Crystal Oscillators

FET Sine -Wave Crystal Oscillators

By FRANK H. TOOKER

Here are a few simple oscillator circuits which engineers can use to obtain a linear sine -wave output with low harmonics.

FNGINEERS who design crystal oscillators know one of in the schematics are practical minimum values and larger the factors contributing to frequency instability is har- values can often be used to advantage. In the circuit of Fig. 1, t monic generation in the oscillator itself. In the past, inductor Ll is preferably a high -quality, 3 -pie (an inductor designing sine -wave generators has not been easy. Vacuum - wound in three distinct layers or pies, side by side) corn - tube oscillators raised the signal level until it was limited -or ponent, wound on a powdered -iron core. clipped and elaborate circuits were needed to obtain linear With crystals of poor activity, the Pierce FET circuit may operation and sine -wave output. Even with junction bipolar not operate and, in some setups, parasitic oscillation can transistors the situation was the same, except clipping was occur at or near the self -resonant frequency of the drain in- sharper. ductor. The gate -to- ground capacitor, Cl, tends to inhibit Field -effect transistors ( FET's) promise to solve this prob- parasitic oscillation. lem. Their characteristics make them a "natural" as sine -wave When a crystal of good activity is in the circuit, drain generators, and with simple circuitry. This article describes current in the unloaded oscillator will be about 30 micro- 100 -kHz and 1000 -kHz sine -wave oscillators in three familiar amperes. Parasitic oscillations tend to produce a much higher configurations: (1) The Pierce circuit; (2) the tuned -drain, drain current, 500 µA or more. Thus, measuring the drain tuned -gate oscillator ( equivalent to the tuned- plate, tuned - current of the unloaded oscillator is generally used as a meth- grid vacuum -tube circuit) ; and (3) the grounded -drain cir- od of determining whether the circuit is operating properly. cuit ( equivalent to the grounded -plate) or Colpitts oscillator. While these circuits do not function perfectly -they do pull some slight gate current in order to establish self -regulating gate bias -they are nevertheless a significant step forward in view of their simplicity. OUTPUT As in its vacuum -tube counterpart, the 1000 -kHz Pierce oscillator shown in Fig. 1 needs a crystal of good activity 100-kHz Mi ( one which requires very little electrical energy to vibrate) . XTAL Clean sine -wave output is obtained when the gate resistor, Rl, has a value of 2.2 megohms and the unbypassed source resistor, R2, a value of 1000 ohms. If waveform distortion +9V occurs in this circuit or any of the other circuits in this article, it can be reduced by increasing the value of R2. Values of R2

Fig. 1. Distortion can be reduced in the Pierce oscillator MPF103 LI by increasing R2. Active crystal is needed for oscillation. D 50 -250pH

+9V 180pF OUTPUT

I 1000-kHz II LI ® R2 I1 XTAL m H 2.2k II 5.0 1000-kHz II

XTAL 1 +9V Ik -0 OUTPUT 0.IpF (B) Fig. 2. Tuned -drain, tuned -gate oscillator can have a split - drain output as in IA) or a link output as shown in M. MPF103 The Pierce FET circuit makes a satisfactory oscillator for some applications but, for the most part, the following circuits are to be preferred. cl RI R2 Tuned- Drain, Tuned -Gate Oscillator IO pF 2.2M Ik The tuned -drain, tuned -gate crystal oscillator circuits of Fig. 2 put out a clean sine wave when properly adjusted, i.e., when the inductor, L1, in the drain circuit is set for minimum drain current ( about 150 pA, unloaded) . This occurs when the drain tank is tuned just slightly higher than the resonant frequency of the crystal. (Continued on page 83) June, 1969 33