Wireless MODEM for 950 MHz Digital Communication

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21 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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1

Wireless MODEM for 950 MHz
Digital Communication


Abhishek Mitra and

Nerdev Sharma

IIIT Allahabad

Fifth Semester Mini Project

19
th

February 2002

Backdrop:Guglielmo Marconi, pioneer of Wireless

Supervised by

Dr. R C Tripathi

2


Communication but
without

wires.


Electromagnetic Waves through Ether


Radio Waves 300KHz to

1GHz



Micro Waves 1GHz to 3THz


Light rays

3THz to 10 PHz


Some Applications


Satellite Communications


Personal Communication Systems


Wireless LAN / Bluetooth


Wireless


3


Identify different methods for digital
data transfer using
Radio

waves.


Search for available technologies.


Design a
Wireless

Modem.


Establish a Wireless Data Link between
two PCs using the Modem.


Develop an Application to control the
device and send / receive
Data
.

Scope of the Project


4

Features:

1.

Maximum Data Rate:
9.6 kbps

Manchester Encoded.


2.

Programmable frequency:
300
-
500MHz.


3.

Output power:
14dBm
, Receiver sensitivity:
-
112dBm
.


4.

PLL

based Detector and Transmitter.


5.

Development Kit Available.


Chipcon CC 400


5

TRF 6900A

Features:

1.

Programmable 850MHz to 950MHz operation.

2.

FSK / ASK

Modulation.

3.

Transmit Power

4.5 dbm
.

85dbm

Receiver Sensitivity
.

4.

High Data Rate of

115.2 kbps

both NRZ and
Manchester.

5.

DDS

based Transmitter
.

6.

Quadrature Demodulator based Detector
.

7.

Received Signal Strength Indicator.

8.

Free Samples

Available from TI.

6

Why TRF 6900A ?


Advantages of TRF6900A



Faster data rates up to

115.2 kbps

versus 9.6 kbps
.



Ability to handle

NRZ

type data versus BiPhase data
.



Direct Digital Synthesizer

based design versus PLL based
.



Easy availability of components for

950 MHz

versus 450 MHz
.



Experiments already done on Chipcon boards hence quest for
something new.



TRF 6900A is a

state of the art

chip has industrial backing
.



Availability of

free samples

from Texas instruments
.

Short Listed Chips



CC 400

from Chipcon.



TRF 6900A

from TI.

7

The Chip

8


Direct Digital Synthesizer

Power
Amplifier

VCO

PLL

RF Out
950MHz

1.
The computer programs the device with various parameters like
frequency of operation, frequency deviation, Tx / Rx mode, etc.

2.
The computer sends serial data to the device.

3.
Modulated Data is transmitted as Radio Waves at 950 MHz
binary(0) and 950.1 MHz binary(1).

Level Converter

9V to 3.3 V

DDS Control Word
From Parallel Port

Data from Serial Port

Crystal Oscillator @
25 MHz

Transmitter Blocks


9

1.
Reference Oscillator (Clock for DDS)


Crystal Oscillator at 25 MHz.

2.

Direct Digital Synthesizer (DDS)


Generate a sine wave in the digital domain as reference to PLL.

3.

Phase Locked Loop (PLL)


Controls frequency of the VCO as per reference supplied by DDS.

4.

Voltage Controlled Oscillator (VCO)


Changes frequency according to the tuning voltage (930 MHz to 960
MHz). LC oscillator with a varactor.

5.
Power Amplifier (PA)


Amplifies the output of the VCO to 4.5dbm (3mW)


Transmitter Section



10

Time

f
1

f
2

f
1

f
2

f
1

f1 = Frequency transmitted for Binary 1 (950 MHz)

f2 = Frequency transmitted for Binary 0 (950.1 MHz)

f1


f2 = Frequency Deviation (100 KHz)

Binary 1

Binary 0

Binary 1

Binary 0

Binary 1

Frequency Shift Keying Basics


11

The two peaks at 915.0 MHz and 915.10 Mhz demonstrate FSK.

FSK Spectrum


12

FSK

Demodulator

LPF

LNA

VCO

10.7 MHz
BPF

IF
AMP

RF Mixer

939.3 MHz

950 MHz

Data Slicer

RF IN

At the mixer output we get sum and difference
of the two frequencies as the mixer multiplies
the signals.

2 COS(A)*COS(B) = COS(A+B) + COS (A
-
B)

10.7 MHz

LPF/Amplifier



Data to Serial Port

Receiver Block Diagram


13

R1
L1
C2
C8
C
1
Ф

Phase Detector

LPF

A

B

Quadrature FSK Demodulator



Tuned Circuit at 10.7 MHz



No phase shift when carrier at 10.7 MHz



Phase Shift at other Frequencies (FM to AM conversion)



Low pass filtering the output recovers the data

Output of
Amplifier fed
to Slicer

Input from IF Stage

Receiver Basics


14

Receiver Characteristics

Demodulated
Data

After
Decision
by Slicer

3.3 V

1.25 V

Reference

15

Our Achievements



A
Plug and Play

Wireless Modem.


Maximum Data Rate of
38.4 Kbps
.


A range of atleast
10 meters
.


WModem Control Panel Software.


IIIT Allahabad’s
first

wireless data
communication equipment, developed
in
-
house.




16



At 950 MHz PCB stray capacitance and inductance
come into play.



Any copper length greater than
λ/4creates trouble.



Use of CAD / CAM is inevitable.

Figures showing the Wmodem

The WModem


17

What Else?



Enhance Data rate up to
115.2 kbps,
speed of
two Dial Up Modems.



Add an Amplifier to boost the output power of
the device, to reach a few
kilometers
.



Packets in the Air
: Enable sockets through
wireless for TCP / IP and Internet connectivity.



Fail Safe Design
: Develop a Robust Error
Correction Mechanism and include a dedicated
Micro
-
controller.



(A Software lends its
reliability

from the
Hardware being programmed underneath) hence
Hardware should be 100% reliable.

18

Which one would you Prefer?

Wireless

Wires

19

Thank You !