Aceste tipuri de filtre au in componenta lor bobine (L), condensatoare (C) si cuarturi (Q). Ele sunt filtre de IF, de banda ingusta, foarte folosite de catre radioamatori in etajele de frecventa intermediara din receptoare, pentru suprimarea unei benzi laterale in emitatoarele BLU (SSB) si, in general, acolo unde este nevoie de o selectivitate mai ridicata.
Cele mai raspandite sunt filtrele in sfert de punte (cu un singur cuart) sau in semipunte (cu doua cuarturi). Filtrele cu cuart in punte pot fi realizale mai greu in conditii amatoresti.
Schema prezentata utilizeaza doua cristale de cuart care trebuie sa aiba frecventele de rezonante serie diferite intre ele, iar diferenta sa fie egala chiar cu banda de trecere dorita. Ll si L2 contin cate l0 spire bobinate pe un tor de ferita. L2 are priza la 1/2 din numarul dc spire.
L3 rezoneaza impreuna cu capacitatile de 330pF in serie cu 1nF in banda de trecere a filtrului. Numarul de spire depinde de valoarea frecventei cuarturilor.
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