In timpul legaturilor telegrafice existe situatii cand doua statii sunt atat de apropiate incat nici cu un filtru de banda foarte ingusta nu pot fi selectate. Si cum, de obicei, tocmai statia mai slaba reprezinta un DX mai interesant cu atat dezamagirea operatorului va fi mai mare.
Solutia prezentata in continuare poate constitui salvarea intr-o situatie ca cea descrisa mai sus. Astfel in lantul de audiofrecventa se intercaleaza un circuit opreste banda a carui schema se prezinta in fig. 1. Caracteristica de frecventa se prezinta in fig. 2. Dupa cum se observa atenuarea unei frecvente nedorite poate atinge valori de peste 60dB. Circuitul prezinta un factor de calitate greu de obsinut cu un alt tip de filtru.
Potentiometrul P permite obtinerea unei atenuari maxime a semnalului. Pentru modificarea frecventei de rezonanta se modifica R5-R6 avand grija ca valorile finale ale acestora sa ramana identice.
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