Franklin Oscillator

Fig. 11 shows the Hartley circuit; its identifying characteristic is the tapped resonator which provides feedback by means of the cathode circuit. This circuit is widely

stability but covers a wide tuning range with very small changes of capacitance.

When a crystal is used as the resonator, the circuit at C results. It is known variously as the grid-plate circuit and as the crystal Colpitis circuit.

All three obtain their feedback from the voltage divider composed o\ capacitors CI and C2 in the grid-cathode circuit. Effectively the circuit is identical to the Hartley arrangement, but the feedback is easier to adjust since C2 can be a trimmer capacitor, adjusted for best operation in any given layout and conditions. This capacitance volt™ age divider is one of the identifying features of this group of circuits; the resonator differences are the other, which distinguishes which member of the group is being shown.

Fig. 12. Capacitance feedback circuit goes under various names, depending on the tuned-circuit arrangement. See Text.

Fig. 13. Typicaf circuit of electron-coupled oscillator. Screen grid serves as "plate" In crystal Colpitts circuit here, while output is taken from the actual plate. Values shown are suitabie for use from 7 through 9 MHz, for output irom 7 through 36 MHz.

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Fig. 13. Typicaf circuit of electron-coupled oscillator. Screen grid serves as "plate" In crystal Colpitts circuit here, while output is taken from the actual plate. Values shown are suitabie for use from 7 through 9 MHz, for output irom 7 through 36 MHz.

All of these circuits are illustrated with triode tubes. Any of them, however, can be made "electron-coupled" by treating the screen grid of the tetrode or a pentode as the triode plate shown in these illustrations. Output can then be taken from the actual plate, with little effect upon oscillator operation, Fig* 13 shows a crystal : olpitts oscillator connected in this manner. The tuned circuit in the plate is adjusted for output at the third harmonic of the crystal frequency, This circuit is ideal for getting 25MHz output from 8.3 MHz crystals, for 50-MHz transmitters.

¡ his brief listing doesn't by any means exhaust the list of possible oscillator circuits. Almost any means of getting f eedback around an amplifier can be, and lias been, used. One example is shown in Fig 14,

Fig, 14. Franklin two-tube oscillator circuit uses ex* tremely small coupling capacitors to eliminate frequency drifts. This circuit, if well made with solid construction, can outperform most crystal oscillators. Output, however, is exceptionally low.

This is known as the 4 Franklin" oscillator; it consists of not one but two stages of amplification, connected in a loop which provides virtually total feedback. In fact, if the resonator were not connected, this would act as a multivibrator rather than as an rf oscillator.

The two capacitors which couple to the resonator are very small. One pF is a typical value for each. The resonator effectively shorts out all energy except that at the frequency to which it is tuned, and the net result is a very low actual feedback fraction —just enough to permit oscillation.

Output is very low, several stages of builer amplification are necessary before the circuit's output can be used for any purpose.

The only advantage of this circuit is that it is a VFO which is more stable than most crystals. Drift is almost undetectable in a well-built Franklin oscillator. Much more circuitry is needed to do this, however, and so the circuit has not gained popularity. The circuits shown in Fig. 8 through 13 should be sufficient to permit perfect scores on this portion oi the license examinations.

Next Installment, This has been an over-length installment because of the material on oscillation and neutralization. Next time out we 11 attempt to even the scales by forgetting transmitter design for a while, and looking at the problems of antennas and transmission lines. Until then, good DX and happy studying.

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