Digital Demodulation of aFractional Fringe Interferometer
Tristan Tayag and Christopher BelkTexas Christian University
Fort Worth, TX 76129
DSPS Fest ‘99Houston, TX 77056
August 5, 1999
Theoretical Output Signal:Time Domain
0 0 .0 0 5 0 .0 1 0 .0 1 5 0 .0 20
0 .5
1
1 .5
2
Tim e [s e c ]
Ph
oto
de
tec
tor
Cu
rre
nt
[A.U
.]
F ra c tio na l F ring e Inte rfe ro m e te r: Tim e Do m a in
LEGENDLEGEND
Reference:Reference: 50 nm 50 nm 100 Hz 100 HzTarget:Target: 50 nm 50 nm 1 kHz 1 kHzUnity FringeUnity Fringe Contrast Contrast
Theoretical Output Signal:Frequency Domain
0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0-1 0 0
-8 0
-6 0
-4 0
-2 0
0
F re q ue nc y [Hz]
Ph
oto
de
tec
tor
Cu
rre
nt
[dB
]
F ra c tio na l F ring e Inte rfe ro m e te r: F re q ue nc y Do m a in
LEGENDLEGEND
Reference:Reference: 50 nm 50 nm 100 Hz 100 HzTarget:Target: 50 nm 50 nm 1 kHz 1 kHzUnity FringeUnity Fringe Contrast Contrast
Computational Efficiency:Comparative Example
Mathematical Direct FFT Decimation GoertzelOperations DFT Time Frequency Algorithm
Complex Multiplications 8 7 7 0Complex Additions 7 7 3 0Real Multiplications 0 0 0 11Real Additions 0 0 4 19Total Multiplications 32 28 28 11Total Additions 30 28 24 19
TOTAL COMPUTATIONS 62 56 52 30Assumptions: 8-point sequenceAssumptions: 8-point sequence
Calculation of 1 frequency pair Calculation of 1 frequency pair
Experimental Output Signal:Frequency Domain
LEGENDLEGEND
Reference:Reference: 100 Hz 100 HzTarget:Target: 1 kHz 1 kHz
Goertzel Frequency Selectivity:658 Hz Square Wave
LEGENDLEGEND
ChCh. 1:. 1: Square Wave Square Wave 658 Hz 658 Hz 2 V 2 V pkpk--pkpkChCh. 2:. 2: 1.25 V DC 1.25 V DC
Goertzel Frequency Selectivity:219 Hz Square Wave
LEGENDLEGEND
ChCh. 1:. 1: Square Wave Square Wave 219 Hz 219 Hz 2 V 2 V pkpk--pkpkChCh. 2:. 2: 0.4 V DC 0.4 V DC
Goertzel Frequency Selectivity:665 Hz Square Wave
LEGENDLEGEND
ChCh. 1:. 1: Square Wave Square Wave 665 Hz 665 Hz 2 V 2 V pkpk--pkpkChCh. 2:. 2: 0 V DC 0 V DC
Goertzel Accuracy:659 Hz Sinusoid
0.07 0.36 0.66 0.95 1.24 1.53 1.83 2.12 2.41 2.71 310
8
6
4
2
0
2
4
6
8
10Goertzel % Error: 3 Vpk-pk AIC Range
Input Voltage [Vpk-pk]
Per
cent
Err
or
%Errori
Vi
Goertzel Linearity:659 Hz Sinusoid
0 0.5 1 1.5 2 2.5 30
2000
4000
6000
8000Goertzel Precision Test (AIC +/- 1.5 V)
Peak-Peak Input Voltage [Volts]
Inpu
t Am
plitu
de [F
loat
ing
Poi
nt]
Goertzeli
yi
Vi
Goertzel Linearity:659 Hz Sinusoid (Low Voltage)
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010
5
10
15
20
25
30Goertzel Precision Test (AIC +/- 1.5 V)
Peak-Peak Input Voltage [Volts]
Inpu
t Am
plitu
de [F
loat
ing
Poi
nt]
Goertzeli
yi
Vi
Summary
• Constructed a fractional fringe interferometerfor the characterization of microelectromechanicalsystem (MEMS) structures.
• Developed stabilization and demodulationarchitectures based on DSP.
• Implemented the Goertzel algorithm on theTMS320C31 DSK to extract the requisite spectralcomponents for stabilization and demodulation.