TCXO: Stability vs Accuracy
Temperature compensated crystal oscillators (TCXOs) are used for frequency reference in systems that require a frequency stability of 10 ppm or better, which typically cannot be achieved by a standard XO (crystal oscillator) or VCXO (voltage controlled crystal oscillator).
The frequency accuracy in these systems must take into account frequency drift due to instability factors including variations in operating temperature, supply voltage, output load, and aging. A set of frequency stability parameters is commonly defined by the industry to quantify the influence of environmental factors and circuit conditions on reference oscillators. This post explains the frequency stability specifications in Vectron TCXO datasheets, focusing on the differences between temperature stability and the initial accuracy of an oscillator.
Initial accuracy/frequency tolerance is generally defined as the difference between the oscillator output frequency and the specified frequency at room temperature (typically 25°C ±3°C) at the time of shipment by the oscillator manufacturer. It is measured under typical power supply voltage and output load conditions. Voltage-controlled TCXOs (VCTCXO) which include a varactor diode and associated circuitry allow the frequency to be changed, minimizing this value to an almost negligible amount.
Temperature stability is a measure of how much the oscillator’s frequency varies over the device’s temperature range. One common approach is to use a ‘plus/minus’ specification (for example: ±2 ppm vs. operating temperature range, referenced to 25°C – with the temperature range typically -40 to 85°C or -20 to 70°C). The specification is telling us that if we take the measured frequency at 25°C to be nominal, then the devices frequency will deviate above or below that nominal frequency no more than 2 ppm. The second way of specifying temperature stability is to use a peak to peak value or just a plus/minus value with no reference point. In the second case we can’t say that we know how far above or below nominal the frequency will change – just that we know what the total range will be. Typically, Vectron products are specified using plus minus values from a defined reference point at 25°C.
For example, say we have a TCXO, whose technical spec states that its nominal frequency (fn) is 10MHz, initial frequency tolerance is ±2 ppm, and temperature stability is ±2 ppm over the operating temperature range of -40 to 85°C. A sample of this TCXO is tested at its nominal voltage at 25°C, outputting a frequency (f1) of 10.00001 MHz. The initial frequency tolerance of this actual part is calculated as:
This device passes the initial frequency tolerance spec. This device is then measured over temperature, and is found to have peak minimum/maximum output frequencies of 9.99999 MHz and 10.00003 MHz respectively. The temperature stability of this actual part is calculated as:
This device passes the temperature stability spec. Note that the temperature stability frequency error is calculated relative to the frequency measured at 25°C and not the nominal frequency. Unlike frequency offset at room temperature, temperature drift cannot be canceled out by using a simple calibration scheme.
Other Stability Factors
Power supply stability specifies how much the oscillator frequency will change as the supply voltage changes. The typical fractional change ranges from ±1 ppb to ±10 ppb for a ±5% change in supply voltage. Voltage sensitivity tends to be largest in TCXOs having a low supply voltage. Since the power supply can be made nearly constant in most applications, power supply stability is usually not significant.
Load stability specifies how much the oscillator frequency will change as the load applied to the output port is varied. The typical fractional frequency change ranges from ±0.2 ppm to ±10 ppb for a load change of ±10% for sine wave outputs. Since the load can be made nearly constant in most applications, load stability is usually not significant.
Even under constant operating conditions, TCXO frequency can shift over time due to internal changes within the device. An additional parameter for frequency shift over time is required in the system budget. The most commonly used parameter is referred to as first year aging. First year aging specifies the limit of frequency shift, with respect to initial frequency, after one year of continuous operation under constant power supply voltage and operating temperature, typically at 25°C.
Temperature is typically the dominate source of frequency error in applications using a TXCO. Vectron defines temperature stability as an oscillator’s frequency variation over temperature relative to the device’s measured frequency at 25°C under nominal operating conditions. Vectron also specifies the more influential factors of frequency drift, such as supply stability, load stability and first year aging. Designers should incorporate all stability criteria when choosing a TCXO for their system application.