The Hartley oscillator is the dual of the Colpitts: instead of a capacitive voltage divider providing feedback, it uses a tapped inductor (or two series inductors) with a single tuning capacitor across the whole coil. It was patented by Ralph Hartley at Bell Labs in 1915 for early radio work, and it's still found in older shortwave receivers, ham-radio VFOs, and RF signal generators where the coil is easy to wind but low-loss capacitors are expensive.

Topology: An amplifier (BJT, JFET, or MOSFET) drives the parallel LC tank. The tank consists of L1 and L2 in series with a tuning capacitor C across the total. The tap between L1 and L2 goes to ground (AC-wise), and the two ends of the coil connect to the transistor's output and input. This gives a 180° phase inversion through the tapped coil, which — combined with the 180° from a common-emitter or common-source amplifier — satisfies the Barkhausen criterion: total loop phase = 360°, loop gain ≥ 1.

Oscillation frequency:

• f = 1 / (2π√(L_total · C)), where L_total = L1 + L2 + 2M (M is mutual inductance if wound on the same core)
• Feedback ratio ≈ L1/L2 (the tap position sets loop gain)

Concrete example — 5 MHz HF VFO: Pick C = 100 pF (mica, for temperature stability). Then L_total = 1/((2π·5×10⁶)²·100×10⁻¹²) ≈ 10 µH. Wind it as, say, L1 = 2 µH (12 turns tapped) and L2 = 8 µH (remaining 24 turns) on a T50-6 powdered-iron toroid. The tap ratio gives about 25% feedback — comfortably above unity loop gain, but not so much that the transistor saturates and produces harmonic-rich output.

Rule of thumb: Target a feedback fraction (L1/L_total) of 20–30%. Too little and it won't start reliably across temperature; too much and you'll get clipping, harmonics, and frequency-pulling as the transistor's nonlinear Cbe modulates the tank.

Design gotchas:

• Coil Q dominates phase noise — wind on low-loss cores (powdered iron for HF, air-core for VHF), use silver-plated wire, and space-wind to reduce proximity losses.
• Frequency drift is the Hartley's Achilles' heel. Use an NP0/C0G tuning cap and a stable core material; polystyrene or silvered-mica caps in parallel help temperature-compensate.
• Buffer the output with an emitter/source follower so load changes don't pull frequency.
• Colpitts vs Hartley: Colpitts is generally preferred above ~30 MHz because parasitic lead inductance ruins Hartley's tap accuracy; Hartley wins when you need continuous wide-range tuning with a single variable cap.