Adjustability
To compensate for long term aging, a frequency adjustment capability is often required. 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. It results in smaller volume and allows the external resistance to be conveniently located for access by the user during calibration. Here a high quality device such as bulk metal foil is preferred.
Aging
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 OCXO’s 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
Also known as short-term stability.
Control Voltage
Also known as tuning voltage. Frequency Accuracy: a measure of the difference between the oscillator frequency and the nominal frequency.
Harmonics
For sine wave outputs 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.
G-sensitivity
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, normally expressed in units of 1E-9/g.
Load Sensitivity
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.
Long-term Stability
Also known as aging.
Magnetic Fields
These 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.
Mid-term Stability
the measure of the oscillator ability to perform under conditions of minor environmental change. Typical target systems cannot guarantee environmental stability, only maximum rates of change. The capability of the oscillator to maintain specification for periods raging from 30 seconds to 30 minutes becomes important. Here OCXO’s perform well when compared to DCXO’s, as they have the ability to meet specifications of ±1ppb per minute for a temperature change of 5°C per hour. DCXO’s have a noisy medium term characteristic, and may only be able to meet ±50 ppb for any period exceeding 5 seconds. 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.
Nominal Frequency (Fnom)
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. Here the rule is, “the greater the mass of quartz, the greater the stability”. The only constraint here is crystal package, and ultimately oscillator package. Frequency ranges down to 10 MHz. Below this frequency dividers and CMOS outputs work best. Above 100 MHz phase-locked loops and frequency multipliers are used to take advantage of the stability of low frequency crystals.
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.

Output Level
For sinewave 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.
Output Type
Outputs can be specified as either sinewave or logic (TTL, CMOS, ECL, etc). In the case of OCXO’s 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.
Overall Stability
The change in frequency due to all external influences, over time. This is a combination of environmental and electrical changes external to the oscillator. Although the stability of a crystal oscillator is largely due to the stability of the crystal, the oscillator is also influenced by the oscillator circuitry. Both sections need to be optimized for best performance.
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 and is denoted by L(f). 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 quadrature 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
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. It is worthwhile to discuss this with the oscillator supplier, to ensure that optimum performance will be achieved.
Pull Range
Aso known as tuning range.
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.
Retrace
Is the frequency error after power is applied, relative to the previous value and aging rate before power was removed. The normal period that the OCXO is powered off is 24 hours. The normal period powered up is to sufficient time to allow complete thermal equilibrium. Good retrace is obtained by proper design of the oscillator, oven mechanics and resonator. All of these require careful processing and assembly to achieve consistent results. This is of the order of ±20 to ±50 ppb. In addition to the crystal related effects described above, thermal stresses from heating and cooling the oven structure can contribute to the retrace, and changes in aging rate.
Room Temperature Offset:
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.
Short-term Stability
The measure of oscillator stability in the time-domain. Also commonly referred to as the Allan variance, it measures the RMS change in successive frequency measurements for short gate times (milliseconds to seconds) and is important in timing applications. It typically improves as the gate time increases until it becomes a measure of the medium to long term drift of the oscillator. This drift is either the result of the temperature coefficient of the oscillator, and/or the aging.
Stability of an oscillator
Consists of multiple components, which include stability over temperature, aging, power supply sensitivity, load sensitivity, thermal hysteresis, retrace, trim skew, sensitivity to acceleration and sensitivity to ionizing and non-ionizing electromagnetic radiation.
Stabilization Time:
For an OCXO , this defined as the time taken to reach a certain level of stability after a long period of being powered down. Oven power reaches the maximum specified, after which it cuts back to reach steady state when the oven has reached its operating temperature. For a DCXO, the time taken for the frequency to reach specification, and is of the order of seconds.
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.
Supply Sensitivity
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.
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.
Thermal Hysteresis
The ability of a TCXO to repeat the frequency versus temperature data over multiple temperature cycles. Here the frequency of a TCXO is measured at one temperature. The temperature is changed and then returned to the original temperature and the frequency is measured again. The two frequencies are not the same. The difference between the two frequencies is called “thermally induced hysteresis”. This is present even if the unit is allowed to stabilize at the same temperature for a long time. This phenomenon is at best unpredictable, but may display a pattern weakly dependant on the directions and rates of temperature change. This is normally of the order of ±0.1 ppm for a good TCXO .
Thermal Transient:
occurs when the rate of temperature change is high enough for the frequency to no longer track the well behaved curve that is generated when measured with slow temperature changes. An acceptable change of temperature would be of the order of 0.5°C per minute. This effect is in a large part due to the transient response of the resonator, and the separation between resonator and temperature sensing devices within the oscillator. In an OCXO , it can also depend on the stability and gain of the error amplifier used in the temperature controller. Typical values are less than ±0.2 ppm.
Trim Skew
When a TCXO is adjusted to the ends of the tuning range, frequency stability of the part is altered. For very large ranges, this can result in the part being out of specification. This effect can be reduced with careful design, and manufacture of parts with good stability margins versus temperature.
Tuning Linearity
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 Range
The total range of frequency adjust available. For DCXO’s and TCXO’s this can be of the order of ±20 ppm. For OCXO’s this is normally of the order of ±2 ppm. This is intended to compensate for long-term drift.
Tuning Sensitivity
The slope of the frequency vs. tuning voltage characteristic. It is expressed in ppm / volt.
Vibration Sensitivity
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. Packaging and design of the oscillator help to reduce this so that the part is still within specification on arrival at the customer. See G-sensitivity.
Warm-up Time
Also known as stabilization time for an OCXO .