In Reply to: Circuit with variable inductor posted by Didier Van Reeth on 10/15/01 at 4:51 PM:
If your 6 ÁH rises or falls by 0.1 ÁH, the change in the resonant frequency of a tank circuit using that coil is only going to be sqr(6.1/6) which is sqr(1.016) which comes to approximately 0.83%. A frequency change that small will probably require a frequency counter to detect. Of course, capacitor stability will be of paramount importance.
If the Colpitts oscillator you tried was unsatisfactory, you might look into other oscillator topologies. I'm thinking in particular of the Clapp oscillator, a circuit that was originated as an improvement in frequency stability / drift properties over the Colpitts versus parametric shifts in the oscillator's gain device, a vacuum tube in those days.
In the Clapp circuit, the single inductance L is replaced by a series LC combination and the Colpitts' split capacitors are made much larger in value, thus shunting the somewhat variable capacitances of the gain device. When the split capacitors were chosen as top notch NPO devices, the gain device's capacitance shifts with temperature and age were far less significant to the frequency of oscillation.
Other oscillator configuations may help too. I recall the name "Vackar" in connection with one, but I don't recall the principles of the Vackar oscillator circuit at all, only that it was highly praised by the author of an article on that circuit which was published in QST magazine sometime back in the 1950's or 1960's.
John Dunn - President