Electronic InstrumentationExperiment 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

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

DIODECATHODE

Introduction to Diodes

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

diode

Introduction to Diodes

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

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

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 RectifierThe 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.

0

R4

10k

D6

D8

V

D5

D1N4148

V+

R3

50

V-

D7

D1N4148

V2

FREQ = 1kVAMPL = 10VOFF = 0

A Full Wave Rectifier

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?

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 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.

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

T i m e

1 1 0 . 0 m s 1 1 0 . 5 m s 1 1 1 . 0 m s 1 1 1 . 5 m s 1 1 2 . 0 m s 1 1 2 . 5 m s 1 1 3 . 0 m sV ( R 1 : 2 ) V ( R 2 : 2 , D 1 : 1 ) V ( R 4 : 2 , D 7 : 1 )

- 1 0 V

- 5 V

0 V

5 V

1 0 V

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

T i m e

0 s 0 . 5 m s 1 . 0 m s 1 . 5 m s 2 . 0 m s 2 . 5 m s 3 . 0 m sV ( R 1 : 1 ) V ( R 1 : 2 )

- 1 0 V

- 5 V

0 V

5 V

1 0 V

( 1 . 2 4 2 0 m , 7 1 8 . 2 7 7 m )

0

D1

D1N4148

VV3

FREQ = 1kVAMPL = 10VOFF = 0 D2

D1N4148

V

R1

1k

In Class Problem: Diode Limiter Circuits

Redraw what the circuit looks like for and give the value of Vout:+2V-2V0V

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= -2V

Vin= 0V

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

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

D1N7

BA

Zener Diodes

Note the voltage limitation for both positive and negative source voltages

D1

D1N750

V1

FREQ = 1kVAMPL = 10VOFF = 0

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

D1N4148 C2

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

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.

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 Communications2. Initial Design3. PSpice Model4. 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

http://www.andor.com/image_lib/lores/introduct ion/introduction%20(light)/intlight%201%20sm all.jpg

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

Transmitting an audio signal using light

Receiver Circuit

Transmitter CircuitIn 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

TTonCycleDuty _

Ton

Toff

PeriodwidthPulseCycleDuty __ 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

GN

D1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VC

C8

R2

27k

R3

1k

C2

.001uF

C34.7uF

V15V

Function_Gen_1

C8

330uR19

100ohms

D1

LED

0

Rpot

100k

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

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

X1

555D

GN

D1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VC

C8

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

GN

D1

TRIGGER2

OUTPUT3

RESET4

CONTROL5

THRESHOLD6

DISCHARGE7

VC

C8

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

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

Add a 100 Ohm resistor in series with the speaker to avoid failures.

56k

Audio Amplifier Details

volume

386 audioamplifier

low pass filter

high passfilter

increasesgain 10X(not needed)

Add a 100 Ohm resistor in series with the speaker to avoid failures.

Special Capacitors

BypassCapacitor

DC BlockingCapacitor

Add a 100 Ohm resistor in series with the speaker to avoid failures.

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 onlyat high frequencies f >>fc.Unfortunately,your amplitude will alsodecrease.

RCf

dtVRC

V

C

inout

21

1

Frequency

1.0Hz 10KHz 100MHzV(R1:2)

0V

250mV

500mV

E

Active Integration

CRf

dtVCR

V

fC

ini

out

21

1

• Integration works at f >>fc• Your gain goes from -Rf /Ri to -1/Ri C• 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.