Frequency Compensated Lc Networks for Oscillators with the Wide Tuning Range

Anyone who have designed a capacitively tuned VFO/VCO with the wide tuning range (>1.5-2x) has noticed that the oscillation threshold changes substantially with the tuning frequency. The usual solution to the problem is a combination of: 1. the AGC circuit, most frequently via the amplifier bias that changes the amplifier conduction angle; 2. shunting the LC circuit with a damping resistor or by a low impedance amplifier terminal; Both methods have a common drawback amplifier gain must be increased to maintain the oscillation condition margin on both sides of the tuning range and/or to compensate for the damping resistor losses. Increasing gain requires either more complex amplifier or tighter coupling of the resonant circuit to the amplifier. The later increases the influence of amplifier parasitic elements on the resonant circuit. The more complex amplifier can increase the phase jitter. Both solutions have a negative impact on the oscillator frequency stability as well as the phase noise.