Transcript Wireless MODEM for 950 MHz Digital Communication

Wireless MODEM for 950 MHz
Digital Communication
Supervised by
Dr. R C Tripathi
Abhishek Mitra and
Nerdev Sharma
IIIT Allahabad
Fifth Semester Mini Project
19th February 2002
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Backdrop:Guglielmo Marconi, pioneer of Wireless
Wireless
 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
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Scope of the Project
 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.
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Chipcon CC 400
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.
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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.
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Why TRF 6900A ?
Short Listed Chips
•
CC 400 from Chipcon.
•
TRF 6900A from TI.
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.
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The Chip
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Transmitter Blocks
Direct Digital Synthesizer
PLL
Crystal Oscillator @
25 MHz
VCO
DDS Control Word
From Parallel Port
Data from Serial Port
Level Converter
9V to 3.3 V
Power
Amplifier
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).
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Transmitter Section
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)
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Frequency Shift Keying Basics
Binary 1
f1
Binary 0
Binary 1
Binary 0
Binary 1
f2
f1
f2
f1
Time
f1 = Frequency transmitted for Binary 1 (950 MHz)
f2 = Frequency transmitted for Binary 0 (950.1 MHz)
f1 – f2 = Frequency Deviation (100 KHz)
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FSK Spectrum
The two peaks at 915.0 MHz and 915.10 Mhz demonstrate FSK.
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Receiver Block Diagram
FSK
Demodulator
IF
AMP
LNA
RF IN
950 MHz
RF Mixer
10.7 MHz
BPF
10.7 MHz
LPF/Amplifier
LPF
VCO
939.3 MHz
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)
Data Slicer
Data to Serial Port
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Receiver Basics
Phase Detector
Input from IF Stage
A
1
Ф
LPF
C8
C
L1
R1
C2
Quadrature FSK Demodulator
B
Output of
Amplifier fed
to Slicer
 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
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Receiver Characteristics
3.3 V
After
Decision
by Slicer
1.25 V
Reference
Demodulated
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Data
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.
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The WModem
Figures showing the Wmodem
 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.
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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.
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Which one would you Prefer?
Wires
Wireless
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Thank You !
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