VFO with variable inductor

56 VFO with variable inductor An LC oscillator as shown in figure 1 is not exactly something new: transistor T1 is connected in a common base configuration and its emitter functions as virtual earth. Transistor T3 is a voltage follower buffer; its emitter voltage, in conjunction with the impedance of the series LC circuit, determines the collector current of Tl. If therefore the resistive losses at resonance of the LC circuit are smaller than the collector resistor R4, the LC circuit wilt oscillate. The level of the oscillator output voltage across R9 is determined by the value of R9, the collector current of T1 and the current through R8. An LC oscillator with variable capacitor tuning has been around for some time as well: change C (C3) in figure 1 to a variable type and you"II be able to adjust the oscillator frequency over a certain range. An LC oscillator with variable inductor tuning as shown in figure 2 is not so usual. Two coils, L1 and L2, are mutually coupled (coupling f...