pydsm.NTFdesign.delsig.ntf_schreier¶
- pydsm.NTFdesign.delsig.ntf_schreier(order=3, osr=64, opt=0, H_inf=1.5, f0=0.0, **options)¶
Synthesizes an NTF for a DS modulator by Schreier’s approach.
Parameters: order : int, optional
the order of the modulator, defaults to 3
osr : float, optional
the oversamping ratio (based on the actual signal bandwidth)
opt : int or list of floats, optional
flag for optimized zeros, defaults to 0
- 0 -> not optimized,
- 1 -> optimized,
- 2 -> optimized with at least one zero at band-center,
- 3 -> optimized zeros (with optimizer)
- 4 -> same as 3, but with at least one zero at band-center
- [z] -> zero locations in complex form
H_inf : real, optional
max allowed peak value of the NTF. Defaults to 1.5
f0 : real, optional
center frequency for BP modulators, or 0 for LP modulators. Defaults to 0. 1 corresponds to the sampling frequency, so that 0.5 is the maximum value. Value 0 specifies an LP modulator.
Returns: ntf : tuple
noise transfer function in zpk form.
Other Parameters: use_optimizer : bool
Use: True for for optimizing the zeros with a fast optimization code, False otherwise. Defaults can be set by changing the function default_options attribute.
Raises: ValueError
‘Frequency f0 must be less than 0.5’ if f0 is out of range
‘Order must be even for a bandpass modulator’ if the order is incompatible with the modulator type.
‘The opt vector must be of length xxx’ if opt is used to explicitly pass the NTF zeros and these are in the wrong number.
RuntimeError
‘Cannot synthesize NTF zeros’ if the synthesis fails for some unspecified reason.
Warns: PyDsmApproximationWarning
‘Creating a lowpass ntf.’ if the center frequency is different from zero, but so low that a low pass modulator must be designed.
‘Unable to achieve specified H_inf ...’ if the desired H_inf cannot be achieved.
‘Iteration limit exceeded’ if the routine converges too slowly.
Notes
This is actually a wrapper function which calls the appropriate version of synthesizeNTF, based on the control flag optimize_NTF which determines whether to use optimization tools.
Parameter H_inf is used to enforce the Lee stability criterion.
If osr or H_inf are low, then the NTF is non optimal. Use synthesizeChebyshevNTF instead.