Comparator Based Crystal Oscillator

Description: A simple crystal oscillator can be constructed using a comparator from the LT1720/LT1721 series; however, this design may encounter several inherent limitations and issues. While the LT1720/LT1721 provides the correct logic output when one input falls outside the common mode range, additional delays can occur under such conditions, leading to the possibility of spurious operating modes. To mitigate this, the DC bias voltages at the inputs must be adjusted to be near the center of the common mode range of the LT1720/LT1721, and a resistor is necessary to attenuate the feedback to the non-inverting input. Although the output duty cycle of this circuit is approximately 50%, it is influenced by resistor tolerances and, to a lesser degree, by comparator offsets and timing variations. For applications requiring a precise 50% duty cycle, the presented circuit generates complementary outputs that are forced to maintain this duty cycle. Crystals, being narrow-band elements, provide resonant positive feedback through the feedback loop connected to the non-inverting input, enabling stable oscillation. The duty cycle can be adjusted by varying the non-inverting reference level. The resistor pair of 2k-680 sets a bias point at the inputs of Comparator IC1a and Comparator IC1b. The complementary input of each comparator is configured with a 2k-1.8k-0.1 µF path to establish an appropriate DC average level based on the output. IC1b generates 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 the negative input of IC1a. The only variable for IC1a is the pulse width, ensuring that the outputs maintain a 50% duty cycle. This circuit operates within a voltage range of 2.7V to 6V. When observing the oscillator output signal, a slight dependence on comparator loading may be noted, necessitating equal and resistive loading in critical applications. The effectiveness of the circuit is attributed to the matched delays and rail-to-rail outputs of the LT1720.

The described crystal oscillator circuit utilizes the LT1720/LT1721 comparator ICs to achieve stable oscillation characteristics. The design leverages the inherent properties of the crystal as a frequency-selective element, which is critical for generating a precise frequency output. The feedback loop is configured such that the output square wave is fed back to the non-inverting input of the comparator, allowing the circuit to maintain oscillation through positive feedback.

The choice of resistors in the biasing network (2k-680) is essential for setting the correct operating point within the common mode range of the comparators. This configuration minimizes the risk of operating in undesirable modes that could arise from improper biasing. The capacitive element (0.1 µF) in conjunction with the resistors creates a low-pass filter effect, smoothing out any rapid fluctuations in the signal and contributing to the stability of the output.

The complementary outputs generated by the two comparators (IC1a and IC1b) ensure that the duty cycle remains at 50%, which is particularly important for applications requiring precise timing and synchronization. The feedback mechanism allows for adjustments to the duty cycle by varying the reference level at the non-inverting input, providing flexibility in the oscillator's performance.

In terms of power supply requirements, the circuit is designed to function effectively within a range of 2.7V to 6V, making it suitable for various low-voltage applications. The rail-to-rail output capability of the LT1720/LT1721 ensures that the output levels are maximized, which is beneficial for driving subsequent stages in digital circuits.

Overall, the described crystal oscillator circuit presents a robust solution for generating stable frequency signals while addressing potential design challenges associated with comparator-based oscillators. The careful consideration of component values and configurations contributes to the reliability and performance of the oscillator in practical applications.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 possib

ility 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ltered 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-680 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. 1 µF 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.