Introduction
A quartz oscillator is a circuit for generating high-frequency alternating electrical voltages. As a frequency-determining component, the oscillator contains an oscillating crystal. Crystal oscillators convince with their frequency accuracy and frequency stability. In practice, the circuits are very often used as clock generators for radio devices, processors and microcontrollers. It is therefore not surprising that crystals and crystal oscillators are considered to be probably the most important components of frequency control in data transmission and telecommunications. Their main advantages include high resonant quality, a wide range of oscillators and high frequency stability.
Professional applications, such as measuring instruments, satellite navigation devices or telecommunications equipment, for example, have very high requirements for the oscillators installed, such as good frequency stability, low phase noise and a long service life. To achieve this, the quartz used must also have improved aging properties in order to be able to achieve the corresponding overall performance. Crystal oscillators can generally be divided into the following groups: Fixed Frequency Oscillators (XO) Voltage Controlled Oscillators (VCXO), Temperature Compensated Oscillators (TCXO) or Temperature Controlled “Oven Controlled Xtal Oscillators” OCXOs.
Crystal Oscillator (XO)
The simplest form of a crystal oscillator is the X-tal oscillator (XO). In general, it consists of an electrical shell which acts as a feedback amplifier. In the feedback network, the oscillating crystal is integrated as a frequency-determining component. If the loop gain is greater than 1, the oscillator starts oscillating out of the noise at the frequency where the phase shift of the whole loop takes a multiple value of 2Ď€.
By inserting a load capacitance in series with the oscillating crystal, the resonance condition can be changed and thus the resonant frequency can be altered within a small range. This can be used to compensate for manufacturing-related deviations in the crystal frequency and to adjust the oscillator to the required nominal frequency.
The oscillating crystal can be operated not only in its fundamental mode, but also in higher overtone modes. Physically, only the odd harmonics (3, 5, 7, etc.) are possible here. (Note: The naming of the harmonics in quartz oscillators is historical and does not correspond to the classical counting of the harmonics of an oscillation. A quartz oscillator in the 3rd overtone actually oscillates in the 3rd harmonic, which physically corresponds to the 2nd overtone.) Both the quartz crystal and all electronic components change their electrical properties more or less depending on the ambient temperature. Simple XOs are directly exposed to the ambient temperature, so the output frequency shows a significant temperature dependence in the range of up to 100 ppm over the temperature range -40 °C – +85 °C.
Voltage Controlled Crystal Oscillator (VCXO)
The load capacitance is used with the uncontrolled crystal oscillator (XO) to adjust the oscillator to the nominal frequency with a built-in capacitance during production. However, the natural frequency of the system can change over time due to aging effects. In order to be able to change the frequency of the oscillator to a small extent afterwards, a so-called capacitance diode can be installed in series with the quartz oscillator instead of a fixed capacitance. This diode has a variable capacitance depending on the voltage applied to it, the so-called draw voltage. An oscillator that can be pulled, i.e. one whose frequency can be changed, is called a VCXO (Voltage controlled X-tal oscillator). Typically, the capacitance diode is designed in such a way that by changing the load capacitance, all frequency deviations due to temperature differences and aging can be compensated for.
Temperature Compensated Crystal Oscillator (TCXO)
The ability of a VCXO to be drawn can be used to produce oscillators that use an internal compensation network to compensate for temperature influences and thus have a significantly lower temperature drift. To do this, the voltage across the capacitance diode is changed in such a way that the change in capacitance counteracts the temperature-related change in frequency.
The uncompensated temperature response can be approximately described with a 3rd order polynomial. This results in a compensation voltage that is generated either by an analog compensation network value or a corresponding integrated circuit and is also described by a polynomial. In modern ICs, polynomials up to degree 6 are common. The compensated temperature curve shows a deviation of only a few ppm for a good TCXO. A combination of temperature-compensated and voltage-controlled quartz oscillators is also possible. These combinations are offered as VCTCXO.
Temperature Stabilized Crystal Oscillator (OCXO)
The best frequency accuracies can be achieved with temperature-stabilized crystal oscillators (OCXO, Oven controlled X-tal oscillator). The quartz crystal and the main components of the electrical oscillator circuit are kept at a constant temperature by a heating control. This eliminates the influence of the ambient temperature on the frequency stability. The heating of the oscillator is designed in such a way that the quartz is heated to the temperature of its upper turning point and is kept at this temperature (usually approx. 80-100°C). The temperature dependency of the quartz is lowest at the upper turning point, which means that small temperature fluctuations only have a small influence on the frequency. The stability of an OCXO depends heavily on the quality of the heating control, which must react quickly and without overshoot to changes in the outside temperature.
Fundamental Terms
Nominal Frequency: Average operating frequency for which the oscillator was designed
Frequency Tolerance: Deviation of the actual frequency after manufacture from the nominal frequency
Frequency Stability: Frequency change within the working temperature range
Aging: Maximum permissible relative change in frequency within a specified time interval
- Pull Voltage: allowable voltage range at the oscillator’s pull input, which can be used to correct the output frequency
- Supply Voltage: Voltage range at which the oscillator can be reliably operated within specifications.
- Warm-up Time: Time after switching on, after which the oscillator shows a defined deviation from the regular operating frequency