Patent ID: 6944423

Claim:
A structure for preventing intermodulation interference in satellite transmission, comprising: a receiving antenna for receiving vertical and horizontal polarized signal sent from a satellite; an RF amplifying system for down-converting and amplifying the received vertical and horizontal signals; a first and a second band-pass filter for filtering signals output from said RF amplifying system; a first mixer for receiving a signal output from said first band-pass filter and executing a frequency subtraction operation; a local oscillator, from an oscillating frequency of which a frequency of said signal output from said first band-pass filter is subtracted in the frequency subtraction operation at said first mixer to generate a first mid-band; a high-pass filter for filtering and isolating a signal within said first mid-band and output from said first mixer, and outputting the filtered and isolated signal; a second mixer for receiving the signal filtered at and output from said second band-pass filter and executing a frequency subtraction operation; a first-stage local oscillator, from an oscillating frequency of which a frequency of said signal output from said second band-pass filter is subtracted in the frequency subtraction operation executed at said second mixer to obtain a reverse-phase signal of said signal output from said second band-pass filter; a third band-pass filter for filtering and isolating said reverse-phase signal output from said second mixer; a third mixer for receiving the signal output from said third band-pass filter and executing a frequency addition operation; a second-stage local oscillator, to an oscillating frequency of which a frequency of said signal output from said third band-pass filter is added in the frequency addition operation executed at said third mixer to obtain a second mid-band; a low-pass filter for filtering and isolating a signal within said second mid-band and output from said third mixer, and outputting the filtered and isolated signal; and a multiplex adder for receiving and combining said signals separately output from said high-pass filter and said low-pass filter; whereby second and third harmonic differences between a frequency obtained by subtracting the oscillating frequency of said second-stage local oscillator from the oscillating frequency of said first-stage local oscillator and the oscillating frequency of said local oscillators do not fall in said first and said second mid-bands, respectively, and an intermodulation interference can therefore be prevented.