RF oscillator
At the UHF range of frequencies, feedback in the oscillator is obtained by mutual inductance, employing lecher lines to form a Hartley or a Colpitts oscillator. Use may also be made of the inter-electrode capacitors of the transistor. The circuit in Figure 13.5 shows a common-base Colpitts oscil- lator in which inter-electrode capacitors Cce (between collector and emit- ter) and Cbe (between emitter and base) provide the necessary feedback for sustained oscillation. Cce in series with Cbe effectively fall across the output developed between the collector and base. Part of this output, that part across Cbe, is fed back into the input between the emitter and base. R1 is the emitter resistor, R2/R3 is the base bias chain with C2 as the base or bias decoupling capacitor and TL1 is the output lecher line resonating with varicap diode C3. A varicap diode is a reversed-biased diode which looks like a capacitance as far as the circuit is concerned. The value of the capacitance it represents is determined by the reverse-bias voltage.
Mixer-oscillator
The purpose of the mixer-oscillator stage is to change the incoming UHF carrier frequency to a common IF, a technique known as super-heterodyne (superhet for short). Frequency changing may be achieved either through multiplication or addition. Addition is the preferred method for TV tuners. A single transistor is made to oscillate at frequency fo, which is 39.5 MHz above the selected channel frequency fc. The non-linear part of the transis- tor characteristic is then used to produce the sum ( fo + fc) and the difference ( fo – fc) of the two frequencies together with the two original frequencies, fo and fc. A tuned circuit at the output is then made to select the frequency dif- ference, fo – fc = 39.5 MHz.