At high frequencies (say above a megahertz) a favorite method of sine wave generation is to use a resonator of some sort to establish the frequency of oscillation. The resonator itself may be electrical (e.g., an LC circuit), or electromechanical (e.g., a piezoelectric quartz crystal), or even atomic or molecular (e.g., a hydrogen maser).
Some resonators are easily tuned (e.g., LC), whereas others are quite stably fixed (e.g., a quartz crystal). Resonator-based oscillators are fundamentally different from the preceding RC-based oscillators, because they exploit a system that has
an intrinsic resonant frequency (like a crystal resonator), compared with an RC circuit’s non-resonant time constant (or phase shift). Because these resonances can be both narrow in frequency and stable over time, they are well suited for conscription into the noble service of oscillation.
We begin with LC-controlled oscillators, which play an important role in communications, and in which a tuned LC circuit is connected in an amplifier-like circuit to provide gain at its resonant frequency. Overall positive feedback is
then used to cause a sustained oscillation to build up at the LC’s resonant frequency; such circuits are self-starting.
Figure 1 shows two popular configurations. The first circuit is the trusty Colpitts oscillator, a parallel tuned LC at the input, with positive feedback from the output (inverted in phase, because the JFET is inverting). For this circuit, operating at 20 MHz, the distortion is typically less than −60 dB.
The second circuit is a Hartley oscillator, built with an NPN transistor; the variable capacitor is for frequency adjustment. Because a common-base amplifier is non-inverting, the feedback signal is not inverted. Both circuits use output link coupling, just a few turns of wire acting as a step-down transformer.
A third oscillator configuration – the emitter-coupled oscillator (or Peltz oscillator) – is shown in Figure 2, which is used in the 1648 “voltage-controlled oscillator”
chip of the ECL III digital logic family. You can think of this as a feedback non-inverting differential amplifier, with a parallel LC to set the frequency of oscillation. The 1648 will operate to 200 MHz, with the operating frequency set, as usual, by the resonant frequency of the parallel LC: f0 =1/2π√LC.
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