Question

Consider an amateur radio operator transmitting at frequencies in the 20 meters (14-14.3 MHz), 15 meter (21.0-21.45 MHz) or 10 meter (28.0-29.7 MHz) amateur bands using a lowpass filter with 1 dB attenuation at 30 MHz. What is the minimum filter required to attain 30 dB harmonic suppression in any of the low VHF TV channels (54-88 MHz)? (Note, there are amateur bands in the lower HF region, but it is typically the upper HF bands that cause issues)

Answer 1

Ans : let us consider filter with passband F_p=14MHz,

stopband F_S=21MHz

and sampling frequency F_s=28MHz

after translating the specifications into digital frequency domain is given as below:

Passband   

14

stopband

21 3π ws = 2π * 28 2

2 2

So minimize filter can be calculated by Kaiser window method

here given that A =30dB

30 612 2.285 * 0.5 * τ

But here armature band at VHF

so

=2π * 54 = 1427

42 * π 54 21 54

14 * π 54 7π 0.26π 27

30-8 = 11.78 2.285 0,26 T