When you build a TCXO oscillator, you are creating a device that measures the frequency and a system that will also regulate the device’s temperature. The heart of a TCXO oscillator is its controlling signal generator. The controlling signal generator observes changes in ambient temperature and translates them into a specific and proportional controlling voltage.
What is a TCXO Oscillator?
The TCXO oscillator is an effective way to measure changes in frequency. Its unique circuitry uses a cofntrolling voltage that reflects the temperature perception of the crystal blanks. Unlike other types of oscillators, a TCXO is especially accurate and precise. However, it is more expensive than most other types of oscillators. Here are some key benefits of a TCXO oscillator:
TCXOs are generally superior to normal crystal oscillators in terms of frequency stability. They are capable of quoting in parts per million and are typically ten to forty times more accurate. Typical TCXOs display stability performance figures in parts per million (ppm) over a wide temperature range. For example, a TCXO with a frequency stability of +1.5 ppm over a -20degC temperature range would fall into the high precision category. However, such a TCXO is expensive and requires a special calibration method.
Uses of TCXO Oscillator:
A TCXO oscillator is ideal for frequency measurement in electronic devices. It has excellent frequency stability, is small, and can be started and stopped rapidly under low current. In addition, a TCXO’s temperature range has the capacity to expand tenfold over a conventional clock oscillator. The TCXO also utilizes a thermistor as the sensing element, a device whose resistance is dependent on ambient temperature.
The TCXO’s thermal time constant is not the same as a crystal’s, and the thermistor/resistor network that compensates for it may change its tuning sensitivity. Some TCXOs are designed with a constant compensation voltage, while others do not. The cost of a TCXO is directly related to its frequency versus temperature specification. The cost of a TCXO depends on the complexity of the design and the thermistor/resistor network.
OCXO Oscillator:
The OCXO oscillator is more expensive and requires a voltage regulator to prevent + shifts due to crystal aging. The difference in performance is most noticeable in the frequency stability, which can be up to ten times higher than that of a normal crystal oscillator.
An OCXO is compared to a thermostat, which senses a temperature change and controls the furnace to keep it at the desired temperature. It has a “turning point” where the frequency versus temperature curve is zero, meaning that any temperature change will have minimal effect on the frequency.
Another important feature of an OCXO is its ability to adjust the oscillator’s frequency in response to temperature changes. In a TCXO, for example, a temperature sensor applies a small voltage to a varactor, producing the opposite temperature frequency. The OCXO can maintain its frequency by adjusting the temperature with counterbalance-based circuitry.
VCXO Oscillator:
A VCXO oscillator is an electronic device that produces a sinewave frequency. There are various types of VCXOs. Different types have different frequencies and can have slightly different tuning ranges. You can measure the frequency by measuring a sinewave signal with a peak equal to the control voltage and then demodulating the output signal. Vectron defines the modulation rate as the maximum frequency achieved within 3 decimal places of a 100 Hz modulating signal.
These modules are generally available in leaded, through-hole, or surface-mount packages. VCXOs are small and compact, so you can use them in various electronic circuit designs. The basic RF circuit design includes a crystal oscillator and back-to-back diodes across the crystal to tune the frequency.
The aging rate of an OCXO is low, 0.20ppm per year, and they require adjustment at +25degC. Most OCXO oscillators are adjustable through mechanical frequency control, ranging from +2 ppm to +0.20 ppm. The crystal cut is another important factor affecting the stability of an OCXO oscillator. Different crystal cuts have different slopes of frequency versus temperature at turning points.
Advantages of TCXO Oscillator:
Circuit Design:
A TCXO oscillator is a circuit for maintaining the frequency of an output signal regardless of temperature. A TCXO has two basic types of output: HCMOS and Clipped Sine. These output types are characterized by their frequency stability and short start-up time. For the most basic form of TCXO, one can achieve temperature compensation using a pressure plate.
A TCXO oscillator is a type of radio frequency (RF) circuit. This type of circuit is common in wireless communication systems. The frequency of this type of oscillator is usually within the microwave range. This type of circuit also incorporates extra LC circuits. The RF circuit design for a TCXO oscillator includes many LC circuits.
Easy to manufacture:
Unlike OCXO oscillators, the voltage-compensated TCXO is easy to manufacture, and one can feed it into a VCO for higher frequency multiplication. It has a wide operating range and is ideal for RF and high-speed digital systems. The VCO can also be voltage-controlled and adjust the frequency of a TCXO oscillator to ppm levels.
Temperature Compensation:
The compensation network of a TCXO is the main component of the circuit. The compensation network handles the smooth operation of the entire system. A three-order polynomial represents the approximate temperature frequency response, while a five-order polynomial is much more accurate. The compensation network senses the temperature and produces a voltage inversely proportional to the temperature.
TCXO can implement temperature compensation by measuring multiple frequency-temperature curves and checking the jitters near the hysteresis frequency f0. The hysteresis error measures the amount of output frequency offset due to hysteresis during a temperature cycle. For example, a 10MHz TCXO may produce a frequency shift of one Hz every ten to fifteen degrees Celsius.
