Composite filter circuit

A composite filter circuit comprising:

• a first resistor to one end of which an input signal is applied;
• a first series resonant circuit comprising a first coil connected between the other end of said first resistor and a first capacitor, said first resonant circuit resonating with a first frequency, and exhibiting a low-pass filtering characteristic;
• a second capacitor connected in parallel with said first coil and constituting in conjunction with said first coil a parallel resonant circuit resonating with a second frequency;
• a low-pass filter circuit comprising a second resistor connected between said first series resonant circuit and a third capacitor, said low-pass filter circuit having a cut-off frequency equal to a third frequency;
• a second series resonant circuit comprising a fourth capacitor connected to the other end of said first resistor and a second coil, said second resonant circuit resonating with the second frequency, and exhibiting a high-pass or band-pass filtering characteristic;
• and
• a high-pass or band-pass filter circuit comprising a fifth capactior connected to said second series resonant circuit and a third resistor, said high-pass or band-pass filter having a cut-off frequency equal to the first frequency,
• said first series resonant circuit, parallel resonant circuit, and low-pass filter circuit constituting a low-pass filter circuit section for filtering signals of a first specified frequency band,
• said second series resonant circuit and high-pass or band-pass filter circuit constituting a high-pass or band-pass filter circuit section for filtering signals of a second specified frequency band,
• said low-pass filter circuit section exhibiting a low-pass filtering characteristic which is generally flat from zero frequency to said first frequency, decreases steeply at frequencies above said first frequency, and has a great dip at said second frequency, said low-pass filtering characteristic resulting from combined effects of the low-pass filter characteristic having a peak at the first frequency of said first series resonant circuit, a dip characteristic at said second frequency of said parallel resonant circuit, a decreasing characteristic at said second frequency due to the existence of said second series resonant circuit, and a low-pass filtering characteristic wherein the cut-off frequency of said low-pass filter circuit is the third frequency,
• said high-pass filter circuit section exhibiting a high-pass or band-pass filtering characteristic having a dip at said first frequency and a peak at said second frequency resulting from a total of the high-pass filter characteristic having a peak at the second frequency of said second series resonant circuit, the high-pass or band-pass filtering characteristic wherein the cut-off frequency of said high-pass or band-pass filter circuit is the first frequency, and the decreasing characteristic at the first frequency due to the existence of said first series resonant circuit.