Figure 1’s circuit uses the LT1720 dual comparator in a 50% duty cycle crystal oscillator. Output frequencies of up to 10 MHz are practical.
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| Figure 1. | Crystal oscillator has complementary outputs and 50% duty cycle. A1’s feedback maintains output duty cycle despite supply variations. |
The circuit of Figure 1 creates a pair of complementary outputs with a forced 50% duty cycle. Crystals are narrowband elements, so the feedback to the noninverting input is a filtered analog version of the square wave output. Changing the noninverting reference level can therefore vary the duty cycle. The 2k–620Ω resistor pair sets a bias point at the comparator’s noninverting input. The 2k–1.8k–0.1µF path sets the inverting input at the node at an appropriate DC-average level based on the output. The crystal’s path provides resonant positive feedback, and stable oscillation occurs. The DC bias voltages at the inputs are set near the center of the LT1720’s common mode range and the 220 Ω resistor attenuates the feedback to the noninverting input. C2 creates a complementary output by comparing the same two nodes with the opposite input polarity. A1 compares band-limited versions of the outputs and biases C1’s negative input. C1’s only degree of freedom to respond is variation of pulse width; hence, the outputs are forced to 50% duty cycle.
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| Figure 2. | Output skew varies only 800 ps over a 2.7 V–6 V supply excursion. |
The circuit operates from 2.7 V to 6 V and the skew between the edges of the two outputs is as shown in Figure 2. There is a slight duty-cycle dependence on comparator loading, so equal capacitive and resistive loading should be used in critical applications. This circuit works well because of the two matched delays and rail-to-rail–style outputs of the LT1720.
