Interference filter with high degree of selectivity for tvro receiver system

Claim 1

A TVRO receiver for receiving frequency modulated video signals, the receiver comprising: a tuner including a super heterodyne circuit having a voltage-controlled oscillator (VCO), means for supplying a controlling input voltage to said VCO, and a mixer for combining incoming 1st intermediate frequency (II) signals within a first predetermined IF frequency range with the output of said VCO to reduce the frequency of the 1st IF signals so as to generate 2nd IF signals within a second predetermined IF frequency range, and a filter for selectively blocking out interference signals, said filter comprising, first trap means for blocking out interference signals having a selected bottom trap frequency, second trap means for blocking out interference signals having a selected top trap frequency, each of said first and second trap means comprising a frequency resonant element, said first and second trap means respectively blocking out signals below said bottom trap frequency and above said top trap frequency with a predefined attenuation level or "trap depth", tuning means for adjusting said bottom and top frequencies of said first and second trap means, an active negative resistance network for offering a negative resistance to said resonant element in said trap means for varying the trap depth thereof, so as to increase the selectivity with which said interference signals are rejected, and compensating means for adjusting the negative resistance offered by said active network so as to controllably counteract any variation due to temperature of said negative resistance and, hence, said trap depth and selectivity of said first and second trap means.