Title: Wireless MODEM for 950 MHz Digital Communication
1Wireless 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
2Wireless
- 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
3Scope 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.
4Chipcon 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.
5TRF 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.
6Why 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.
7The Chip
8Transmitter 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
- The computer programs the device with various
parameters like frequency of operation, frequency
deviation, Tx / Rx mode, etc. - The computer sends serial data to the device.
- Modulated Data is transmitted as Radio Waves at
950 MHz binary(0) and 950.1 MHz binary(1).
9Transmitter 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)
10Frequency 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)
11FSK Spectrum
The two peaks at 915.0 MHz and 915.10 Mhz
demonstrate FSK.
12Receiver 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
13Receiver 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
14Receiver Characteristics
After Decision by Slicer
3.3 V
1.25 V Reference
Demodulated Data
15Our 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.
16The 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.
17What 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.
18Which one would you Prefer?
Wires
Wireless
19Thank You !