NRZ Optical Transceiver

Arachchi A.U.W 130036T

Arunkumar.P 130042H

Athukorala A.A.A.C 130048G

Bandara K.A.J.P 130057H

EN2092: Laboratory Practice II

Department of Electronic and Telecommunication Engineering

University of Moratuwa

14thAugust 2015

Design and construction of wireless NRZ optical transceiver

1. Abstract

In this project two computers are used to transfer

data in free space optical communication system

using transmitter and receiver circuits over a

distance of 3 m. In this project we have designed

and constructed an optical communication

system between two computers in practice using

theoretical design circuits. The transmitter part

of the optical communication system converts the

binary stream into optical signal through laser

diode and the receiver sense it using a photo

transistor and converts back to the digital bit

stream. The COM port (USB) in the computer is

used to send and receive data with “Eltima

advanced serial port terminal” program version

6. The transmission was successful for 12m with

a baud rate of 9600bps. Baud rate reduced with

distance.

KEY WORDS: Baud rate, TTL, free space,

wireless, optical communication, Transceiver.

2. Introduction

In recent world computing power are migrating

from desktop to portable laptops and smart

phones. Even though it allows to process vast

amount data, the interchange of data being a

challenge due to small size and cost. High

performance links are needed to exchange data

between these devices, also the ability to form

ad-hock networks between the devices remains

an attractive application. These links can be

categorized into short range data interchange

links and long range wireless networks.

In the present day optical links are available for

digital transmission. They can transmit 4 Mbps

over short distance and 100Gbps using optical

fibers. Wireless optical links transmit data by

an optoelectronic modulator, typically a light

emitting diode (LED). Optical radiation in

visible and IR range can be sent through opaque

boundaries which contributes to the security of

transmission.

Purpose of this project is to build an error free

transmission using optical in the wireless

medium.

2.1 The Optical Transmitter

The major part in the transmitter is an optical

source. Optical communication often uses LED

and semiconductor laser diodes (LD). The role

of optical transmitter is to convert electrical

signal into optical form and launch them in free

space creating a channel.

Fig 01: The Transmitter

As in the Fig 01 there are two important parts

in the transmitter optical source and optical

driver. Lasers are fast and low error occurrence

and on the other hand LEDs are cheap and easy

to handle. Light source must lit up accurately

based on the data received and it is done by

driving circuit. Optical signals can be directly

transmitted (direct modulation), but it will be

cost effective to use modulation techniques

such as Wave Division Multiplexing (WDM)

techniques in multi-channel transmission. In

Fig 01 modulator block is separately shown.

2.2 The Optical Receiver

The main task of the receiver is to convert light

signal back to electrical signal and differentiate

between two logic levels. It has two main parts

photo detector and logic level detector. Photo

detector operates by converting light signal that

hit the junction into voltage signal, Absorption

of light crates electron hole pairs. Photo

transistor and photo diode can be used to detect

the optical signal. Photo diodes has high

refreshing rate than photo transistor while

photo transistor has an internal gain. Voltage at

receiving end won’t be 0V and 5V due to noise,

so a proper threshold voltage should be selected

to differentiate between logic levels 1 and 0s.

Driving circuit in Fig 02 will do that function.

Demodulator block separates the signal to the

proper channels.

Fig 02: The Receiver

2.3 USB to TTL module

It is a module used to convert serial bit stream

into Transistor Logic (TTL) levels of 5V for 1s

and 0V for 0 by using ICs like CH340 and PL

2303. It has two ports, USB COM port and a

four pin terminal namely Tx, Rx, 5V and

Ground .An external program can be used to

write the data to com port (USB) of the

computer and transferred to the transmitter

circuit through the module by Tx pin.

On the other end received voltage at the

receiver is given to the RX pin of the module

and which again read by the software by the

same program through the com port.

Eltima Advanced Serial Port Terminal can be

used as the software.

3. Method

3.1 Transmitter Circuit

Fig 04: Transmitter circuit

The transmitter can be connected to a device

using an USB port. The data stream

communicated between the transmitter and the

connected device is converted to a serial bit

stream using a USB to TTL module with a

CH340IC. The transmit terminal of the USB to

TTL module is then connected to a 2N2222

transistor which is operated in switching mode,

as shown in Fig 04. For each bit with a logic

value of ‘1’, the transistor will be saturated and

the laser will be turned on. For each bit with

logic ‘0’ value, the transistor will be cut off and

the laser will be turned off.

3.2 Receiver Circuit

Fig 05: Receiver circuit

The laser beam emitted from the transmitter is

used to bias a phototransistor in the receiver.

The collector voltage of the phototransistor is

used as the input for an inverting voltage

comparator, constructed using a 741 OpAmp.

The op-amp is fed with a voltage supply of

+Vs= +5V and –Vs= -4V. Thus, when the laser

beam is observed, the op-amp output will be

+5V and -4V (Practical values were +3.5V and

-2.6V). The output of the op-amp is then

inputted to a pair of inverted Schmitt triggers in

series, in order to broaden the pulse width of the

received signal at the receiver. The output of the

second Schmitt trigger is fed to another USB to

TTL module to be converted back to a USB

data stream.

3.3 PCB design and manufacturing

The PCB schematic design was done using

Orcad 9.3 Capture software (Fig 07), and the

PCB layout design was done using Orcad 9.3

Layout software. PCB was manufactured using

ironing method with HCl, H2O2 as the etching

regents. The PCB is a single sided PCB.

3.4 Enclosure

The enclosure was made using dark blue

Perspex sheets (Fig 06). The dark color was

chosen to minimize the effect of ambient

lightening to the light sensitive electronic

components in the circuit. The enclosure was

fixed with 3 adjustable mounts, to adjust the

orientation of the transceiver. The orientation

adjustment is critical, as both the transceivers

should be perfectly focused at each other, in

order to communicate with each other.

Fig 06: Optical Transceiver

Fig 07: The final circuit for the transceiver

4. Results

Transmission was success between two

computers at a baud rate of 9600 bps.

Computers were separated by 3m. There

weren’t any errors in the transmission. As the

laser which was used in this project has an

adjustable focus, it was possible to focus up to

a length of 25m. So it has been expected the

distance can be increased up to a length of 50m

which cannot be practically checked. Because

when the distance is too long it’s difficult to

focus smoothly by hand. So we successes in 12

meters. As this is a project based of digital

communication there was a rare need to use

laboratory oscilloscope and multi meters

satisfied the needs.

In this project USB to Serial TTL module was

used. This was checked to confirm the

transmission. Transmitter pin of one module

was directly connected to the receiver pin of the

module and text files were transmitted using

Eltima Advanced Serial port Terminal. It was

successful. But the transmitter module connects

long wires it gives errors due to resistance of

connecting wires. Noise margin of the module

was found to be 0.4V to 2.6V through its data

sheet. There was almost a voltage of 1.8V in the

transmitter pin when no transmission occurred,

which is a noise.

Fig 08: IC741

Transmitter circuit was not that much difficult

as it worked as laser glowing with transistor

switch. But the receiver was complex to

calibrate and handle. The received light

switched the photo transistor, but the collector

voltage was 1.4V and 2.5V for 0s and 1s which

was in the noise range.

A threshold must be chosen to differentiate the

voltages, as a solution IC741 (Fig 08) op-amp

was chosen to be used in comparator mode.

Firstly we used single power supply operation

mode to feed the power for op amp as 0V and

9V. By separating the transmitter and receiver

at a distance of 3m, we supplied voltages

between 1.4V - 2.5V to inverting input pin of

IC741 op-amp. An oscilloscope was used to get

the threshold voltage by taking several

readings. We got the best value as 1.8V.

Threshold voltage was supplied using potential

divider with high resistances were used.

Bounding voltages were 5V and 0V. But the

output voltages were 3.7V for 1s and 1.5V for

0s. Using the op amp 1s could be identified but

0s are in the noise margin. This is due to offset

voltages and currents and CMRR ratio of the

op-amp. Then we use plus and minus voltages

to give power for op amp. In order to bring the

0s voltage down a negative bounding voltage

was planned to supply. IC 7905 was used and

+5V and -4V was supplied.

When the circuit was checked after doing this,

there were steeps for 1s in the oscilloscope and

several errors occurred in the transmission.

Oscilloscope show us when transition from one

state to another signal had little incline and plus

width were too small for 1s more than the 0s.

So there was a need for Schmitt triggering in

order to increase the width of the pulse to be

read by the receiver pin of the USB to Serial

TTL module. IC 7414N (Fig 10) was used for

the Schmitt triggering.

Fig 09: IC 7414N Schmitt trigger

The signal from IC 741 was given to pin 1 and

output which was taken from pin 2 was again

given to pin 3 and final output was taken from

pin 4 and was given to the module. Finally

successfully transmit data computers were

separated by 3m. The distance was increased to

12m and was checked no errors occurred.

Successfully transmitted at a baud rate of 9600

bps. But we increase the baud rate to 14400, we

send text some word was destroyed. There were

many op amp we checked at 12 meters. Some

op amp supported to higher baud rate mare than

9600.

5. Discussion

When distance between the transceivers was

increased, there were some errors occurring.

The main cause for this was the difficulty of

focusing the lasers so that the other transceiver

receives the laser beam with a high intensity.

BJTs are relatively faster than op-amp.

Therefore, op-amp is the bottleneck of the

circuit. Op-amp with low response time like LT

1016 with 9ns response time will better be a

solution to this application. Some other groups

had used MOSFET instead of BJT which had

resulted in a reduction in the speed of

transmission.

Furthermore power of the laser is also an

important scenario. If the power of the laser was

above 26mw and a proper green color laser

beam could be obtained and the laser would

have been capable of causing a high intensity

even at larger distances between the

transceivers.

Laser light and the phototransistor should be in

the line of sight for proper transmission, which

might be a draw back as light waves have low

deflection comparing to radio waves.

The voltage received at the collector of the

transistor in Receiver circuit was 1.4V and 2.5V

which was in the noise margin (0.4V to 2.6V)

of the USB to TTL module. As a solution a

secondary transistor was used to separate logic

levels but due to high impedance of the photo

transistor base current of the second transistor

was not adequate to bring it to the saturated

region. A component with very sharp threshold

needed be used to differentiate logic levels thus

Op-amp was taken as a solution. The Op-Amp

was used in open loop comparator mode. In

order to find the threshold, receiver circuit was

checked using oscilloscope with 3m apart from

transmitter and a threshold value of 1.82V was

chosen. While building the comparator circuit

power consumption of the circuit was taken into

consideration and high resistor values were

used in potential divider for the threshold

voltage. +5V and 0V were given as the

bounding voltages for the op-amp. However,

later it was found that, due to imperfection in

op-amp 3.7V for logic 1 and 1.5V for logic 0

were received at the output. Now logic1s wave)

became steeps and the time needed by the

receiver module to read data was higher than

the pulse width. A Schmitt trigger was decided

to be used in the circuit so that it would

increases the pulse width and reduced noise can

be identified by the module but logic0s were

still in the noise margin. So it was decided to

give negative voltage less than -1.1V for lower

bound. A 9V battery and 7905 regulator was

used to get required + and – bounding voltages.

Due to a delay and imperfection in electronic

components the data originally sent by the

computer (a square wave) became steeps and

the time needed by the receiver module to read

data was higher than the pulse width. A Schmitt

trigger was decided to be used in the circuit so

that it would increases the pulse width and

reduced noise.

Reference

Mohammed Fawazi, Mohammed Kadim

and AL-Temini, “Design and

Construction an Optical System of Data

Transfer in Air using Laser Technology”,

International Journal of Current

Engineering and Technology, April 2014

Vol 04, No 02.

Datasheets

USB to Serial TTL module

https://www.olimex.com/Products/Bre

adboarding/BB-

CH340T/resources/CH340DS1.PDF

IC 741 op-amp

http://www.ti.com/lit/ds/symlink/lm74

1.pdf

IC 7905 Voltage regulator

http://www.hep.upenn.edu/SNO/daq/p

arts/lm7915.pdf

IC7414N Schmitt trigger IC

http://images.ihscontent.net/vipimages

/VipMasterIC/IC/TXII/TXIID004/TXI

ID004-3-85.pdf

Appendix

Fig 10: Operation of a Schmitt trigger

Fig 11: Properties of LED and LD

Fig 12: Checking with Eltima Advanced serial port terminal