Experiment 8: Diodes continued Project 4: Optical Communications Link

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Experiment 8 Diodes (continued)Project 4
Optical Communications Link
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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

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What you will know

• What a Zener diode is used for
• How a signal is modulated to carry information
• How what youve learned to this point in this
  course can be used for the optical link
• What is expected in Project 4
• What the PSpice model is representing
• What the simulated output tells you

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Introduction to Diodes

• 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

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Introduction to Diodes

• In effect, diodes act like a flapper valve
• Note this is the simplest possible model of a
  diode

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Introduction to Diodes

• Only positive
• current flows

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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

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i-v characteristic of a real diode

• Real diode is close to ideal

Ideal Diode
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Diode Circuits

• Rectifiers
• Voltage Limiters (Clippers)

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A Half Wave Rectifier
Since the diode only allows current in one
direction, only the positive half of the voltage
is preserved.
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Smoothing Capacitors

• Filtering can be performed by adding a capacitor
  across the load resistor
• This RC combination is a low pass filter
• It smoothes out the output to make it more like DC

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A Full Wave Rectifier

• The rectifier we have just seen is called a
  half-wave rectifier since it only uses half of
  the sinusoidal voltage. A full wave rectifier
  uses both the negative and positive voltages.

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A Full Wave Rectifier
1.4V (2 diodes)
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.
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Full Wave Rectifier With Smoothing
Capacitor holds charge
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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

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Voltage Limitation
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Zener Diodes

• Introduction
• i-v curve for a Zener diode
• Zener diode voltage regulation

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Zener Diodes

• Up to this point, we have not taken full
  advantage of the reverse biased part of the diode
  characteristic.

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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.

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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

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Zener Diodes Circuits

• 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).

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Zener Diodes

• Note the voltage limitation for both positive and
  negative source voltages

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Wall Warts
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Transformer Rectifier

• 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 gt
  170Vpeak)

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Transformer Rectifier
Filtered
Unfiltered
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Zener Diode Voltage Regulation
Note stable voltage
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Diodes and Light

• Light Emitting Diodes (LEDs)
• Photodiodes and Phototransistors

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Light Emitting Diodes

• The Light-Emitting Diode (LED) is a semiconductor
  pn junction diode that emits visible light or
  near-infrared 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.

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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 reverse-biased pn junction,
a very small current proportional to the light
intensity flows. The sensitivity depends on the
wavelength of light.
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Phototransistor Light Sensitivity

• The current through a phototransistor is
  directly proportional to the intensity of the
  incident light.

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Project 4 Optical Communication Link

• 1. Optical Communications
• 2. Initial Design
• 3. PSpice Model
• 4. Final Design
• 5. Project Report

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Why use optics? Advantages of optical
communication(over Radio Frequency)

• Wider bandwidth
• Larger capacity
• Lower power consumption
• More compact equipment
• Greater security against eavesdropping
• Immunity from interference
• More directed energy

http//www.andor.com/image_lib/lores/introduction/
introduction20(light)/intlight20120small.jpg
http//spie.org/x8857.xml
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1. Optical Communications
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Lighting the way to a revolutionhttp//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)
• Cell phones
• Satellite communication
• Others?

1966 Dr. Kao and George Hockham fiber optics to
carry information with light
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Transmitting an audio signal using light
In free space (air)
Transmitter Circuit
audio signal
Receiver Circuit
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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.

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Modulation
Modulating Input signal
Carrier signal
Output (modulated carrier) depends on the type
of modulation used
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Modulation Types

• General
• Frequency Modulation
• Amplitude Modulation
• Pulse
• Pulse Width Modulation
• Pulse Position Modulation
• Pulse Frequency Modulation

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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
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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
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Pulse Modulation

• Remember duty cycle definition and equation
• 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

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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/modpw
m.htm
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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/modpp
m.htm
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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.
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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.

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Transmitter Circuit
555 Timer Similar to astable multivibrator
configuration Pin five input alters frequency of
pulses
RRC with variable resistor Changes sampling
frequency (of carrier signal)
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Transmitter CircuitInput and Modulated Output
Output signal Light modulation from LED
Input signal function generator or audio
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Special Capacitors
DC Blocking Capacitor (High Pass Filter) Keeps
DC offset from 555 Timer from interfering with
input
Bypass Capacitor (Low Pass Filter)
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Sample Input and Output

• When input is higher, pulses are longer
• When input is lower, pulses are shorter

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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.

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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

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Receiver Circuit
56k
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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Receive Light Signal
56k
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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Inverting Amplifier (Pre-Amp)
56k
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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Audio Amplifier
56k
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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Audio Amplifier Details
increases gain 10X (not needed)
386 audio amplifier
high pass filter
volume
low pass filter
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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Special Capacitors
56k
Not needed
DC Blocking Capacitor
Bypass Capacitor
Add a 100 Ohm resistor in series with the speaker
to avoid failures.
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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.

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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?

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Comparing Output of Blocks

• wide-angle view
• Shows overall shape and size of input and output

• close-up view
• Output divided by 10
• Shows sampling frequency
• Shows shape of samples

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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.

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Passive Integration
E
Integration works only at high frequencies f
gtgtfc. Unfortunately, your amplitude will
also decrease.
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Active Integration
F
E

• Integration works at f gtgtfc
• Your gain goes from -Rf/Ri to -1/RiC
• The amplitude of your signal will decrease or
  increase depending on components

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Input at A vs. Output at H
Before addition of integrator
After addition of integrator
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Effect of Smoothing Capacitor
Recall what the smoothing capacitor did to the
output of the half wave rectifier.
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Input at A vs. Output at H
Before smoothing capacitor
After smoothing capacitor
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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

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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

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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.

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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.