In a series-fed Hartley oscillator, what happens if the frequency-determining capacitance is increased?

In a series-fed Hartley oscillator, what happens if the frequency-determining capacitance is increased?

• A. The oscillator frequency will decrease.
• B. The oscillator frequency will remain the same.
• C. The oscillator frequency will increase.
• D. The oscillator will stop oscillating.

Answer: (A)

In an LC tank, the oscillation frequency is set by f ≈ 1/(2π√(Leq C)). In a Hartley oscillator, the tank uses the two inductors in series (the tapped coil) together with the frequency-determining capacitor, so the effective inductance is the total L of the coil combination. If you increase the capacitance, the product Leq × C grows, making the square root larger and the frequency smaller. So the oscillator runs at a lower frequency when the capacitance increases.

In an LC tank, the oscillation frequency is set by f ≈ 1/(2π√(Leq C)). In a Hartley oscillator, the tank uses the two inductors in series (the tapped coil) together with the frequency-determining capacitor, so the effective inductance is the total L of the coil combination. If you increase the capacitance, the product Leq × C grows, making the square root larger and the frequency smaller. So the oscillator runs at a lower frequency when the capacitance increases.