Divided into the following 4 types according to major categories

1. Simple crystal oscillator (XO)

This is the most basic type, and its stability is completely determined by the inherent characteristics of the crystal resonator itself. The higher frequency crystals in the MHz range are made of quartz rods, which are manufactured in such a way that they provide relative stability even if the ambient temperature varies from -55°C to +125°C (-67°F to +257°F) Frequency of. Even in such a wide temperature range, properly cut quartz crystals can achieve a stability of ±25ppm. Compared with other passive resonators, such as LC oscillator circuits that change with temperature up to 1% (10,000ppm) or higher, the performance of crystal oscillators has been greatly improved. But for some applications, even 25ppm is not good enough, so additional measures must be taken.

2. Temperature compensated crystal oscillator (TCXO)

If the natural frequency and the temperature stability of the quartz crystal cannot meet the application requirements, a temperature compensation unit can be used. TCXO uses temperature sensing elements and a circuit that generates a voltage curve. In the entire temperature range, the voltage curve is completely opposite to the frequency change trend of the crystal, so it can ideally offset the drift of the crystal. According to the type and temperature range of TCXO, the typical stability specification range of TCXO is less than ±0.5ppm to ±5ppm.

3. Oven controlled crystal oscillator (OCXO)

For some applications, the frequency-temperature stability index of TCXO still cannot meet the requirements. In these cases, OCXO may be required. As the name suggests, an oscillator with a baking cavity heats the crystal to a higher temperature, but it is still controlled so that the temperature of the crystal remains stable even if the ambient temperature changes greatly. Since the crystal temperature and the sensitive part of the oscillator change very little, the frequency-ambient temperature stability is significantly improved. In the ambient temperature range, the stability of OCXO can reach 0.001ppm. However, this increase in stability comes at the expense of increased power consumption. Of course, energy is required to provide heat to the oven. A typical OCXO may require 1 to 5W of power to maintain internal temperature. After starting up, you still need to wait for the warm-up time when the temperature and frequency are stable. Depending on the type of crystal oscillator, the warm-up time usually ranges from 1 minute to more than 10 minutes.

4. Voltage Controlled Crystal Oscillator (VCXO)

In some applications, it is desirable to be able to tune or adjust the frequency of the oscillator in order to phase-lock it to a reference in a phase-locked loop (PLL), or possibly to adjust the waveform. VCXO provides this function through electronic frequency control (EFC) voltage input. For some special components, the tuning range specification of VCXO may be ±10ppm to ±100ppm (or even higher).