Magazine

form gain.

Temperature variations which result in frequency drift may be largely neutralized by proper biasing techniques or by using temperature sensitive capacitors in the tank circuit. Although the design of bias networks is beyond scope ot this article, (here have been several excellent articles and books written on the subject.1,2 Essentially, the bias resistors must be chosen so that the operating point remains relatively fixed with changes in the outside environment. This may often be done economically by using temperature-sensitive resistors; the temperature dependence necessary to stabilize the frequency may be determined quite easily.

A variable resistance is simply inserted into the circuit in place of the temperature-sensitive element, i hen the circuit is exposed to the projected temperature range and this resistance is varied to keep the frequency constant. The temperature dependence of the temperature-sensitive resistor is then selected to match the measured temperature curve, Phis resistance may not necessarily Keep the bias point constant, but it will change in such a way that it maintains a constant frequency of oscillation, compensating for more than one fluctuation in the circuit as a function of temperature.

A temperature sensitive capacitor in the tank circuit may be selected by the same technique—a variable capacitor is placed across the tank and adjusted for constant frequency output at the temperature extremes. The compensating capacitor should be chosen to follow the same curve.

Although temperature considerations and circuit loading are both very important to frequency stability, low drift is primarily dependent upon the Q of the tank circuit. All other things being equal, the higher Q circuit always results in lower drift When the effects of temperature ancl circuit loading are neglected, the percent of drift is a direct function of Q as shown in Fig, 7, With proper temperature compensation and very light loading, the frequency stability obtained in a practical circuit will very closely approach this curve.

In addition, the tank L/C ratio should be low; this results in a larger value of capacitance in the tank circuit to filter out harmonics which tend toward frequency instability. Also, the self-resonant frequency of ilie inductors and capacitors in the tank circuit should be at least ten times the operab-

Fig. 7. The affect of tuned-circuit Q on frequency drift in an oscillator. For maximum frequency stability !n a practical circuit the Q should be as high as possible, ing frequency of the oscillator, and where possible, even larger. Otherwise, the internal parasitic parameters of these tank components will seriously degrade oscillator performance to the point that stability will be unsatisfactory.

Colpitts oscillator design

Since frequency stability is usually the first consideration, circuit Q is a good place to start the design of a transistor oscillator. From the graph of Fig. 7, you can choose a value of Q that is compatible with practical components and will provide the frequency stability required. With this value of Q in mind, the frequency of operation and the desired impedance of the tuned circuit at resonance, the correct value of tank capacitance may be found from c- Q

" 27rfZ

Again, the math in this formula, although not completely formidable, is inconvenient, so the nomograph of Fig. 8 was prepared to give you an almost instant answer; the nomograph lias the added advantage that you can quickly check die effect of various values

0 0

Post a comment