Hartley vs Colpitts for FM carrier

Choice: Colpitts oscillator is preferred for generating the carrier in an FM transmitter (together with a crystal or a variable-capacitance diode for modulation).

Reasons

1. Better frequency stability. The Colpitts frequency depends on a fixed inductor and capacitors, and is less affected by stray effects. The Hartley needs a tapped coil, and the mutual inductance M changes with temperature and construction, causing drift.
2. Purer sine wave. The capacitor divider shunts the harmonics to ground, so the waveform has lower harmonic distortion than Hartley.
3. Works well at high frequency (up to hundreds of MHz). The FM band (88 to 108 MHz) is high. A single coil without a tap is easy to make at VHF, while a tapped coil is difficult.
4. Easy tuning and FM modulation. A varactor diode can replace or sit across C1 or C2. The audio voltage changes its capacitance, and so changes the carrier frequency (frequency modulation). The feedback ratio C1/C2 remains almost constant.
5. Stable amplitude with feedback ratio set by capacitor ratio.

Circuit diagram (Colpitts)

 Collector o----+------+
                |      |
               [C1]    |
                |     [L]
 Emitter/GND o--+      |
                |      |
               [C2]    |
                |      |
 Base        o--+------+

f = 1/(2π √(L C1 C2/(C1 + C2))), with a CE transistor stage with bias network and an RF choke in the collector. In an FM transmitter a varactor diode is added across C2 or in parallel with the tank.