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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
ISM Band Transmitter/Receiver
  • Bob Sosack
  • Chris Lettow
  • Justin Quek
  • TA: Julio Urbina
  • July 27, 2001
introduction
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
project features and goals
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)
signal generation and modulation the transceiver chip
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
programming the microcontroller
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
power amplifier
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)

slide10

Power Amplifier Chip

with Biasing and RF Matching Network

power amplifier impedance matching
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
slide13

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

slide14

Reflection Loss for output impedance matching

network. C1 = 1000 pF, C2 = 8.2 pF

amplifier testing problems solutions
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
solution 1 surface mount capacitors
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
slide17

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

solution 2 sma connectors
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
solution 3 change capacitor values
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
microstrip patch antennas
Microstrip Patch Antennas
  • Used for two reasons:
    • Flat surface makes them ideal for mounting on airplane
    • Impedance matching fairly simple

Calculating Patch Length:

impedance matching inset feeds
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
antenna testing problems solutions
Antenna Testing – Problems/Solutions
  • Problem #1: First design did not resonate at correct frequency ( 950 MHz)
    • Increase patch size  increase /2  decrease resonant frequency
antenna testing problems solutions23
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
antenna testing problems solutions26
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
final conclusions and recommendations
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
final conclusions and recommendations30
Final Conclusions and Recommendations
  • Transceiver:
    • Determine the cause of frequency drift (PLL)
    • Update board layout for better size matching
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