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.