0
1
Vout 
Vin dt

RC
1
fC 
2RC
250mV
0V
1.0Hz
V(R1:2)
10KHz
100MHz
Frequency
Integration works only
at high frequencies f >>fc.
Unfortunately,
your amplitude will also
decrease.
Active Integration
F
500mV
E
250mV
0V
1.0Hz
V(R1:2)
10KHz
100MHz
Frequency
Vout
1

Vin dt

Ri C
1
fC 
2R f C
• Integration works at f >>fc
• Your gain goes from -Rf/Ri to
-1/RiC
• The amplitude of your signal
will decrease or increase
depending on components
Input at A vs. Output at H
10V
5V
0V
-5V
8.0ms
V(R1:1)
Before addition of integrator
8.4ms
V(L1:2)
8.8ms
9.2ms
9.6ms
10.0ms
Time
4.0V
2.0V
0V
-2.0V
8.0ms
V(V4:+)
8.4ms
V(L1:2)
8.8ms
9.2ms
After addition of integrator
Time
9.6ms
10.0ms
Effect of Smoothing Capacitor
D1
D1N4148
V
V
V1
R1
VOFF = 0
VAMPL = 5v
FREQ = 1k
1k
C1
5u
0
Recall what the smoothing capacitor did to the output of
the half wave rectifier.
Input at A vs. Output at H
4.0V
2.0V
0V
-2.0V
8.0ms
V(V4:+)
8.4ms
V(L1:2)
8.8ms
9.2ms
Before smoothing capacitor
9.6ms
10.0ms
9.6ms
10.0ms
Time
2.0V
0V
-2.0V
8.0ms
V(V4:+)
8.4ms
V(L1:2)
-v(L1:2)
8.8ms
9.2ms
After smoothing capacitor
Time
Project Packet

Initial Data with Function Generator
•
•
•
•
•
PSpice
Mobile Studio plots from circuit
Brief Comparison
Block Description
For
• Blocks: A-B, A-C, A-D, A-E, A-F, A-G
• Overall System: A-H

Initial Data with Audio
• Mobile Studio plots from circuit
• For E-F and A-H
Project Packet

Final Data (integrator only) with Function
Generator
•
•
•
•

PSpice
Mobile Studio plots from circuit
Brief Comparison
For E-F and A-H
Final Data (integrator and smoothing) PSpice
only
• PSpice
• Compare to without smoothing
• For E-F and A-H
Project Packet

Final Data with Integrator (and possibly
Smoothing) with Audio
• Mobile Studio plots from circuit
• For E-F and A-H

Extra Credit
• Mobile Studio picture of A-H with input from function
generator and integrated, smoothed output. Indicate
values of components and where used.
Work in teams

Put the transmitter on one protoboard and the
receiver on a second.
• One pair do the transmitter circuit
• This is the easier circuit, so maybe also start the PSpice
simulation.
• The other pair build the receiver circuit

One report for the entire team
• Report is closer to an experiment report than a project
report
• See details in handout.