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Wireless MODEM for 950 MHz Digital Communication

Abhishek Mitra andNerdev SharmaIIIT AllahabadFifth Semester Mini Project19th February 2002 Backdrop:Guglielmo Marconi, pioneer of Wireless

Supervised by

Dr. R C Tripathi

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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

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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

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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

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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 ?

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.

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The Chip

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Direct Digital Synthesizer

Power AmplifierVCO

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 Converter9V to 3.3 V

DDS Control Word From Parallel Port

Data from Serial PortCrystal Oscillator @ 25 MHz

Transmitter Blocks

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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

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Vol

tage

Time

f1f2 f1 f2 f1

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

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

FSK Spectrum

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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

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R1L1C2

C8C

1 Ф

Phase Detector

LPFA

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

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Receiver Characteristics

Demodulated Data

After Decision by Slicer

3.3 V

1.25 V

Reference

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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 Allahabad’s first wireless data communication equipment, developed in-house.

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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

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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?

Wireless

Wires

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Thank You !