English: Typical dial layout of a tuned radio frequency (TRF) receiver from the 1920s. A TRF receiver had several tuned radio frequency amplifier stages before the detector, each consisting of a vacuum tube amplifier and a tuned circuit with a variable capacitor. The multiple tuned circuits each had to be tuned to the frequency of the desired station before the radio would work. Later TRF receivers had the capacitors mounted on a common shaft so they could be tuned together with a single knob, but early receivers like this one had separate tuning knobs for each capacitor.