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Hartley Oscillator Calculator

Enter the two inductor values and the capacitor to find the oscillation frequency.

How it is calculated

The tool adds the two coil sections and applies the tank-circuit formula: f = 1 / (2π × √(L_total × C)) with L_total = L1 + L2. Enter L1 and L2 in microhenries (µH) and C in nanofarads (nF); they are converted to henries and farads internally and the frequency is shown in kHz. Mutual inductance between the two coils is ignored, so a real tapped coil with coupling will oscillate somewhat away from this figure. The tool also shows the total inductance in µH and the ratio L2 / L1 as a feedback figure. All three inputs must be greater than zero, otherwise there is no result.

Worked example

Take L1 = 10 µH, L2 = 40 µH and C = 100 nF. Total inductance is 10 + 40 = 50 µH, which is 50 × 10−6 H. The capacitor is 100 × 10−9 F. The product is 5 × 10−12, whose square root is about 2.236 × 10−6. Multiplying by 2π gives about 1.405 × 10−5, and the reciprocal is roughly 71 177 Hz. The tool shows 71.18 kHz, a total inductance of 50.00 µH and an L2 / L1 ratio of 4.00.

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Common mistakes

  • Entering the capacitor in microfarads or picofarads. The field expects nanofarads, so 0.1 µF must be typed as 100 and 470 pF as 0.47.
  • Entering inductances in millihenries or henries. The fields are in µH, so 1 mH must be typed as 1000.
  • Typing only one coil section or the whole coil value into both fields. The frequency depends on the sum L1 + L2, not on either one alone.
  • Expecting swapping L1 and L2 to change the frequency. It does not, because only the sum is used; it only changes the L2 / L1 ratio (4.00 becomes 0.25 above).
  • Treating the result as the exact bench frequency. Mutual coupling between the sections, stray capacitance and transistor loading are not modelled, so use it as a starting value and trim the circuit while measuring.

About this calculator

A Hartley oscillator generates a steady sine wave by feeding part of an LC tank circuit’s output back into an amplifier, using a tapped inductor (split into two coils, L1 and L2) instead of the tapped capacitor found in its cousin, the Colpitts oscillator. Ignoring mutual inductance between the coils, the oscillation frequency follows the standard tank-circuit formula f = 1 / (2π√(L_total × C)), where L_total = L1 + L2.

Invented by Ralph Hartley in 1915, the circuit’s simple, robust design and wide tuning range made it a workhorse in early radio and continue to earn it a place in RF engineering today. Radio transmitter and receiver designers, ham-radio builders, and test-equipment makers use it to generate a stable carrier or local-oscillator signal from just two coils, a capacitor, and an active device.

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