Signal Processing Toolbox

cheb2ord

Chebyshev Type II filter order

Syntax

• [n,Wn] = cheb2ord(Wp,Ws,Rp,Rs)
  [n,Wn] = cheb2ord(Wp,Ws,Rp,Rs,'s')
  

Description

cheb2ord calculates the minimum order of a digital or analog Chebyshev Type II filter required to meet a set of filter design specifications.

Digital Domain

[n,Wn] = cheb2ord(Wp,Ws,Rp,Rs) returns the lowest order n of the Chebyshev Type II filter that loses no more than Rp dB in the passband and has at least Rs dB of attenuation in the stopband. The scalar (or vector) of corresponding cutoff frequencies Wn, is also returned. Use the output arguments n and Wn in cheby2.

Choose the input arguments to specify the stopband and passband according to the following table.

Description of Stopband and Passband Filter Parameters

Parameter	Description
Wp	Passband corner frequency Wp, the cutoff frequency, is a scalar or a two-element vector with values between 0 and 1, with 1 corresponding to the normalized Nyquist frequency, radians per sample.
Ws	Stopband corner frequency Ws, is a scalar or a two-element vector with values between 0 and 1, with 1 corresponding to the normalized Nyquist frequency.
Rp	Passband ripple, in decibels. This value is the maximum permissible passband loss in decibels.
Rs	Stopband attenuation, in decibels. This value is the number of decibels the stopband is down from the passband.

Use the following guide to specify filters of different types.

Filter Type Stopband and Passband Specifications

Filter Type	Stopband and Passband Conditions	Stopband	Passband
Lowpass	Wp < Ws, both scalars	(Ws,1)	(0,Wp)
Highpass	Wp > Ws, both scalars	(0,Ws)	(Wp,1)
Bandpass	The interval specified by Ws contains the one specified by Wp (Ws(1) < Wp(1) < Wp(2) < Ws(2)).	(0,Ws(1)) and (Ws(2),1)	(Wp(1),Wp(2))
Bandstop	The interval specified by Wp contains the one specified by Ws (Wp(1) < Ws(1) < Ws(2) < Wp(2)).	(0,Wp(1)) and (Wp(2),1)	(Ws(1),Ws(2))

If your filter specifications call for a bandpass or bandstop filter with unequal ripple in each of the passbands or stopbands, design separate lowpass and highpass filters according to the specifications in this table, and cascade the two filters together.

Analog Domain

[n,Wn] = cheb2ord(Wp,Ws,Rp,Rs,'s') finds the minimum order n and cutoff frequencies Wn for an analog Chebyshev Type II filter. You specify the frequencies Wp and Ws similar to those described in the Table , Description of Stopband and Passband Filter Parameters table above, only in this case you specify the frequency in radians per second, and the passband or the stopband can be infinite.

Use cheb2ord for lowpass, highpass, bandpass, and bandstop filters as described in the Table , Filter Type Stopband and Passband Specifications table above.

Examples

Example 1

For data sampled at 1000 Hz, design a lowpass filter with less than 3 dB of ripple in the passband defined from 0 to 40 Hz, and at least 60 dB of attenuation in the stopband defined from 150 Hz to the Nyquist frequency (500 Hz):

• Wp = 40/500; Ws = 150/500;
  Rp = 3; Rs = 60;
  [n,Wn] = cheb2ord(Wp,Ws,Rp,Rs)
  n =
       4
  Wn =
      0.2597
  [b,a] = cheby2(n,Rs,Wn);
  freqz(b,a,512,1000); 
  title('n=4 Chebyshev Type II Lowpass Filter')
  
  
  

Example 2

Next design a bandpass filter with a passband of 60 Hz to 200 Hz, with less than 3 dB of ripple in the passband, and 40 dB attenuation in the stopbands that are 50 Hz wide on both sides of the passband:

• Wp = [60 200]/500; Ws = [50 250]/500;
  Rp = 3; Rs = 40;
  [n,Wn] = cheb2ord(Wp,Ws,Rp,Rs)
  n =
       7
  Wn =
      0.1019    0.4516
  [b,a] = cheby2(n,Rs,Wn);
  freqz(b,a,512,1000)
  title('n=7 Chebyshev Type II Bandpass Filter')
  
  
  

Algorithm

cheb2ord uses the Chebyshev lowpass filter order prediction formula described in [1]. The function performs its calculations in the analog domain for both analog and digital cases. For the digital case, it converts the frequency parameters to the s-domain before the order and natural frequency estimation process, and then converts them back to the z-domain.

cheb2ord initially develops a lowpass filter prototype by transforming the stopband frequencies of the desired filter to 1 rad/s (for low- and highpass filters) and to -1 and 1 rad/s (for bandpass and bandstop filters). It then computes the minimum order required for a lowpass filter to meet the passband specification.

See Also

buttord, cheb1ord, cheby2, ellipord, kaiserord

References

[1] Rabiner, L.R., and B. Gold. Theory and Application of Digital Signal Processing. Englewood Cliffs, NJ: Prentice-Hall, 1975. Pg. 241.

cheb2ap

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