Bandpass filters are based on cavity with a high Q-factor. Filtration principle consists in the cavity ability to select only frequencies, which are equal to natural resonance frequency of quarter-wave vibrator(1), enclosed in container. Rotating micrometer screw (2), which control length of quarter-wave cavity, fundamental frequency tuning is made. Rotating turntables (7), where couling loops are situated, insertion loss is adjusted. Mechanical and temperature stability is provided by INVAR material with low linear expansion factor, from which adjusting micrometer screw is made.
High quality of electric contact between moving and fixed part of quarter-wave cavity is ensured by group of flexible lamelas (3), made of beryllium bronze. Special heat treatment provides reliability of electric contact and smooth running during tuning. Inner traveling coupling rod has solid silver coating.
The case of the cavity (4) is produced from the cold-rolled duralium metall. High quality of rolling reduces roughness in RF current transverse flow to minimum.
Top cover (5) is argon-arc welded to the case, which is also result in minimal attenuation in the area of maximum RF current flow.
Cavity's anodized inner surface ensures high characteristics' stability. Case is protected by durable duralium coating from outside.
Coupling loops (6) are installed on turntables (7) for filter characteristics flexibility. Loops itself are coated with silver.
The difference between bandrejection filters and bandpass filters is that frequency, on which cavity is tuned, is cut out. Bandrejection filters physically have only one coupling loop and T-join