OCXO Basics: Types and Crystal Cut Comparisons

This article explains the basics of Oven Controlled Crystal Oscillators (OCXOs), their different types, and compares AT cut, IT cut, and SC cut crystals.

Definition: OCXO stands for Oven Controlled Crystal Oscillator.

As the name suggests, an OCXO is a crystal oscillator that maintains a stable frequency output regardless of variations in the surrounding ambient temperature. While Temperature Compensated Crystal Oscillators (TCXOs) also provide temperature-compensated frequency output, they offer lower frequency stability compared to OCXOs. This limitation makes OCXOs, with their superior frequency stability, ideal for applications demanding high stability.

OCXOs find widespread use in applications requiring very high frequency stability, such as oscillators, synthesizers, transceivers, signal generators, and signal analyzers.

OCXO Working Operation

Figure 1 illustrates the internal structure of a typical OCXO. It consists of an oven-controlled circuit, a temperature sensor, and the crystal oscillator itself. The improved stability stems from the OCXO’s ability to maintain a constant crystal temperature, irrespective of ambient temperature changes. This is achieved through an oven-based heater control circuit.

Based on the temperature sensed by a thermistor or other temperature sensor, the heating element adjusts the crystal’s temperature. The OCXO maintains a constant temperature through a closed-loop feedback network. By keeping the temperature constant, the oscillator’s frequency stability is significantly improved.

As shown in Figure 2, SC cut OCXOs exhibit superior frequency stability over a wider temperature range compared to IT cut and AT cut types.

OCXO Types: AT cut, IT cut, SC cut

The following are common types of OCXO crystal oscillators:

• AT cut
• IT cut
• SC cut

Figure 2 (above) depicts the frequency response of these different OCXO types.

OCXO AT cut

The AT cut crystal type possesses the following characteristics:

• The AT cut is made from a Y bar at a 35-degree theta rotation.
• It represents an entry-level OCXO type.
• It offers good stable performance across a wide temperature range, from -55°C to 125°C.
• It provides frequency output ranging from 0.5 to 300 MHz.

OCXO IT cut

The IT cut crystal type features:

• Similar to the SC cut, it is a double-rotated crystal type, making it suitable for high-stability applications.
• It offers frequency output from 0.5 to 200 MHz.
• It has an upper turning point between 85°C and 105°C.

OCXO SC cut

The SC cut crystal type is characterized by:

• It offers double rotation, defined by a theta of 34.11 degrees and a phi of 21.93 degrees.
• It provides frequency output from 0.5 to 200 MHz.
• The cutting method is complex, leading to higher labor costs.
• Stresses observed in the AT cut type are reduced or eliminated using a second rotation in the φ direction.

Refer to articles discussing AT cut versus SC cut crystal types and the advantages and disadvantages of SC cut crystals over AT cut crystals for more in-depth information.