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ISM Band Transmitter/Receiver PowerPoint PPT Presentation


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ISM Band Transmitter/Receiver. Bob Sosack Chris Lettow Justin Quek TA: Julio Urbina July 27, 2001. Introduction. Project created for Aerial Robotics Club – Develop main communication link between airplane and ground base station Design system to work in ISM Band (902-928 MHz)

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ISM Band Transmitter/Receiver

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Ism band transmitter receiver l.jpg

ISM Band Transmitter/Receiver

  • Bob Sosack

  • Chris Lettow

  • Justin Quek

  • TA: Julio Urbina

  • July 27, 2001


Introduction l.jpg

Introduction

  • Project created for Aerial Robotics Club – Develop main communication link between airplane and ground base station

  • Design system to work in ISM Band (902-928 MHz)

  • RF system consists of microcontroller-driven transceiver chip, a power amplifier, and transmit and receive antennas


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Project Features and Goals

  • Transmit and receive ISM Band signals

  • High power transmission with low power consumption

  • Obtain at least 11.6 dB gain in the power amplifier

  • Antennas have 2:1 VSWR Bandwidth in entire ISM Band (902-928 MHz)


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General Design Schematic


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Signal Generation and Modulation – The Transceiver Chip

  • Texas Instruments TRF6900A transceiver chip

  • Modulator on chip takes digital binary signal generated from microcontroller

  • Digital word input into Direct Digital Synthesizer (DDS) which outputs an analog sine wave


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Transmitter/Receiver Block Diagram


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MSP430 Microcontroller


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Programming the Microcontroller

  • Started with sample code from TI:

    • Transmit at 869 MHz and receive at 859 MHz

    • Provides checksums, RS232 communication

  • Adapted for our own use:

    • Transmit or receive at 915 MHz

    • Sets the TRF6900 mode appropriately


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Power Amplifier

  • Maxim power amplifier chip used

  • Goal: obtain 11.6 dB Gain (typical value obtained from data sheets)

  • Transceiver outputs 4.5 dBm signal (2.8 mW). Goal is to obtain about 40.47 mW output power

11.6 = 10log(Pout/Pin)


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Power Amplifier Chip

with Biasing and RF Matching Network


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Power Amplifier – Impedance Matching

  • Goal: Match 50  output impedance of transceiver board and 50  antenna feed impedance to internal source and load impedances of amplifier chip

    • Zs = (5.025+j2.173)  at 915 MHz

    • Zl = (5.939 + j1.629) at 915 MHz

  • Impedance Match especially critical on input side because of very low power input – Must have VERY little reflection loss


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Amplifier Input Impedance Matching Network


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Reflection Loss for input impedance matching network

Actual used values C2=7 pF

C1 = 5 pF

“Ideal” values C2= 7.3 pF,

C1 = 5.37 pF


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Reflection Loss for output impedance matching

network. C1 = 1000 pF, C2 = 8.2 pF


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Amplifier Testing – Problems/Solutions

  • Initial testing of amplifier on network analyzer:

    • Wideband gain ( 12dB) from 300-800 MHz

    • Gain dropped off significantly after 800 MHz

    • Gain at 915 MHz  0dB


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Solution #1: Surface Mount Capacitors

  • Smaller size, reduced parasitic inductance from lumped element capacitor wires

  • Capacitor values changed slightly due to part availability, but still acceptable impedances matches obtained


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Reflection Loss of impedance matching networks

using surface mount capacitors

Input matching network

C2 = 6.8 pF, C1 = 5 pF

Output matching network

C1 = 1200 pF, C2 = 8.2 pF


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Solution #2: SMA Connectors

  • Much more reliable for RF applications than BNC: conductor directly from connector housing to copper feedline

  • Test results after these improvements showed very little change, gain still only around 1dB


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Solution #3: Change Capacitor Values

  • Trial and error process: Capacitor values on input and output matching network lowered

  • Testing showed gain of 12.2 dB at 915 MHz

  • Amplifier Conclusions:

    • Gain surpassed design goal

    • RF parasitic effects most likely caused theoretical values to be ineffective


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Microstrip Patch Antennas

  • Used for two reasons:

    • Flat surface makes them ideal for mounting on airplane

    • Impedance matching fairly simple

Calculating Patch Length:


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Impedance Matching – Inset Feeds

  • Patch edge has impedance  150 . Matching to 50  would require a long, thin /4 feedline

  • Alternative: Inset feed – Obtain 50  impedance at patch edge

    • No need for impedance transformer

    • Thicker feed line should limit inductance


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Antenna Testing – Problems/Solutions

  • Problem #1: First design did not resonate at correct frequency ( 950 MHz)

    • Increase patch size  increase /2  decrease resonant frequency


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Antenna Testing – Problems/Solutions

  • Problem #2: Patch edge impedance not low enough  69.27 

    • Increase inset – Impedance drop more gradual as it tends to 50 

  • Problem #3: High Reactive Capacitance degrades impedance match, Bandwidth

    • Replace BNC connectors with SMA Connectors


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Antenna Design #2 – Resonant at 918 MHz with

Z = (69.27-j32.06) 


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Antenna Design #2 – 2:1 VSWR Bandwidth (13 MHz)


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Antenna Testing – Problems/Solutions

  • Problem #4: New design showed high reactive inductance

    • Decrease inset gap spacing to add capacitance – negligible effect

  • Antenna Conclusions:

    • Resonates at the correct frequency

    • Achieved 50  at patch edge

    • Over half desired bandwidth obtained

    • More bandwidth could be achieved by neutralizing the inductive effects


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Final Antenna Design Resonant at 916 MHz with

Z = (49.77+j13.80) 


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Final Antenna Design VSWR Bandwidth (15.75 MHz)


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Final Conclusions and Recommendations

  • Antennas: Use higher quality substrate, higher dielectric to decrease size, find way to increase antenna gain

  • Amplifier: Determine exact cause of mismatch from theoretical values, cascade together to increase overall gain


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Final Conclusions and Recommendations

  • Transceiver:

    • Determine the cause of frequency drift (PLL)

    • Update board layout for better size matching


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