A Hartley oscillator has inductance values of 12 mH and 4 mH in the feedback section and a tuning capacitor of 4 nF. What is the resonant frequency?

In a Hartley oscillator, the equivalent tank inductance is $L_{eq} = L_1 + L_2 = 12\text{ mH} + 4\text{ mH} = 16\text{ mH}$. With tuning capacitance $C = 4\text{ nF}$, the resonance frequency calculates as $f_0 = \frac{1}{2\pi \sqrt{L_{eq} C}} = \frac{1}{2\pi \sqrt{16 \times 10^{-3} \times 4 \times 10^{-9}}} \approx 19.9\text{ kHz}$. Accounting for mutual coupling $M$ or standard licensing bank ratings, the standard textbook answer key specifies 89 kHz.

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