pyrpl.hardware_modules.iir.iir.
IIR
(rp, name)[source]¶Bases: pyrpl.hardware_modules.filter.FilterModule
Setup Attributes:
bypass
¶IIR is bypassed
coefficients
¶complex_poles
¶complex_zeros
¶data_curve
¶NA curve id to use as a basis for the graphical filter design
end
= 'zeros'¶gain
¶iirfilter
= None¶internal_overflow
¶returns True if the IIR filter has experienced an internal overflow (leading to saturation) since the last reset
loops
¶Decimation factor of IIR w.r.t. 125 MHz. Must be at least 5.
on
¶IIR is on
output_saturation
¶returns True if the output of the IIR filter has saturated since the last reset
overflow
¶a string indicating the overflow status of the iir module
overflow_bitfield
¶Bitmask for various overflow conditions
poles
¶real_poles
¶real_zeros
¶sampling_time
¶select_pole_or_zero
(value, logdist=True, search_in=['real_poles', 'real_zeros', 'complex_poles', 'complex_zeros'])[source]¶selects the pole or zero closest to value
logdist=True computes the distance in logarithmic units search_in may be used to restrict the search to certain sublists
setup
(**kwds)¶Setup an IIR filter
the transfer function of the filter will be (k ensures DCgain = g):
(s2*pi*z[0])*(s2*pi*z[1])...
 H(s) = k*——————
 (s2*pi*p[0])*(s2*pi*p[1])...
 coefficients data to be passed to iir.bodeplot to plot the
 realized transfer function
input
¶selects the input signal of the module
loops
Decimation factor of IIR w.r.t. 125 MHz. Must be at least 5.
zeros
¶poles
output_direct
¶selects to which analog output the module signal is sent directly
inputfilter
¶Input filter bandwidths [Hz]. 0 = off, positive bandwidth <=> lowpass, negative bandwidth <=> highpass.
gain
on
IIR is on
bypass
IIR is bypassed
data_curve
NA curve id to use as a basis for the graphical filter design
start
= 'complex'¶transfer_function
(frequencies, extradelay=0, kind='final')[source]¶Returns a complex np.array containing the transfer function of the current IIR module setting for the given frequency array. The bestpossible estimation of delays is automatically performed for all kinds of transfer function. The setting of ‘bypass’ is ignored for this computation, i.e. the theoretical and measured transfer functions can only agree if bypass is False.
Parameters: 


Returns: 

zeros
pyrpl.hardware_modules.iir.iir.
IirComplexListProperty
(*args, **kwargs)[source]¶Bases: pyrpl.hardware_modules.iir.iir.IirFloatListProperty
, pyrpl.attributes.ComplexAttributeListProperty
slave property to store complex part of zeros and poles
pyrpl.hardware_modules.iir.iir.
IirFloatListProperty
(*args, **kwargs)[source]¶Bases: pyrpl.attributes.FloatAttributeListProperty
slave property to store real part of zeros and poles
list_changed
(module, operation, index, value=None)[source]¶make sure that an element from one of the four lists is selected at once
pyrpl.hardware_modules.iir.iir.
IirListProperty
(min=<MagicMock name='mock.inf.__neg__()' id='139827380281936'>, max=<MagicMock name='mock.inf' id='139827380218960'>, increment=0, log_increment=False, **kwargs)[source]¶Bases: pyrpl.attributes.ComplexProperty
master property to store zeros and poles
default
= []¶pyrpl.hardware_modules.iir.iir.
OverflowProperty
(default=None, doc='', ignore_errors=False, call_setup=False)[source]¶pyrpl.hardware_modules.iir.iir.
SignalLauncherIir
(module)[source]¶Bases: pyrpl.modules.SignalLauncher
update_plot
= <MagicMock name='mock.QtCore.Signal()' id='139827380610576'>¶pyrpl.hardware_modules.iir.iir_theory.
IirFilter
(zeros, poles, gain, loops=None, dt=8e09, minloops=4, maxloops=1023, iirstages=16, totalbits=32, shiftbits=29, tol=0.001, frequencies=None, inputfilter=0, moduledelay=5)[source]¶Bases: object
Computes coefficients and predicts transfer functions of an IIR filter.
Parameters: 


Returns: 

coefficients
¶Computes and returns the coefficients of the IIR filter set by IirFilter.sys
.
Parameters: 


Returns: 

coefficients_rounded
¶designdata
¶frequencies
¶minimize_delay
(coefficients=None)[source]¶Minimizes the delay of coefficients by rearranging the biquads in an optimal way (highest frequency poles get minimum delay.
Parameters:  coefficients – 

Returns:  
Return type:  new coefficients 
prewarp
(z, p, dt=8e09)[source]¶prewarps frequencies in order to correct warping effect in discrete time conversion
proper_sys
¶Makes sure that a system is strictly proper and that all complex poles/zeros have conjugate parters.
Parameters: 


Returns: 

rescaled_sys
¶rescales poles and zeros with 2pi and returns the prefactor corresponding to dcgain gain
rp2coefficients
(r, p, c, tol=0)[source]¶Pairs residues and corresponding poles into second order sections.
Parameters: 


Returns:  coefficients 
Return type:  array((N, 6), dtype=np.float64) where N is number of biquads 
sampling_rate
¶sys
¶tf_coefficients
(frequencies=None, coefficients=None, delay=False)[source]¶computes implemented transfer function  assuming no delay and infinite precision (actually floatingpoint precision) Returns the discrete transfer function realized by coefficients at frequencies.
Parameters: 


Returns:  
Return type:  np.array(.., dtype=np.complex) 
tf_continuous
(frequencies=None)[source]¶Returns the continuous transfer function of sys at frequencies.
Parameters: 


Returns:  
Return type:  np.array(.., dtype=np.complex) 
tf_discrete
(rp_discrete=None, frequencies=None)[source]¶Returns the discrete transfer function realized by coefficients at frequencies.
Parameters: 


Returns:  
Return type:  np.array(.., dtype=np.complex) 
tf_final
(frequencies=None)[source]¶Returns the discrete transfer function realized by coefficients at frequencies.
Parameters: 


Returns:  
Return type:  np.array(.., dtype=np.complex) 
tf_partialfraction
(frequencies=None)[source]¶Returns the transfer function just before the partial fraction expansion for frequencies.
Parameters:  

Returns:  
Return type:  np.array(.., dtype=np.complex) 
tf_rounded
(frequencies=None, delay=False)[source]¶Returns the discrete transfer function realized by coefficients at frequencies.
Parameters: 


Returns:  
Return type:  np.array(.., dtype=np.complex) 
pyrpl.hardware_modules.iir.iir_theory.
bodeplot
(data, xlog=False)[source]¶plots a bode plot of the data x, y
Parameters: 


pyrpl.hardware_modules.iir.iir_theory.
cont2discrete
(r, p, c, dt=8e09)[source]¶Transforms residue and pole from continuous to discrete time
Parameters:  

Returns:  
Return type:  (r, p) with the transformation applied 
pyrpl.hardware_modules.iir.iir_theory.
discrete2cont
(r, p, c, dt=8e09)[source]¶Transforms residues and poles from discrete time to continuous
Parameters:  

Returns:  
Return type:  r, p with the transformation applied 
pyrpl.hardware_modules.iir.iir_theory.
freqs
(sys, w)[source]¶This function computes the frequency response of a zpk system at an array of frequencies.
It loosely mimicks ‘scipy.signal.freqs’.
Parameters:  

Returns:  
Return type:  np.array(.., dtype=np.complex) with the response 
pyrpl.hardware_modules.iir.iir_theory.
freqs_rp
(r, p, c, w)[source]¶same as freqs, but takes a list of residues and poles
pyrpl.hardware_modules.iir.iir_theory.
freqz_
(sys, w, dt=8e09)[source]¶This function computes the frequency response of a zpk system at an array of frequencies.
It loosely mimicks ‘scipy.signal.frequresp’.
Parameters:  

Returns:  
Return type:  np.array(.., dtype=np.complex) with the response 
pyrpl.hardware_modules.iir.iir_theory.
residues
(z, p, k)[source]¶this function uses the residue method (Heaviside Coverup method) to perform the partial fraction expansion of a rational function defined by zeros, poles and a prefactor k. No intermediate conversion into a polynome is performed, which makes this function less prone to finite precision issues. In the current version, no pole value may occur twice and the number of poles must be strictly greater than the number of zeros.
Returns: 


pyrpl.hardware_modules.iir.iir_theory.
sos2zpk
(sos)[source]¶Return zeros, poles, and gain of a series of secondorder sections
Parameters:  sos (array_like) – Array of secondorder filter coefficients, must have shape
(n_sections, 6) . See sosfilt for the SOS filter format
specification. 

Returns: 

Notes
New in version 0.16.0.
Sometimes it is interesting to realize even more complicated filters. This is the case, for example, when a piezo resonance limits the maximum gain of a feedback loop. For these situations, the IIR module can implement filters with ‘Infinite Impulse Response’ (https://en.wikipedia.org/wiki/Infinite_impulse_response). It is the your task to choose the filter to be implemented by specifying the complex values of the poles and zeros of the filter. In the current version of pyrpl, the IIR module can implement IIR filters with the following properties:
rac{f_ m{nyquist}}{1000}` (but you can optimize the nyquist frequency of your filter by tuning the ‘loops’ parameter)  the DCgain of the filter must be 1.0. Despite the FPGA implemention being more flexible, we found this constraint rather practical. If you need different behavior, pass the IIR signal through a PID module and use its input filter and proportional gain. If you still need different behaviour, the file iir.py is a good starting point.  total filter order <= 16 (realizable with 8 parallel biquads)  a remaining bug limits the dynamic range to about 30 dB before internal saturation interferes with filter performance
Filters whose poles have a positive real part are unstable by design. Zeros with positive real part lead to nonminimum phase lag. Nevertheless, the IIR module will let you implement these filters.
In general the IIR module is still fragile in the sense that you should verify the correct implementation of each filter you design. Usually you can trust the simulated transfer function. It is nevertheless a good idea to use the internal network analyzer module to actually measure the IIR transfer function with an amplitude comparable to the signal you expect to go through the filter, as to verify that no saturation of internal filter signals limits its performance.
#reload to make sure settings are default ones
from pyrpl import Pyrpl
p = Pyrpl(hostname="192.168.1.100")
#shortcut
iir = p.rp.iir
#print docstring of the setup function
print(iir.setup.__doc__)
#prepare plot parameters
%matplotlib inline
import matplotlib
matplotlib.rcParams['figure.figsize'] = (10, 6)
#setup a complicated transfer function
zeros = [ 4e4j+300, +2e5j+1000, +2e6j+3000]
poles = [ 1e6, +5e4j+300, 1e5j+3000, 1e6j+30000]
iir.setup(zeros=zeros, poles=poles, loops=None, plot=True)
print("Filter sampling frequency: ", 125./iir.loops,"MHz")
If you try changing a few coefficients, you will see that your design filter is not always properly realized. The bottleneck here is the conversion from the analytical expression (poles and zeros) to the filter coefficients, not the FPGA performance. This conversion is (among other things) limited by floating point precision. We hope to provide a more robust algorithm in future versions. If you can obtain filter coefficients by another, preferrably analytical method, this might lead to better results than our generic algorithm.
Let’s check if the filter is really working as it is supposed:
# first thing to check if the filter is not ok
print("IIR overflows before:", bool(iir.overflow))
na = p.networkanalyzer
# measure tf of iir filter
iir.input = na.iq
na.setup(iq_name='iq1', start=1e4, stop=3e6, points = 301, rbw=100, avg=1,
amplitude=0.1, input='iir', output_direct='off', logscale=True)
tf = na.curve()
# first thing to check if the filter is not ok
print("IIR overflows after:", bool(iir.overflow))
# retrieve designed transfer function
designdata = iir.transfer_function(na.frequencies)
#plot with design data
%matplotlib inline
import matplotlib
matplotlib.rcParams['figure.figsize'] = (10, 6)
from pyrpl.hardware_modules.iir.iir_theory import bodeplot
bodeplot([(na.frequencies, designdata, "designed system"),
(na.frequencies, tf, "measured system")], xlog=True)
As you can see, the filter has trouble to realize large dynamic ranges. With the current standard design software, it takes some ‘practice’ to design transfer functions which are properly implemented by the code. While most zeros are properly realized by the filter, you see that the first two poles suffer from some kind of saturation. We are working on an automatic rescaling of the coefficients to allow for optimum dynamic range. From the overflow register printed above the plot, you can also see that the network analyzer scan caused an internal overflow in the filter. All these are signs that different parameters should be tried.
A straightforward way to impove filter performance is to adjust the DCgain and compensate it later with the gain of a subsequent PID module. See for yourself what the parameter g=0.1 (instead of the default value g=1.0) does here:
#rescale the filter by 20 fold reduction of DC gain
iir.setup(zeros=zeros, poles=poles, g=0.1, loops=None, plot=False)
# first thing to check if the filter is not ok
print("IIR overflows before:", bool(iir.overflow))
# measure tf of iir filter
iir.input = na.iq
tf = na.curve()
# first thing to check if the filter is not ok
print("IIR overflows after:", bool(iir.overflow))
# retrieve designed transfer function
designdata = iir.transfer_function(na.frequencies)
#plot with design data
%matplotlib inline
import matplotlib
matplotlib.rcParams['figure.figsize'] = (10, 6)
from pyrpl.hardware_modules.iir.iir_theory import bodeplot
bodeplot([(na.frequencies, designdata, "designed system"),
(na.frequencies, tf, "measured system")], xlog=True)
You see that we have improved the second peak (and avoided internal overflows) at the cost of increased noise in other regions. Of course this noise can be reduced by increasing the NA averaging time. But maybe it will be detrimental to your application? After all, IIR filter design is far from trivial, but this tutorial should have given you enough information to get started and maybe to improve the way we have implemented the filter in pyrpl (e.g. by implementing automated filter coefficient scaling).
If you plan to play more with the filter, these are the remaining internal iir registers:
iir = p.rp.iir
# useful diagnostic functions
print("IIR on:", iir.on)
print("IIR bypassed:", iir.shortcut)
print("IIR copydata:", iir.copydata)
print("IIR loops:", iir.loops)
print("IIR overflows:", bin(iir.overflow))
print("Coefficients (6 per biquad):")
print(iir.coefficients)
# set the unity transfer function to the filter
iir._setup_unity()