ADC characterization in the frequency domain by dual tone testing

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ADC characterizationin the frequency domainby
dual tone testing
David Slepicka Dominique Dallet Vladimir
Shitikov Francois Barbara
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Content

• Phase determination
• (Incoherent sampling, cosine windows)
• ADC transfer function determination
• Experimental results
• Simulation
• Measurement
• Conclusion

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Cosine windows Phase characteristics
Phase determination ADC transfer
function Experimental results
Conclusion
In the range of ?L? (L is the window order) can
be written
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Phase determination of windowed DFT
Phase determination ADC transfer
function Experimental results
Conclusion
Windowed DFT of a pure cosine wave
is
For positive frequency spectrum X(k) and
a is integer, ??0.5
Maximum of the amplitude frequency spectrum is at
bin a
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Frequency estimation
Phase determination ADC transfer
function Experimental results
Conclusion

• Parametric frequency estimators (root-MUSIC,
  ESPRIT) are generally sensitive to distortion
  that is usual in practice
• Proposed FFT frequency estimator computes
  weighted average of amplitude frequency spectrum
  M(k) squared within the main lobe of the
  fundamental harmonic component

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Dual tone signal
Phase determination ADC transfer
function Experimental results
Conclusion

• ADC test signal consists of sum of two sine wave
  signals
• Highly linear sum circuit (power combiner) has to
  be used
• Intermodulation distortion (IMD) depends only on
  ADC nonlinearity

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Dual tone ADC transfer curve estimation (1)
Phase determination ADC transfer
function Experimental results
Conclusion
ADC signal
with ??1tf1 and ??2tf2
Power decomposition
Intermodulation components
r,s?N
The terms containing intermodulation
coefficients Cr,s for the power of d
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Dual tone ADC transfer curve estimation (2)
Phase determination ADC transfer
function Experimental results
Conclusion
The coefficient Dd,i of the term cos(i?) coming
from the decomposition of the function
cosd(?) can be derived Using the formula
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Dual tone ADC transfer curve estimation (3)
Phase determination ADC transfer
function Experimental results
Conclusion
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Dual tone ADC transfer curve estimation (4)
Phase determination ADC transfer
function Experimental results
Conclusion

• Absolute values of coefficients are estimated
  from the amplitudes of intermodulation components
• ?
• Independency of test signal distortion
• Coefficient signs have to be reconstructed using
  the phases of intermodulation components
• Computation of phases of intermodulation
  components from the phase frequency spectrum
• Computation of phases of intermodulation
  components using phases of fundamental components
• Comparison of both results and changing
  coefficient sign in case of opposite phases

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Simulation results
Phase determination ADC transfer
function Experimental results
Conclusion

• ADC transfer function nonlinearity
• ADC Gaussian white noise 200 LSB
• ADC bits 24, record length 64 kSa

Values are in a good agreement
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Experimental results
Phase determination ADC transfer
function Experimental results
Conclusion

• 24-bit S-? ADC AD7793, record length 64 kSa
• Sampling frequency of 500 Hz
• SR DS360 signal generators, passive linear power
  combiner

Experimental result of single and dual tone tests
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Conclusion
Phase determination ADC transfer
function Experimental results
Conclusion

• Phase analysis of incoherently sampled signals
  performed
• Symbolical formula of intermodulation components
  analyzed
• ADC transfer function independent of test signal
  harmonic distortion derived
• Proposed methodology verified by simulations as
  well as experimental measurement