Fundamental-frequency-crystal-oscillator

Description: For frequencies below 20 MHz, a fundamental-frequency crystal can be utilized, eliminating the need for a resonant tank. At these lower frequencies, the typical MECL 10,000 propagation delay of 2 ns becomes negligible compared to the oscillation period, necessitating the use of a non-inverting output. Consequently, the MC10116 oscillator section operates primarily as an amplifier. A 1.0 kΩ resistor biases the line receiver near "Vbb," while a 0.1 µF capacitor serves as a filter capacitor for the "Vbb" supply. Additionally, the capacitor in series with the crystal allows for minor frequency adjustments. The second section of the MC10116 is configured as a Schmitt-trigger circuit, ensuring clean MECL edges from a relatively slow input signal below 20 MHz. The third stage of the MC10116 functions as a buffer and provides complementary outputs from the crystal oscillator circuit. The circuit supports a maximum operating frequency of approximately 20 MHz and a minimum of around 1 MHz, designed for use with a crystal that operates in the fundamental mode of oscillation.

The circuit described operates effectively within a frequency range that capitalizes on the properties of fundamental-frequency crystals. The elimination of the resonant tank simplifies the design, allowing for a compact and efficient oscillator circuit. The MC10116's role as an amplifier in this configuration enhances the signal integrity, particularly in environments where propagation delays may influence performance. The biasing resistor (1.0 kΩ) is strategically placed to stabilize the line receiver at a suitable voltage level, ensuring that the output remains within the operational limits of the MECL logic family.

The inclusion of a 0.1 µF capacitor as a filter for the "Vbb" supply plays a critical role in maintaining the stability of the power supply, thereby reducing noise that could adversely affect the oscillator's performance. The series capacitor with the crystal allows for fine-tuning of the oscillation frequency, which is essential for applications requiring precise frequency control.

The Schmitt-trigger configuration in the second section of the MC10116 provides a robust solution for signal conditioning, improving the rise and fall times of the output signal. This is particularly beneficial when dealing with slower input signals, as it ensures that the output maintains the sharp transitions characteristic of MECL logic levels.

The buffer stage in the third section is essential for isolating the oscillator circuit from the load, ensuring that the crystal oscillator can operate without being adversely affected by the characteristics of the subsequent circuitry. The complementary outputs facilitate interfacing with other digital logic components, enhancing the flexibility of the design.

Overall, this oscillator circuit is well-suited for applications requiring stable and reliable frequency generation in the sub-20 MHz range, leveraging the advantages of fundamental-mode crystal oscillators while maintaining compatibility with MECL standards.For frequencies below 20 MHz, a fundamental-frequency crystal can be used and the resonant tank is no longer required. Also, at this lower frequency range the typical MECL 10,000 propagation delay of 2 ns becomes small compared to the period of oscillation, and it becomes necessary to use a noninverting output.

Thus, the MC10116 oscillator section functions simply as an amplifier. The 1.0 Kohm resistor biases the line receiver near "BB and the 0.1-uF capacitor is a filter capacitor for the "Vbb supply. The capacitor, in series with the crystal, provides for minor frequency adjustments. The second section of the MC10116 is connected as a Schmitt-trigger circuit; this ensures good MECL edges from a rather slow, less than 20MHz input signal. The third stage of the MC10116 is used as a buffer and to give complementary outputs from the crystal oscillator circuit.

The circuit has a maximum operating frequency of approximately 20 MHz and a minimum of approximately 1 MHz; it is intended for use with a crystal which operates in the fundamental mode of oscillation.