Wireless MODEM for 950 MHz Digital Communication

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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
BackdropGuglielmo Marconi, pioneer of Wireless
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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
• Maximum Data Rate 9.6 kbps Manchester Encoded.
• Programmable frequency 300-500MHz.
• Output power 14dBm, Receiver sensitivity
  -112dBm.
• PLL based Detector and Transmitter.
• Development Kit Available.

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TRF 6900A

• Features
• Programmable 850MHz to 950MHz operation.
• FSK / ASK Modulation.
• Transmit Power 4.5 dbm. 85dbm Receiver
  Sensitivity.
• High Data Rate of 115.2 kbps both NRZ and
  Manchester.
• DDS based Transmitter.
• Quadrature Demodulator based Detector.
• Received Signal Strength Indicator.
• 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
DDS Control Word From Parallel Port
Direct Digital Synthesizer
PLL
Crystal Oscillator _at_ 25 MHz
Data from Serial Port
Level Converter 9V to 3.3 V
Power Amplifier
VCO
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

• 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.
• Power Amplifier (PA)
• Amplifies the output of the VCO to 4.5dbm (3mW)

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Frequency Shift Keying Basics
Binary 1
Binary 0
Binary 1
Binary 0
Binary 1
f2
f1
f2
f1
f1
Voltage
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
10.7 MHz BPF
10.7 MHz
RF Mixer
LPF
LPF/Amplifier
VCO
939.3 MHz
Data Slicer
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(AB) COS (A-B)
Data to Serial Port
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Receiver Basics
Phase Detector
Input from IF Stage
LPF
A
?
B
Quadrature FSK Demodulator
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
After Decision by Slicer
3.3 V
1.25 V Reference
Demodulated Data
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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 Allahabads 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 ?/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 !