Tuning systems for radio and television receivers typically include a tunable RF stage for selecting the RF signal corresponding to a desired station or channel from a plurality of received RF signals, a tunable local oscillator for generating a local oscillator signal having a frequency corresponding to the desired station or channel and a mixer for heterodyning the selected RF signal with the local oscillator signal to produce an IF signal corresponding to the RF signal. Typically, both the RF stage and local oscillator include respective varactor diodes which are responsive to a tuning voltage.
In many applications, the RF stage and local oscillator should be capable of being tuned over a relatively wide tuning range in substantial tracking relationship to ensure mixing of the appropriate RF signal and local oscillator signal for each channel with each other throughout the tuning range.