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Pseudo-Doppler Direction FinderAmanda Ke, Melissa Li, Jimmy Mawdsley
MIT | 6.101 Analog Electronics Laboratory | April 14 2016
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Background and Project Goals
Determines direction toward signal source (RF, acoustic, etc.)
RDF applications: aerial navigation, tracking, locating pirate radio stations...
. . .
UPW DFSource
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Pseudo-Doppler Direction Finding
Synthesize a rotating effective antenna by switching between elements in an array
Effective antenna moves toward/away from source → Doppler shift
Typically use a soft-switching profile to reduce phase discontinuities
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Motivation
Originally idea: Radio Direction Finder (RDF) operating in FM band…
Same principle of operation holds for audio frequency
Analog solutions for most of the functionality¼ wavelength = 8.5 cm for 1 kHz tone
c = 340 m/s
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High-Level Block Diagram
Microphone array and signal conditioning
Electronically weighted audio mixer Control unit
FM RX Bandpass filter Phase comparator
Offset correction
DisplayPseudo-Doppler DF
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Overview: Signal Acquisition Module
Microphone array and signal conditioning
Electronically weighted audio mixer Control unit
pseudo-Doppler signal (FM, 1kHz) reference tone (SW, 100Hz)
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Close-Up: Signal Acquisition ModuleSignal Conditioning - Block Diagram
Microphone
Preamp
Bandpass Filter
Peak Detector
Compressor
Mic x
Considerations:
○ Why is AGC needed?
○ Why is BPF needed?
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Close-Up: Signal Acquisition ModuleSignal Conditioning - Implementation
mic input stage band-pass filter with gain compressor (with gain)
Complications with multipath?
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Close-Up: Signal Acquisition Module
Control Unit
reference sinusoid
control bias waveforms
Control Unit - Block Diagram
Considerations:
○ Shape of bias waveform?
○ How to make quadrature?
Implementation: reference sinusoid → square wave → triangle (zero offset) → comparator → quadrature
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Close-Up: Signal Acquisition ModuleVoltage Controlled Audio Summer - Block Diagram
ΣMicrophone Signal Conditioner
Control Unit
mic signals
gain control signals
FM pseudo-Doppler
Controllable Gain
Voltage Summer
Audio Mixer
Implementation: JFET operated in linear regime for voltage controlled gain
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Phase Extraction
Control unit
VCO
Phase Comparator
FM Demodulator
DC Control Sq. Wave
Sq. Wave
to Display
Schmitt Trigger
FM
from Audio Mixer
Mixer
Mixer Integrator
Integrator
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FM Demodulator
1st-order Phase locked loop
FMInput Phase
Detector
Output proportional to phase difference
VCO
Loop Filter
Demodulated signal
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Close-Up: FM Demodulator
Challenges:
- Tuning the low pass filter- Jitter
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Gilbert Cells and Mixing
Schematic
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Phase comparison
Challenges:
- Asymmetry of components
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Display
Offset correction
Oscilloscope XY
LED Display
16 Comp LED
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LED Display
Negative Terminals● 0, Vref/15, 2Vref/15, ...
14Vref/15, Vref
Positive Terminals● All connected at phase
comparator output
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Oscilloscope XY Display
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Oscilloscope XY Display
555 Integrator
Phase Comparator Output
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Schedule
Week of Tasks
April 11 Begin building individual modules
April 18 Continue building individual modules. Begin integrating/debugging modules
April 25 Continue integrating/debugging modules. Begin working on stretch goals
May 2 Continue working on stretch goals. Demonstration of complete system
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Concluding Remarks
Expected outcome:
● Complete PD-DF system that can point toward speaker with good accuracy● Extensions: lay out a PCB, redesign for RF, more elaborate interface
Next steps:
● Finish up individual modules (design, construction, test)● Test interconnected modules
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Questions?