Components/IP/SD

Sigma-Delta modulator

This IP can be found on the EDA Repository: svn: https://repos.hevs.ch/svn/eda/

The modulator architecture is of Cascaded Integrators with Distributed Feedback (CIDF).

Design

The coefficients can be chosen for the Signal Transfer Function (STF) to match any desired all-pole transfer function. This is done by writing the algebraic form of the modulator's STF and by comparing the denominator coefficients with the one of the desired transfer function. A Maxima script delivers the required coefficients.

An additional parameter, shiftBitNb is used to ensure stability. Adding 1 to shiftBitNb reduces the STF by one octave (a factor of 2). This value has to be adjusted within a simulation loop.

VHDL Entity

The VHDL entity of the modulator shows the generics, the inputs and the outputs:

ENTITY sigmaDeltaModulator IS
   GENERIC( 
      signalBitNb : positive := 16;
      shiftBitNb  : natural  := 1
   );
   PORT( 
      reset      : IN     std_ulogic;
      clock      : IN     std_ulogic;
      en         : IN     std_ulogic;
      parallelIn : IN     signed (signalBitNb-1 DOWNTO 0);
      serialOut  : OUT    std_ulogic
   );

The en would allow to work at a smaller frequency than the clock's. As Sigma-Delta modulators should work at high rates, en is usually set to 1.

Testbench

The testbench generates a sinewave input to the modulator. Clock and signal frequencies are defined as constants in the top-level architecture and passed to the tester via generics.

The modulator output is fed to a lowpass filter which reconstructs the original signal. The lowpass we use also has a parameter shiftBitNb which can be used to shift the cutoff frequency octave by octave. As a matter of fact, the testbench filter has the same structure as the sigma-delta modulator. The only difference is that it hasn't a comparator at the output which reduces it to a single bit.

Lowpass filter setup

The lowpass filter VHDL code allows to select filter order (presently Bessel 2, 3 or 16) by commenting/uncommenting constants at the top of the architecture. For the choice of the filter order, one has to consider the following points:

• Choosing a higher order provides a better Signal-to_Noise Ratio (SNR), but a larger delay
• Selecting a lower order shows ripples in the reconstructed signal, but augmenting filterShiftBitNb reduces these ripples. On the other hand, choosing filterShiftBitNb too large also leads to a longer delay.