Design Ideas:October 27, 1994
The circuit in Fig 1 delivers a pure, buffered, and level-controlled sine wave. With no tuning requirements, the circuit works with all crystals up to about 30 MHz.
The typical crystal oscillator is an amplifier with a gain that is sufficiently higher than the loss of the crystal and feedback network and guarantees sustained oscillation. Because the amplifier's gain is greater than the network's loss, oscillation amplitude grows until the amplifier's output clips. The crystal filters the large harmonic content of the output and returns a fairly pure feedback sine wave to the amplifier. This circuit's main failings are its uncontrolled and unbuffered sinusoidal amplitude and its very high distortion levels--even when filtered by the crystal. Furthermore, the overload-recovery delays of the amplifier can warp the oscillation frequency away from the crystal's real resonant frequency, resulting in frequency error and drift.
Fig 1's circuit uses a variable-gain-control IC as the amplifier that continuously servos its gain to maintain a constant oscillation amplitude. The circuit connects the feedback terminal (FB) to the gain control's output. This connection establishes a gain range of 0 to 2 for a gain-control range of 0 to +2V. The gain-control inputs are differential and allow for flexible biasing and selection of phase sense. Dis a simple peak detector. If the output amplitude increases, the circuit applies an increasing, rectified, and filtered voltage to the negative-gain-control input, VGAIN , which reduces the amplifier's gain. When the oscillation amplitude drops, the Vinput voltage drops, and the amplifier's gain increases to just that required for a controlled oscillation amplitude. The bias network at the positive-gain-control input Vsets the output amplitude at 500 mV rms. Dcompensates for the offset and temperature drift of D1. Overtone-mode crystals require a series-tuned circuit to force oscillation to the right harmonic frequency.
With a 2-MHz crystal, output distortion is a low 0.22%. The distortion at the input of the amplifier, VIN+, is even lower at 0.03% because of the filtering by the crystal. You can use this amplifier input as an output, preferably after buffering it with a low-distortion amplifier. The narrowband spectral jitter is unmeasurable. The available spectrum analyzer has a 3-Hz filter; the spectral width of either output is only 14Hz wide and 90 dB down from the center frequency, corresponding to the filter's shape factor. (DI #1610)