In satellite communication 6/4 GHz band used in many
countries. Commercial communication satellites use a frequency band of 500 MHz
bandwidth near 6 GHz for uplink transmission and another
500 MHz bandwidth near near 4 GHz for down link transmission. While an uplink
of 5.725 to 7.075 GHz and a downlink of 3.4 to 4.8 GHz is used. For both
uplink and downlink band a single dual bandpass filter can be used instead of
two band-pass filters.

To design microwave filter well known classical
methods like Butterworth, Chebyshev are used but these methods are
applicable on single band filters. To design the multipassband filter some
limited analytical techniques have been proposed. These all
techniques are based on optimization method. Apart from these techniques
another technique to design of microwave filter with arbitrary frequency
response also proposed which is based on iterative quasi-Newton algorithm. The methodology proposed in this paper
follow this technique and rests on the translation of analog
specification of both bands of 6/4 GHz band into digital specification since in
digital domain the design is little bit easy. A designer can also take
advantage of these sophisticated and continuously developing digital techniques
in digital domain.

Following program can be used to design a dualpassband filter for satellite communication:

%% clear the command window & workspace

clear all

clc

%% define the besic perameter related to filter

% order of numerator % denominator

n=10;

d=10;

% frequency range of passbands & stopbands

f1=0:1/1000:.9406;

f2=0.9406:.0001:.9578;

f3=0.9579:.0001:.9645;

f4=.9646:.0001:.9714;

f5=[.9715:1/1000:1 1];

f=[f1 f2 f3 f4 f5];

edges=[0 f(1278:1280)];

% magnitiude to define passbands & stopbands

m1=[repmat(-40,1,900) -40:1:0];

m2=repmat(0,1,173);

m3=[0:-1:-32 -33:1:0];

m4=repmat(0,1,69);

m5=[0:-1:-27 -28.5 -30];

m=[m1 m2 m3 m4 m5];

%% desigining of filter

% conversion of magnitude from dB to normal value

a=db2mag(m);

% define the weight of error

w=repmat(50,1,1280);

% numerator & denominator of digital filter

[num,den]=iirlpnorm(n,d,f,edges,a,w);

% frequency responce of digital filter

figure(1),freqz(num,den)

% transfer function of digital filter

s=tf(num,den,10^(-9));

% pole zero plot of designed digital filter

figure(2),pzplot(s)

% conversion of digital filter from digital to analog

S=d2c(s,'tustin');

% bode plot of analog filter

figure(3),bode(S)

% numerator & denominator of analog filter

[NUM,DEN]=tfdata(S,'v');

% conversion of negative coefficent of numerator & denominator to make

% filter stable

if NUM<0

NUM=-1*NUM;

end

if DEN<0

DEN=-1*NUM;

end

% transfer function of analog filter with new coefficents

S1=tf(NUM,DEN);

% bode plot of analog filter with new coefficents

figure(4),bode(S1)

%% end of designing program

The frequency response for designed filter will be: