Compact Semiconductor Laser Microphone and RF Transmitter

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Compact Semiconductor Laser Microphone and RF
Transmitter

• Project Number 15
• Team Members
• Michael Bastin
• Patrick Cox
• TA
• Justin Foster

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Project Introduction

• Design and engineer an accurate and compact
  optical listening device
• Couple optical device with a secure RF
  transmitter
• Receive transmitted RF signal at an off site
  location and reproduce audio signal originally
  detected by optical device

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Reasons for Project Selection

• Combines the specialty interests of both team
  members (optics and RF)
• Opportunity to incorporate the newest
  technologies of each field in our project
• Opportunity to improve on previous designs used
  for the same purpose

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Project Objective

• The end goal of the project is to be able to
  detect sound waves hitting a reflective object at
  one location optically, and then reconstruct
  those sound waves at a remote location via RF
  transmission

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Uses of end product

• Eaves dropping device
• Room audio recorder for presentations
• Economical Physics Educational Demonstrations

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Project Block Diagram
Reflective Surface
Interferometer Optics
High Gain Audio Amp
Voice
RF Transmitter
RF Receiver
Listening Equipment
Front End
Back End
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Front End Components-Voice

• Our entire system must be able to work over the
  frequency range of the human voice (500 Hz to
  2000 Hz) at a minimum
• Must also be sensitive to loudness (amplitude) of
  voice sound waves

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Front End Components- Reflective Surface

• For our demonstration and testing purposes we
  used a four pane-glass window mounted in a wooden
  frame for our standard
• Any reflective surface can be used to pick up
  sound
• Design and results will vary with different
  surfaces depending on type of surface and mounting

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Front End Components- Optical Theory

• A Michelson Interferometer uses constructive and
  destructive interference to measure distance
• Sound waves impact the window and change the
  distance the light travels
• A direct change in distance in converted to a
  change in light intensity at the output of the
  interferometer

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Front End Components-OpticsOriginal Setup
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Front End Components-OpticsIntermediate Setup
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Front End Components-OpticsBOM and Details

• An 850 nm VCSEL used for size and beam quality
  considerations
• 780 nm cube beam splitter
• Graded index mirror for equal reference and
  incident beam strengths
• Pane of glass for incident plane
• Variety of photo-detectors

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Front End Components-OpticsTesting Procedures
and Results

• Beams must be aligned using infrared viewing
  cards
• Light intensity from each leg of the
  interferometer must be measured at output
• Movement in glass should be evident by voltage
  change across pull down resistor after
  photo-detector

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Front End Components-Optics Conclusions

• Non-ideal components lead to poor signal
• Low gain PIN gives very quiet voice
• Large aperture Darlington introduces noise
• 780 nm Beam splitter degrades light quality
• Difficult to use on moving reflective surface
• Opportunity for surface to move more than 1 ?
• No contrast for sharp syllables

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Front End Components- Audio Amplifier

• The audio amplifier must convert a light beam of
  time varying intensity to a time varying voltage
  wave
• Intensity changes are directly proportional to
  sound waves so the voltage wave should also be
  directly related
• Changes in light intensity are very small and
  must be highly amplified

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Front End Components- Original Design Problems

• Photo-detector could not be effectively decoupled
  from rest of circuit caused current and voltage
  delivered to rest of circuit to vary
• Voltage controlled attenuation of signal to fixed
  gain amplifier over complicated since maximum
  gain was needed

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Front End Components- Audio Amplifier Final
Design Results

• Capacitors heavily used to stabilize circuit
• Cross /- terminals to improve DC
• Small shunt capacitors prevent oscillations
• Decouple AC voice component from DC signal
• Circuitry had severe shielding problems
• Simple amplifier used at extremely high gain
  caused distortion
• Switched power to batteries to clear up high
  frequency noise

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Front End Components- Bluetooth Protocol

• Short range radio link being developed to
  connect portable and/or stationary devices
• Uses the 2.4 GHz Industrial Science and Medicine
  Band (FCC non-licensed ISM band)
• Capable of replacing all communication wires
• LAN
• Audio
• COM Ports
• USB

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Front End Components- Bluetooth Protocol

• High Level Security Encryption via
• Frequency Hopping (1600 Hops/sec)
• Individual Component address
• 128 bit private user keys for authentication and
  encryption
• Three-two way 64 kb/s audio channels and 723 kb/s
  asynchronous data channel
• Output power regulated as distance changes to
  extend battery life

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Front End Components- Bluetooth Transmitter

• GN Netcom 9000 Wireless Bluetooth Headset
  (donated to us by Motorola PCS)
• Connected to the output of the Audio Amplifier
  via the headset microphone connection
• Participates as a slave in the Bluetooth Piconet

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Front End Components- Bluetooth Transmission
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Back End Components- Bluetooth Receiver

• Motorola Bluetooth PCMCIA PC Card (Donated by
  Motorola)
• Used with a Laptop for off site listening
• 10 meter range
• Shorter distance depending upon location

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Back End Components- Bluetooth Software

• Used Digianswers Bluetooth Software Suite
• Established a General Audio Connection
  connection from Laptop (master) to Headset (slave)

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Back End Components- Listening Equipment

• Able to listen to transmitted audio signal in a
  variety of ways
• Laptop Speakers
• Headphones
• Record signal for later playback
• Transmit audio file via Bluetooth LAN to another
  offsite computer
• Transmit audio file via Bluetooth to a PDA

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Bluetooth Transmission Testing

• Used CATC Merlin BT Protocol Analyzer to record
  and analyze a transmission between the BT headset
  (our transmitter) and the PCMCIA card (our
  receiver)

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System Tests

• Function Generator connected to speaker
  outputting a 1 KHz tone at 20 Vpp
• Was not able to hear the sound at the Back End
• The PC speaker may not have been able to create
  large enough sound waves to vibrate the large
  piece of glass
• Most likely explanation Voice comes from
  distance change. Non-affixed window was moved
  and held by sound waves.

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System Tests

• Person Talking
• Yielded the best results
• Was able to distinguish major syllables of word
  spoken
• Not able to distinguish smaller one syllable
  words
• Somewhat hard to hear because of noise floor
• HVAC System
• Cooling fans of Power Supplies and Computers

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Outcome

• Successfully engineered a compact optical
  listening device and with RF transmission to an
  off site location
• Successfully utilized two new and up in coming
  technologies to attain our goals

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Outcome cont.

• Front end not as small as planned
• Can be easily accomplished with proper funding
  (smaller optics and stands, mounting)
• Background noise more significant than originally
  planned for (wind, rain etc)
• Voice reproduction not as clear as wanted
• Distortion from window looseness
• VCSEL was not of high enough quality
• Large aperture photo-detectors

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Outcome cont.

• Sound source must be close to window
• May have to do with sensitivity of PIN
• Extremely lossy beam splitter and not designed
  for the wavelength of our beam
• Could be caused by not sensitive enough PIN

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Miscellaneous Things we Learned From Project

• Necessity of having good Power Supplies
• Decoupling Power Supplies from circuits
• Effects of shielding and objects around you on
  circuitry
• Datasheets are not always accurate
• Quality components lead to quality systems

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Ideas for Improvement

• Use a more efficient beam splitter that has a
  bandwidth centered at 850 nm
• Use a higher quality VCSEL
• Use a higher quality PIN
• Build facing much more secure reflective surface

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Questions?