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The ELT Locator

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  1. The ELT Locator by Jeremy Jenkins and Rich Rittis Project #32

  2. Introduction • Objective of project • To detect and display the relative position of an ELT signal relative to the RCVR • Practical Use • Used at air fields to detect malfunctioning ELT’s • Mounted on a vehicle • Relative range about 300 feet

  3. Intro, cont… Displays relative position of aircraft transmitter

  4. Intro, cont... • Standard Airplane ELT • 243 MHz Pulse Signal • Wavelength = 48.6 inches • Typical Power = 5 Watts or 37 dBm • Simulated ELT Signal Using Function Generator • 243MHz Pulse Modulated Signal • Signal Power = 16dBm • Transmitted through a /4 antenna

  5. Antenna Array RCVR Data Storage Intro, cont… Block Diagram of Entire ELT locator Display

  6. Antenna Array and Control

  7. Antenna Array and Control Antenna Array Controller Antenna Array

  8. Antenna Controller • Allows the use of one receiver with many antennas • Consists of off-the-shelf IC’s • reduce costs • improve availability of parts • Functions of controller broken into: • Timer • State machine (6 states) • Control logic • Output to antenna array switching circuit

  9. Antenna Control Flow 555 Timer State Machine Six states for controlling antenna switching and data storage MC1455P1 9ms/cycle 3-bit counter 3-8 decoder SN74LS169N SN74LS137N

  10. Antenna Control Flow 555 Timer State Machine Control Logic OR gates 2-4 decoders SN74LS139N

  11. Antenna Control Flow 555 Timer State Machine Control Logic Antenna Array

  12. Antenna Array • Control signals used to switch pairs of /4 dipoles to receiver. • Three /4 dipoles phased to create an antenna array capable of sensing 360º in 60º steps.

  13. /4 Dipole Antennas • /4 dipole • 1.013 feet long (243mhz) • Spaced /4 apart (1.103 feet) • Made from stripped coax cable • Arranged in triangle to provide 360º sensing • -/2 delay line • Switched between pairs of /4 dipoles • Produces array sensing in six directions

  14. Phased Antenna /2 Dipole(Using Two /4 Dipoles) /4 “Delayed” Dipole /4 Dipole /4 Distance -/2 delay /4 = 1.013 feet for 243MHz

  15. Relative Signal Strength Pattern “Delayed” /4 Dipole /4 Dipole

  16. Sensed Directions for Dipole Pairs “delayed” dipole

  17. Sensed Directions for Dipole Pairs “delayed” dipole

  18. Sensed Directions for Dipole Pairs “delayed” dipole

  19. Sensed Directions for Dipole Pairs “delayed” dipole

  20. Sensed Directions for Dipole Pairs “delayed” dipole

  21. Sensed Directions for Dipole Pairs “delayed” dipole

  22. Circuit Design RCVR -/2 delay

  23. Circuit Design RCVR Sensing direction -/2 delay

  24. Circuit Design RCVR Sensing direction -/2 delay

  25. Antenna Switches • Initially proposed using PIN diode switches • Eliminates moving parts from system • Requires surface mount components and board design. • Demonstrated system using reed relays • 50-ohm shielded • Prove viability of design • Facilitate project completion without PC board.

  26. PIN Diode Switching Circuit

  27. Reed Relays • Pros • Replaces four components with one reed relay • Does not require SMT for implementation • Price comparable • Low contact resistance (0.2 ohms) • Cons • Not suitable for continuous use • Expected lifetime is 1 million cycles or 7.5 hours of operation in our circuit (37 cycles per second)

  28. Phased Antenna Testing • Initially tested a single phased dipole pair • Reception pattern typical of /2 dipole ant • “End fired” pattern • Two null points at 90º and 270º • Produces two location indications, one correct and one 180º opposite to correct location • Tested under less than ideal conditions • In the ECE 353/363 lab

  29. Problem Illustration Correct Indication Incorrect Indication

  30. Antenna Array • Coax dipoles • Wood mount • Aluminum ground plane

  31. Recommended Fixes • Proper tuning of antenna array • Solid state switching using PIN diode switches • Improved antennas • Durability • Closer manufacturing tolerances (instead of the “looks about right” method)

  32. ELT Receiver

  33. * the BW limiting filter in the Linx Module is 280kHz Frequency Spectrum Data Out * 1-if signal present * 0-if signal not present 171MHz 315MHz 315MHz 243MHz 315MHz LINX Receiver 243MHz ELT Signal (From Array) X 72MHz Receiver • RCVR Functionality • Original Design *SAW based Superhet *AM Demodulation *Data Rate 5Kbps

  34. Data Out * 1-if signal present * 0-if signal not present 243MHz ELT Signal (From Array) Mixer 315MHz LINX Receiver RF IF X LO IF Amp Pre-Amplifier Bandpass Filter Attenuator 72MHz Oscillator RCVR, cont... • Final RCVR Design

  35. Pre- Amplifier • Necessary to Amplify 243MHz Signal from Array • Input from Antenna Array by Coaxial Cable • Two Cascaded MAR-1 Amplifiers • 47dB Gain at 243Mhz • +5V bias on emitter • 17-20 mA current • Caps used for DC Block

  36. 120 ohm 72MHz Oscillator 68 68 GND LO Signal • 72 MHz • Necessary to UP-convert RF signal to 315MHz • Citizen Clock Oscillator • 16dBm maximum output at 72MHz • 14dB Antenuator needed • Mixer Input Max is +7dBm • Resistive Network Used • LO Amplitude cut to 2dBm Zin = 50 50

  37. LO Signal, cont… • Bandpass Filter Needed • to attenuate harmonics of 72MHz signal • Can have large BW but not greater than 70Mhz • Butterworth Design Equation: • | S21(jw) |2 = 1 / (1 + w2n) • normalized wc = 1 rad/s, • n= # of lossless components in filter • ADS simulation of Butterworth BPF, BW=50MHz • Final BPF used was BW=20MHz

  38. LO Signal, cont… ADS Circuit of 70MHz Bandpass Filter BW= 50MHz

  39. LO Signal, cont… • Advantage of BPF • 1st Harmonic (144MHz) attenuated 30 dB more, -60 dB from fundamental • 2nd Harmonic (216MHz) attenuated 40dB more, -60 dB from fundamental +2 dBm, 72MHz Oscillator Bandpass Filter 14 dB Attenuator 120 ohm LO, to mixer 68 68 GND

  40. LO Signal, cont… LO Signal Un-Filtered LO Signal Filtered

  41. SRA-1 Mixer 243MHz RF Signal, from Antenna Array RF IF 315 MHz IF Signal, to LINX RX Chip X LO +2dBm 72 MHz LO Signal Mixer • SRA-1 Doubly Balanced Mixer • Necessary to Combine Signals to 315 MHz • Conversion loss RF-IF is -7dB • Max IF output is -13dBm MAR-1

  42. IF Amplifier • IF amplifier is necessary • Due to conversion loss through the Mixer and circuit • MAR-1 amplifier chip used • 14dB gain at 315MHz • Frequency IF Plot • IF filter found in Linx Chip

  43. PRE-selector Filter 10.7 MHz IF Filter Filter, AM Detector, and Hard Limiter RF in Data Out SAW Local Oscillator Linx RX Chip • Necessary to detect if ELT signal is present • SAW based Receiver • Maximum RF input is 0 dBm • Inside the Linx Chip

  44. LINX MODULE Signal Generator Link Oscilloscope Linx RX Chip, cont… • Overall Effect of Chip • Provides a 280KHz IF Filter • Makes RX as a whole a Double Conversion Super Het • Minimum Detectable Signal = -90.8 dBm (conducted), -50dBm (wireless) • Tested • Range 315.21 MHz - 314.81 MHz

  45. + 5V Linx RX Chip, cont… • Linx Output when Signal is Present • Periodic, Vpp = 5V • 0 volts outputted when signal NOT present

  46. RCVR, cont...

  47. Data Storage & Display

  48. Data Storage and Display • Data storage takes output from LINX module (logic 1 or 0), inverts and stores into “D” latches • Chip enable on latches controlled by the state machine • Each latch stores signal information from a phased antenna pair

  49. Data Storage and Display • Output of latches sent through combinational logic to determine output to be displayed • Display consists of 12 red LEDs arranged in “clock positions” relative to antenna array alignment