Total posts : 45366
Carl is correct. The adding of a temperature compensating element near the tuning coil, or physically attached to the shield casing of the tuning coil can vastly improve the stability. I did just that with my FM 10 unit using a temperature compensator case that fit snug over a HC-6 crystal
from a very old and scrapped RCA FM tube exciter. All it needed was some voltage and a way to sense the temperature to control the voltage feeding the heating element.
Shaped and fitted the temperature “oven” right over the shield case of the tuning coil itself and wala.
The sensor was made with a simple diode, op amp and a differential amp. Kept the FM 10’s frequency quite stable.