Hartley RF oscillator

 

The Hartley RF oscillator, illustrated in Figure , is centered around the commonly available 2N4416A FET transistor. This general purpose VFO oscillator operates around 5100 kHz. The frequency determining components are L1, C1, a 10 pF trimmer and capacitors

C2, C3, C4, C5 and C6. Note capacitor C6 is a 10-100 pF variable trimmer type. Capacitor C7 is to reduce the loading on the tuned circuit components. Its value can be small but be able to provide sufficient drive to the succeeding buffer amplifier stage. You can experiment using a small viable capacitor trimmer, such as a 5-25 pF.

The other components, such as the two resistors, silicon diode at D1 are standard types, nothing particularly special. The Zener diode at D2 is a 6.2 volt type. Capacitor C8 can be selected to give higher/lower output to the buffer amplifier. Smaller C6 values give lower

output and conversely higher values give larger output. In order to get the circuit to work properly, you need to have an inductive reactance for L1 of around about 180 ohms. At 5 MHz this works out at about 5.7 uH. The important consideration, is that the feedback point

from the source of the JFET connects to about 25% of the windings of L1 from the ground end. An air cored inductor is shown in the diagram. It could be, for example, 18-19 turns of #20 gauge wire on a25.4 mm (1′′) diameter form spread evenly over a length of about 25.4 mm (1′′). The tap would be at about 41⁄2 turns.