Greetings everyone, I have a small question regarding filters. I work as a technician in Ericsson (have worked for 6 months or so, have studied electronics for 2 years, working and schooling myself at the same time), and one of our main products is a bandpass filter. My job there is to maintain the testing stations, to make sure that they work properlly. I am full aware of the hardware side of things, which is all that is required. And a fair bit of the software side to make sure thigns run smoothly. But my question is about the filters. I never really thought about them before, but I got sort of interested in them after seeing the insides. Ive never really asked any of the working engineers about them, as we speak different languages and they have their own things to do. This filter, except for the small PCB inside (for troubleshooting and testing) has no electrical components that make it work as a classic passive or active bandpass filter (electronic components), instead it is a hollow metal slab with mushroom shaped resonators inside, that you have to tune it to specs with the screw on the "roof" of the chassis. The Rx/Tx inputs/outputs are essentially connected to a network analyzer, through a system of switching relays. How exactly is this filter called ? I would like to learn a fair bit more about it. Either by searching for the name or perhaps recieveing a small explanation here ? Another question I have is regarding a certain test, called IM-test, I assume its impedance test ? Workers are equipped with hammers, if needed they have to hammer the sides of the tuned filter to reach the required measurement values. What exactly does hammering do to the filter ? Thank you for taking your time reading this and perhaps answering.