For example, the signal at frequency $F_{C,2}$ is selected through tuning the bandpass filter to frequency $F_{C,2}$ that removes all other spectral contents, as illustrated in the figure below. If an adjacent channel were to be chosen, the tuning control mechanism consisting of circuit elements like variable capacitors and inductors just altered the center frequency of this filter. The major problem with a TRF is designing tunable bandpass filter with constant bandwidth and sufficient frequency selectivity over the entire tunable range.