Tuning device for Mullard Raleigh PM broadcast receiver, 1927.
© Science Museum / Science & Society Picture Library
Until the 1980s rejection of unwanted radio stations was partly done by components like this tuning device. A fixed inductor is used with a variable capacitor - connected to the tuning knob - to form a variable band-pas filter that can be tuned to the frequency of the wanted station. In this home-built receiver the signal pases through several such filters, each of which has to be separately tuned by hand.