a tuner uses a bank of tracking filters to preselect a channel to be received . each tracking filter covers a range of frequencies . the tracking filters are tunable in frequency using switched capacitors and are tunable in gain by using switched resistors . the switched resistors can be controlled by an automatic gain control circuit that monitors the selected signal and adjust the tracking filter gain to achieve a desired signal level . a switch directs the received signal or signal from a test tone generator into the tracking filters . the test tone , generated by a frequency agile circuit , can be used to calibrate the filters , both in frequency and gain .