Comparator Based Crystal Oscillator

Although a simple
crystal oscillator may be built from one comparator of an LT1720/LT1721,
this will suffer from a number of inherent shortcomings and design
problems. Although the LT1720/LT1721 will give the correct logic output
when one input is outside the common mode range, additional delays may
occur when it is so operated, opening the possibility of spurious
operating modes. Therefore, the DC bias voltages at the inputs have to
be set near the center of the LT1720/LT1721’s common mode range and a
resistor is required to attenuate the feedback to the non-inverting
input. Unfortunately, although the output duty cycle for this circuit is
roughly 50%, it is affected by resistor tolerances and, to a lesser
extent, by comparator offsets and timings.

If a 50% duty cycle is required, the circuit shown here creates a
pair of complementary outputs with a forced 50% duty cycle. Crystals are
narrow-band elements, so the feedback to the non-inverting input is a
filtered analogue version of the square-wave output. The crystal’s path
provides resonant positive feedback and stable oscillation occurs.
Changing the non-inverting reference level can vary the duty cycle. The
2k-680Ω resistor pair sets a bias point at the comparator + (Comparator
IC1a) and – (Comparator IC1b) input. At the complementary input of each
comparator, the 2k-1.8k-0.1µF path sets up an appropriate DC average
level based on the output.

IC1b creates a complementary output to IC1a by comparing the same
two nodes with the opposite input. IC2 compares band-limited versions of
the outputs and biases IC1a’s negative input. IC1a’s only degree of
freedom to respond is variation of pulse width; hence the outputs are
forced to 50% duty cycle. The circuit operates from 2.7V to 6V. When
‘scoping the oscillator output signal, a slight dependence on comparator
loading, will be noted, so equal and resistive loading should be used
in critical applications. The circuit works well because of the two
matched delays and rail-to-rail outputs of the LT1720.

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