electronic instrumentation experiment 8: diodes (continued) project 4: optical communications link

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

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 What the PSpice model is representing What the simulated output tells you

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 …

ANODED1

DIODE

CATHODE


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

diode


Only positivecurrent flows

0

VV

R1

1k

D1

D1N4002V1

FREQ = 1k

VAMPL = 10V

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(D1:1) V(D1:2)

-10V

-5V

0V

5V

10V

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

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

This RC combination is a low pass filter It smoothes out the output to make it more

like DC

0

R1

1k

D1

D1N4148

V2C1

47uF

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.

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(D5:2) V(R4:2,D7:1)

-10V

-5V

0V

5V

10V

A Full Wave Rectifier1.4V (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.

Full Wave Rectifier With Smoothing

D3

D4

D1N4148

C1

0.1uF

R1

50

D2

R2

10k

D1

D1N4148

0

V1

FREQ = 1kVAMPL = 10VOFF = 0

Capacitor holds charge

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(R1:2) V(R2:2,D1:1) V(R4:2,D7:1)

-10V

-5V

0V

5V

10V

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

0

V1

R1

1k

D1

D1N4148

D2

D1N4148

A B

Voltage Limitation

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(R1:1) V(R1:2)

-10V

-5V

0V

5V

10V

(1.2420m,718.277m)

0

D1

D1N4148

VV3

FREQ = 1kVAMPL = 10VOFF = 0 D2

D1N4148

V

R1

1k

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

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

Knee Current

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).

0

V11V

R1

1k

D1

D1N750

BA

Zener Diodes Note the voltage

limitation for both positive and negative source voltages

D1

D1N750

V1


V V

0

R1

1k

Time

0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0msV(R1:1) V(D1:2)

-10V

-5V

0V

5V

10V

Wall Warts

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 => 170Vpeak)

D2

D1N4148

V

0

V1

FREQ = 60VAMPL = 120VOFF = 0

R1

5

TX1

D4

D1N4148

0

R2

1k

D3

D1N4148

D1

D1N4148C2

33uF

V

Transformer Rectifier

Time

10.000s 10.005s 10.010s 10.015s 10.020s 10.025s 10.030s 10.035s 10.040s 10.045s 10.050sV(R1:2) V(R3:2) V(D2:2) V(R4:1)

-120V

-80V

-40V

-0V

40V

80V

120V

Filtered

Unfiltered

Zener Diode Voltage Regulation

Note stable voltage

Time

110.0ms 110.5ms 111.0ms 111.5ms 112.0ms 112.5ms 113.0msV(D10:1) V(R8:1,R8:2)

-8.0V

-4.0V

0V

4.0V

8.0V

0

C4

1mF

R5

50

D1N4148

D10

D1N4148

R7

10

V+

V3


V-

D9

D1N4148

R8

10k

V

D14

D1N750

D12

D1N4148

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 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.

• Photodiodes are designed to detect photons and can be used in circuits to sense light.

• Phototransistors are photodiodes with some internal amplification.

IV

+

R

Photodiode Light-detectorCircuit

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.

Photodiodes and Phototransistors

Phototransistor Light Sensitivity

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

Electronic InstrumentationProject 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 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/introduction%20(light)/intlight%201%20small.jpg

http://spie.org/x8857.xml




http://spie.org/x8857.xml

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)

• Cell phones

• Satellite communication Others?

1966 Dr. Kao andGeorge Hockham:

fiber optics to carry information with light

http://news.bbc.co.uk/1/hi/sci/tech/4671788.stm

Transmitting an audio signal using light

Receiver Circuit

Transmitter Circuit

In free space (air)

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

Output (modulated carrier) depends on the type of modulation used

ModulatingInput signal

Carriersignal

Time

0s 4.0msV(R2:1) V(R1:1)

-4.0V

-3.0V

-2.0V

-1.0V

0V

1.0V

2.0V

3.0V

4.0V

5.0V

6.0V

7.0V

8.0V

Modulation Types

General• Frequency Modulation• Amplitude Modulation

Pulse• Pulse Width Modulation• Pulse Position Modulation• Pulse Frequency Modulation

Amplitude Modulation

http://cnyack.homestead.com/files/modulation/modam.htm

Frequency of carrier remains constant.

Input signal alters amplitude of carrier.

Higher input voltage means higher carrier amplitude.



Frequency Modulation

http://cnyack.homestead.com/files/modulation/modfm.htm

Amplitude of carrier remains constant.

Input signal alters frequency of carrier.

Higher input voltage means higher carrier frequency.



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

T

TonCycleDuty _

Ton

Toff

Period

widthPulseCycleDuty

__ ToffTonT

Pulse Width Modulation

http://cnyack.homestead.com/files/modulation/modpwm.htm

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.



Pulse Position Modulation

http://cnyack.homestead.com/files/modulation/modppm.htm

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.



Pulse Frequency ModulationDuty 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

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.

transmitter receiver

Transmitter Circuit

X1

555D

GND

1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VCC

8

R2

27k

R3

1k

C2

.001uF

C34.7uF

V15V

Function_Gen_1

C8

330uR19

100ohms

D1

LED

0

Rpot

100k

555 Timer Similar to astable multivibrator configuration:Pin five input alters frequency of pulses

RRCwith variable resistor:Changes sampling frequency (of carrier signal)

X1

555D

GND

1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VCC

8

R2

27k

R3

1k

C2

.001uF

C34.7uF

V15V

Function_Gen_1

C8

330uR19

100ohms

D1

LED

0

Rpot

100k

Transmitter Circuit:Input and Modulated Output

Input signal: function generator or audio

Output signal:Light modulationfrom LED

X1

555D

GND

1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VCC

8

R2

27k

R3

1k

C2

.001uF

C34.7uF

V15V

Function_Gen_1

C8

330uR19

100ohms

D1

LED

0

Rpot

100k

Special Capacitors

Bypass Capacitor(Low Pass Filter)

DC Blocking Capacitor(High Pass Filter)Keeps DC offset from 555 Timerfrom 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


Inverting Amplifier (Pre-Amp)

56k


Audio Amplifier


56k

Audio Amplifier Details

volume

386 audioamplifier

low pass filter

high passfilter

increasesgain 10X(not needed)


Special Capacitors

BypassCapacitor

DC BlockingCapacitor


Not needed56k

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

Time

8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0msV(R1:1) V(L1:2)

-5V

0V

5V

10V

“close-up” view Output divided

by 10 Shows sampling

frequency Shows shape of

samples

Time

8.400ms 8.500ms 8.600ms 8.700ms8.301ms 8.799msV(R1:1) V(L1:2)/10

-1.0V

0V

1.0V

“wide-angle” view Shows overall

shape and size of input and output

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

C1

R1 VoutVin

0

Integration works only at high frequencies f >>fc. Unfortunately,your amplitude will alsodecrease.

RCf

dtVRC

V

C

inout

2

1

1

Frequency

1.0Hz 10KHz 100MHzV(R1:2)

0V

250mV

500mV

E

Active Integration

CRf

dtVCR

V

fC

ini

out

2

1

1

• 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

Frequency

1.0Hz 10KHz 100MHzV(R1:2)

0V

250mV

500mV

EF

Input at A vs. Output at H

Time

8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0msV(R1:1) V(L1:2)

-5V

0V

5V

10V

Time

8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0msV(V4:+) V(L1:2)

-2.0V

0V

2.0V

4.0V

Before addition of integrator

After addition of integrator

Effect of Smoothing Capacitor

Recall what the smoothing capacitor did to the output of the half wave rectifier.

0

V

D1

D1N4148V

V1

FREQ = 1kVAMPL = 5vVOFF = 0 C1

5u

R1

1k

Time

8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0msV(V4:+) V(L1:2)

-2.0V

0V

2.0V

4.0V

Time

8.0ms 8.4ms 8.8ms 9.2ms 9.6ms 10.0msV(V4:+) V(L1:2) -v(L1:2)

-2.0V

0V

2.0V

Input at A vs. Output at H

Before smoothing capacitor

After smoothing capacitor

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.

electronic instrumentation experiment 8: diodes (continued) project 4: optical communications link

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