 It is difficult to build multistage amplifiers, filters, and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable oscillators.Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same fixed frequency, the IF.Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers.