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fdesign.differentiator

Differentiator filter specification object

Syntax

D = fdesign.differentiator
D = fdesign.differentiator(SPEC)
D = fdesign.differentiator(SPEC,specvalue1,specvalue2, ...)
D = fdesign.differentiator(specvalue1)
D = fdesign.differentiator(...,Fs)
D = fdesign.differentiator(...,MAGUNITS)

Description

D = fdesign.differentiator constructs a default differentiator filter designer D with the filter order set to 31.

D = fdesign.differentiator(SPEC) initializes the filter designer Specification property to SPEC. You provide one of the following strings as input to replace SPEC. The string you provide is not case sensitive.

    Note:   Specifications strings marked with an asterisk require the DSP System Toolbox™ software.

  • 'N' — Full band differentiator (default)

  • 'N,Fp,Fst' — Partial band differentiator

  • 'N,Fp,Fst,Ap' — Partial band differentiator *

  • 'N,Fp,Fst,Ast' — Partial band differentiator *

  • 'Ap' — Minimum order full band differentiator *

  • 'Fp,Fst,Ap,Ast' — Minimum order partial band differentiator *

The string entries are defined as follows:

  • Ap — amount of ripple allowed in the pass band in decibels (the default units). Also called Apass.

  • Ast — attenuation in the stop band in decibels (the default units). Also called Astop.

  • Fp — frequency at the start of the pass band. Specified in normalized frequency units. Also called Fpass.

  • Fst — frequency at the end of the stop band. Specified in normalized frequency units. Also called Fstop.

  • N — filter order.

By default, fdesign.differentiator assumes that all frequency specifications are provided in normalized frequency units. Also, decibels is the default for all magnitude specifications.

Use designopts to determine the design options for a given design method. Enter help(D,METHOD) at the MATLAB® command line to obtain detailed help on the design options for a given design method, METHOD.

D = fdesign.differentiator(SPEC,specvalue1,specvalue2, ...) initializes the filter designer specifications in SPEC with specvalue1, specvalue2, and so on. To get a description of the specifications specvalue1, specvalue2, and more, enter

get(d,'description')

at the Command prompt.

D = fdesign.differentiator(specvalue1) assumes the default specification string N, setting the filter order to the value you provide.

D = fdesign.differentiator(...,Fs) adds the argument Fs, specified in Hz to define the sampling frequency to use. In this case, all frequencies in the specifications are in Hz as well.

D = fdesign.differentiator(...,MAGUNITS) specifies the units for any magnitude specification you provide in the input arguments. MAGUNITS can be one of

  • 'linear' — specify the magnitude in linear units

  • 'dB' — specify the magnitude in dB (decibels)

  • 'squared' — specify the magnitude in power units

When you omit the MAGUNITS argument, fdesign assumes that all magnitudes are in decibels. Note that fdesign stores all magnitude specifications in decibels (converting to decibels when necessary) regardless of how you specify the magnitudes.

Examples

Use an FIR equiripple differentiator to transform frequency modulation into amplitude modulation, which can be detected using an envelope detector.

Modulate a message signal consisting of a 20-Hz sine wave with a 1 kHz carrier frequency. The sampling frequency is 10 kHz .

t = linspace(0,1,1e4);
x = cos(2*pi*20*t);
Fc = 1e3;
Fs = 1e4;
y = modulate(x,Fc,Fs,'fm');

Design the equiripple FIR differentiator of order 31.

d = fdesign.differentiator(31,1e4);
Hd = design(d,'equiripple');

Filter the modulated signal and take the Hilbert transform to obtain the envelope.

y1 = filter(Hd,y);
y1 = hilbert(y1);
% Plot the envelope
plot(t.*1000,abs(y1));
xlabel('Milliseconds'); ylabel('Magnitude');
grid on;
title('Envelope of the Demodulated Signal');

From the preceding figure, you see that the envelope completes two cycles every 100 milliseconds. The envelope is oscillating at 20 Hz, which corresponds to the frequency of the message signal.

 

Design an FIR differentiator using least squares and plot the zero phase response.

d = fdesign.differentiator(33); % Filter order is 33.
hd = design(d,'firls');
fvtool(hd,'magnitudedisplay','zero-phase',...
'frequencyrange','[-pi, pi)')

Design a narrow band differentiator. Differentiate the first 25 percent of the frequencies in the Nyquist range and filter the higher frequencies.

Fs=20000; %sampling frequency
d = fdesign.differentiator('N,Fp,Fst',54,2500,3000,Fs);
Hd= design(d,'equiripple');
% Weight the stopband to increase attenuation
Hd1 = design(d,'equiripple','Wstop',4);
hfvt = fvtool(Hd,Hd1,'magnitudedisplay','zero-phase',...
'frequencyrange','[0, Fs/2)');
legend(hfvt,'Without stopband weighting',...
'With stopband weighting');

See Also

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