Oven Controlled Crystal Oscillators: Terms and Definitions
The OCXO device usually contains an oven block where the temperature sensor, heating element, oven circuitry, and insulation function to maintain a stable temperature. By keeping the temperature of the crystal and other temperature sensitive components, great improvements in oscillator performance are realized such that the crystal's F vs. T has zero slope. OCXOs can use either AT-, SC-, or IT- cut crystals depending on temperature range and aging performance. Typical OCXO can provide a >1000X improvement over the crystal's f vs. T variation.
Nominal Frequency: The desired frequency of the oscillator. For any given crystal cut, lower frequency crystals exhibit superior stability. For a given frequency, the highest possible overtone will provide the better stability.
Frequency Accuracy: A measure of the difference between the oscillator frequency and the nominal frequency.
Reference Frequency: The frequency used to calculate the maximum deviation. This can be the nominal frequency, or a frequency measured at a given temperature, usually 25°C.
Temperature Stability: The measure of the frequency change due to temperature changes. It is measured by placing the oscillator in a temperature chamber and allowing it to stabilize. After the frequency is measured, the temperature is changed and the sequence is repeated until the desired temperature range has been covered. Here the steps between temperatures are chosen to obtain a sufficiently detailed picture of the oscillator performance. Intervals of 5°C are commonly used. The stability is calculated by finding the difference between the reference frequency, and maximum and minimum frequencies. The accuracy is found by finding the frequency furthest from the reference frequency. The specification should state whether the stability specification is peak-to-peak over the entire range or whether it is relative to the reference frequency. The most common method of specification is from the room temperature value, as the oscillator is normally calibrated at room temperature.
Aging (Long-term Stability): The slow change of oscillator frequency with time, if all other influences are held constant. The primary causes are mass transfer and stress relaxation mechanisms in the crystal unit. These can be reduced by maximizing the ratio of quartz resonator mass to contamination mass by increasing the number of the overtone, and by careful design and processing of the resonator. In a good oscillator the aging rate will tend to decrease with time. Aging rates in OCXOs below 0.5 ppb per day can be achieved after initial aging of 30 to 60 days. Aging shifts will occur whether or not the unit is powered up, and may be significant if the units are left in stock for extended periods before installation in target systems.
Allan Variance (Short-term Stability): The measure of oscillator stability in the time-domain. It is is similar to phase noise except that it is based in the time domain instead of the frequency domain.
Phase Noise: A frequency domain measure of stability and is usually expressed as the SSB spectral density in dBc/Hz. This is the single-sideband noise. It is important in many applications and has direct correlation to the short term stability. Low levels of phase noise are achieved through careful circuit design and use of high-Q resonators. Typically, to measure the phase noise of a crystal oscillator an identical tunable oscillator is used as a reference and phase locked in phase to the oscillator being tested. This allows removal of the carrier signal while leaving the sidebands to be measured with a low frequency FFT analyzer. If the two oscillators have identical noise, the noise of each oscillator is 3 dB better than that measured for both.
Power Supply Noise: It can be a common source of externally induced noise. High quality oscillators normally have internal voltage regulators. This may be adequate for line rejection, but can cause problems. Regulators can receive a small amount of RF energy from the oscillator, which can be modulated with its own noise, and if this gets back into the oscillator, can degrade the noise performance.
Supply Sensitivity: It measures when the oscillator frequency changes as the supply voltage changes. The typical fractional frequency change ranges from ±1 to ±50 ppb for a ±10% change in supply voltage. Voltage sensitivity tends to be higher in TCXO's and DigiXO’s having a low supply voltage. Ovens with higher supply voltages are able to make use of double regulation to reduce this sensitivity.
Load Sensitivity: It measures when the oscillator frequency changes as the load applied to the output pin varies. The typical fractional frequency change ranges from ±0.1 to ±50 ppb for a load change of ±10%. Since the load can be made nearly constant in most applications, load sensitivity is usually not a significant parameter.
G-sensitivity: It is a measure of the sensitivity to acceleration. This is related to the vibration sensitivity, but is generally lower, as this is a static measure of change. The most notable test is the Two G Tip-over test. Here the G-sensitivity is measured by allowing the oscillator to stabilize, and the frequency is measured. The oscillator is then turned upside down, 180°. The frequency is measured again. This test is repeated for each major axis of the oscillator. The difference in frequency is divided by 2, yielding the static G-sensitivity.
Vibration Sensitivity: It is the measure of the oscillator sensitivity to vibration. This is viewed in two ways: dynamic and static. Dynamic sensitivity refers to degradation of phase noise due to vibration while the unit is powered in the target system. This can be different from the static G-sensitivity number in that the oscillator may posses an internal structural resonance which will have a higher sensitivity at certain frequencies. In most cases this specification is ignored, as typical oscillators are rack mounted, and not subjected to significant vibration levels. A more important measure is the static sensitivity, or sensitivity to transportation. Vibration levels in transit from manufacturer to customer are normally outside the control of the manufacturer. The shock and vibration can result in shifts in the calibration frequency, resulting in an offset at the customer.
Operating Temperature Range: A range of temperatures over which the oscillator will meet the specified frequency stability. Outside of this range, the frequency will change rapidly, as the oscillator may no be able to deal with the extremes. No functional damage will result as long as this is within the storage temperature range. For temperatures much higher than the maximum, increased aging rates will occur, and internal component damage may occur.
Storage Temperature Range: Range of temperatures over which the oscillator may be stored. Exceeding these temperatures can result in increased aging rates, and internal component damage may occur.
Magnetic Fields: Magnetic fields may be present and can be a potential noise source as they will result in frequency modulation, and degraded phase noise performance. Oscillator housings are not normally designed with this in mind.
Output Type: Outputs can be specified as either sinewave or logic (TTL, CMOS, ECL, etc). In the case of OCXOs, these are limited to low level sinewave, and CMOS, capable of driving TTL loads, or low current loads. Load sensitivity will depend on the output type.
Output Level: For sinewave output, this is limited to +0dBm into a 50 Ohm load, and other parameters include harmonic and spur levels. For CMOS, the load is limited to 15 pF, and the number of gates, duty cycle, and rise and fall times are specified. TTL levels are specified as a subset of CMOS levels, as the latter is able to drive the former.
Harmonics: For sinewave outputs, it is the measure of the next highest-level frequency component which is an integer multiple of the output frequency, relative to the output frequency level. Measured in dBc, or dB relative to the carrier.
Adjustability: To compensate for long term aging, a frequency adjustment capability is often required in OCXOs. This adjustment also allows for shifts that may occur during transport and subsequent processing of the oscillator. The frequency tuning range and resolution are specified. An external resistance or stable voltage is used, as the oscillators are hermetically sealed.
Tuning Range (Pull Range): The total range of frequency adjust available. For DCXOs and TCXOs, this can be of the order of ±20 ppm. For OCXOs, this is normally of the order of ±2 ppm. This is intended to compensate for long-term drift.
Control Voltage: Also known as tuning voltage. Frequency Accuracy: a measure of the difference between the oscillator frequency and the nominal frequency.
Monotonic: Refers to the nature of the change of the tuning voltage. This is normally positive or negative, and does not change direction for a given slope.
Tuning Linearity: It can be expressed in several ways. In the method of MIL-0-55310, a best-fit straight line is drawn and the ratio of the deviation of the worst point on that line to the maximum deviation is used as the specification. This is normally specified as a percentage, with ±10% being the most common. Other measures are based on the resulting modulation distortion or the slope variation.
Tuning Sensivity: The slope of the frequency vs. tuning voltage characteristic. It is expressed in ppm / volt.
Room Temperature Offset: This allows optimum peak to peak temperature deviation. The oscillator frequency is often deliberately offset at room temperature to minimize the largest deviation from nominal frequency over the whole temperature range. This results in the maximum positive and negative frequency deviations being equally spaced about the nominal frequency.
Setability: The required voltage on the tuning voltage pin necessary to set the unit at reference frequency. This is made as wide as possible to allow for correlation errors, aging shifts, hysteresis, retrace, and shifts due to vibration and any other environmental changes. Aging shifts will occur whether or not the unit is powered up, and may be significant if the units are left in stock for extended periods before installation in target systems.
Stabilization Time (Warm-up Time): This defines as the time taken to reach a certain level of stability after a long period of being turned off. Oven power reaches the specified maximum, after which it cuts back to reach steady state when the oven has reached its operating temperature. Power consumption for OCXOs is typically around 5W at warm-up and 1.5W at steady state, depending on size.