Here you go:
Fs = 1000; % Sampling Frequency (Hz)
Fn = Fs/2; % Nyquist Frequency (Hz)
Wp1 = [4.0 8.0]/Fn; % Passband Frequencies (Normalized)
Ws1 = [3.9 8.1]/Fn; % Stopband Frequencies (Normalized)
Rp = 10; % Passband Ripple (dB)
Rs = 50; % Stopband Ripple (dB)
[n,Ws] = cheb2ord(Wp1,Ws1,Rp,Rs); % Filter Order
[z,p,k] = cheby2(n,Rs,Ws); % Filter Design
[sosbp1,gbp1] = zp2sos(z,p,k); % Convert To Second-Order-Section For Stability
figure(1)
freqz(sosbp1, 2^17, Fs)
set(subplot(2,1,1), 'XLim',[0 15]) % ‘Zoom’ X-Axis To See Passband
set(subplot(2,1,2), 'XLim',[0 15]) % ‘Zoom’ X-Axis To See Passband
theta_eeg = filtfilt(sosbp1, gpb1, original_eeg); % Filter EEG Signal
