For me, the VFO is the
most important circuit in a transceiver.
Many circuits have been tested, but the above one has given me the best results, mainly a very good stability.
A piece of PVC tube, the one used in electrical installations, has been used as the support of coil L1.
Coil L2 is an RF choke with about 100 uH or more, it is frequency dependent. Best results obtained with the value shown in the components description shown above.
Q1 is MPF102 but others can be used.
Capacitors C3, C5, C6 e C8 are polystyrene. It is a good practice to use two or more capacitors in place of C6 to increase thermal stability.
C7 must also be a series /parallel arrangement, it depends on its value, the working frequency and the wanted ratio.
D1 is used to limit positive oscillations.
A power supply with any value between 5 and 9 volts and a high degree of regulation, in order to avoid slipping, can be used.
A VARICAP circuit is being used in place of C7. No changes have been noted in stability, mainly after the initial ten minutes.
A buffer amplifier must be used at the output of the VFO in order to avoid that it is overloaded. The circuit below can be used.