Crystal Industry Innovation
In an OXCO the crystal and other temperature sensitive circuity is placed in a temperature controlled structure. The idea is to keep the crystal at a stable temperature higher than the highest ambient temperature to which the OCXO will be exposed. For best results, the oven is set to the resonator's turnover temperature. Either an AT or SC cut crystal may be used. The SC cut rensonator offers best overall performance; the AT cut offers lower costs.
The primary reson behind controlling the temperature is to remove the effect of temperature induced anomalies. All crystal passes phenomena which are at best unpredictable, and only allow compensation(or predictability) to within ±0.1 ppm. The other reason allows the use of higher overtone crystals which are not very pullable (but very stable), to be set at frequency by controlling the temperature. The former also results in improved short term stability resulting form higher Q of the crystal, and improved long term performance resulting from the increased quartz mass.
The greastest advantage of an OCXO is its stability, which is unequalled by other crystal oscillator types. Typical fractional stability can range from ± 20 ppb(±20E-9) to ± 100 ppb. This stability can be valid for a temperature range of -40°C to +85°C. Improved stability can be abtained over narrower temperature ranges.
The main disadvantages of OCXO's in general, are power, size, warm-up time and cost. The amount of oven power required is determind mainly by the quality of insulation used and the temperature differential between the oven and the external environment. Increasing the amount of insulation to reduce heat loss requires an increase in size, resulting in a tradeoff between power and size. Warm-up is the time required for the oven to reach operating temperature and the frequency to stabilize. It is largely dependent on available power, the thermal mass of the oven, and the quality of insulation, and ambient temperature. Typical warm-up times are from 15seconds to 5minutes.
While these disadvantages plague OCXO's in general, Valpeyfisher has developed a line of oscillators to address these disadvantages, while retaining as many advantages as possible. The power, size and warm-up have all been reduced to produce parts that are competitively priced against larger parts similar specifications.
OCXO's require power for two stages of operation:warm-up and steady state. When power is first supplied to the unit, this can be as high as 10W. Power requirement are between 500mW and 3W at steady state. Power supply voltage can from 5 to 12 Volts.
Most OCXO's can be made tunable over a small frequency range, using a voltage control. A typical frequency tuning range is ± 1 ppm ( ±1E-6). This is intended primarily as calibration and aging adjust over the life of the part.
Temperature-compensated crystal oscillators(TCXO's), although less stable in frequency, have advantages over OCXO's in warm-up time(typicall 50 to 1000ms), power (10-150 mW), size and cost. The cost of a TCXO is usually a fration of an OCXO's.
TCXO's use a temperature-sensitive compensation network, which must be custom-bulit for each unit, to tune the oscillator just enough to offset the uncompensated frequency change with temperature. The stability of the oscillator is improved so as to meet the required specification. With standard compensation techniques, fractional stabilities of around ± 1 ppm for a temperature range of -40°C to +8.5°C can be achieved. Better stabilities can be achieved over narrower temperature ranges. Frequency-temperaturehysteresis limits the ultimate attainable stability of a TCXO. The resonator is a primary source of this hysteresus, which can be minimized but not eliminated. Another anomaly found in crystals, which can be difficult to compensate, are perturbations. These can also be minized, but normally at the cost of degraded crystal parameters.
To allow for aging, most TCXO's are made tunable over a small frequency range, using a voltage control (VCTCXO).
A typical fractional tuning range is ± 5ppm. Ranges up to ± 50ppm can be readily accommodated ; however, a large tuning range usually degrades the temperature stability.
A VCXO (voltage controlled crystal oscillator) is a crystal oscillator which includes a varactor diode and associated circuitry allowing the frequency to be changed by application of a voltage across that diode. This can be accomplished in a simple clock or sinewave crystal oscillator, a TCXO (resulting in a TC/VCXO-temperature compensated voltage controlled crystal oscillator), or an oven controlled type (resulting in an OC/VCXO-oven controlled voltage crystal oscillator).
There are several characteristics peculiar to VCXOs. In generating a VCXO specification these apply in addition to the characteristics which define fixed frequency crystal oscillators. Primary among the specifications which are peculiar to VCXOs are the following:
Control Voltage - This is the varying voltage which is applied to the VCXO input terminal causing a change in frequency. It is sometimes referred to as Modulation Voltage, especially if the input is an AC signal.
Deviation - This is the amount of frequency change which results from changes in control voltage. For example. a 5 volt control voltage might result in a deviation of 100 ppm, or a 0 to + 5 volt control voltage might result in total deviation of 150 ppm.
Transfer function (sometimes referred to as Slope Polarity) - This denotes the direction of frequency change vs control voltage. A positive transfer function denotes an increase in frequency for an increasing positive control voltage, as in Figure 1 A. Conversely, if the frequency decreases with a more positive (or less negative) control voltage, as in Figure 1 B, the transfer function is negative.
Linearity - The generally accepted definition of linearity is that specified in MIL-0-55310. It is the ratio between frequency error and total deviation, expressed in percent, where frequency error is the maximum frequency excursion from the best straight line drawn through a plot of output frequency vs control voltage. If the specification for an oscillator requires a linearity of ±5% and the actual deviation is 20 kHz total as shown in Figure 2, the curve of output frequency vs control voltage input could vary ± 1 kHz (20 kHz ±5%) from the Best Straight Line "A". These limits are shown by lines "B" and "C". "D" represents the typical curve of a VCXO exhibiting a linearity within ±5%.
In Figure 3, the maximum deviation from Best Straight Line "A" is - 14 ppm and the total deviation is 100 ppm, so the linearity is ±14ppm/100 ppm = ±14%.
The VCXO which produces the characteristic indicated in Figure 2 uses a hyper-abrupt junction varactor diode, biased to accommodate a bipolar (±) control voltage. The VCXO which produces the characteristic in Figure 3 uses an abrupt junction varactor diode with an applied unipolar control voltage (positive in this case).
Good VCXO design dictates that the voltage to frequency curve be smooth (no discontinuities) and monotonic. All Vectron VCXOs exhibit these characteristics.
Modulation rate (sometimes referred to as Deviation Rate or Frequency Response) - This is the rate at which the control voltage can change resulting in a corresponding frequency change. It is measured by applying a sinewave signal with a peak value equal to the specified control voltage, demodulating the VCXO's output signal, and comparing the output level of the demodulated signal at different modulation rates.
The modulation rate is defined by Vectron as the maximum modulation frequency which produces a demodulated signal within 3 dB of that which is present with a 100 Hz modulating signal. While non-crystal controlled VCOs can be modulated at very high rates (greater than 1 MHz for output frequencies greater than 10 MHz), the modulation rate of VCXOs is restricted by the physical characteristics of the crystal. While the VCXO's modulation input network can be broadened to produce a 3 dB response above 100 kHz, the demodulated signal may exhibit amplitude variations of 5-15 dB at modulation frequencies greater than 20 kHz due to the crystal.
Slope/Slope Linearity/Incremental Sensitivity - This can be a confusing area as these terms are often mis-applied.
Slope should be really called average slope if it is intended to define the total deviation divided by the total control voltage swing. For the VCXO depicted in Figure 2, the average slope is -20 kHz * 10 volts = -2 kHz/volt. Incremental sensitivity, often misnomerred Slope Linearity means the incremental change in the frequency vs control voltage.
Thus, while the average slope in this example is -2 kHz per volt, the slope for any segment of the curve may be considerably different from -2 kHz/volt. In fact, for VCXOs with Best Straight Line linearity of ±1% to ±5%, the Incremental Sensitivity is approximately (very approximately) 10 times as great as the Best Straight Line linearity.
Thus a VGXO with ±5% Best Straight Line linearity can exhibit a slope change of ±50% on a per volt basis. Therefore, a specification which reads "Slope: 2 kHz/volt ± 10%" requires clarification as it could mean either Average Slope or Incremental Sensitivity. If it were intended to define average slope, it simply specifies a total deviation of 18 kHz to 22 kHz and would more properly have stated, "Total Deviation: 20 kHz - 10%." However, if it were intended that the frequency change for each incremental volt must fall between 1.8 kHz and 2.2 kHz, a highly linear VCXO is being specified as a ±10% Incremental Sensitivity relates approximately to a ±1% Best Straight Line linearity. That element of the specification should read, "Incremental Sensitivity: 2 kHz ± 10% per volt."
Stability - A quartz crystal is a high Q device which is the crystal oscillator's stability determining element. It inherently resists being "pulled" (deviated) from its designed frequency. In order to produce a VCXO with significant deviation, the oscillator circuit must be "de-Q'd". This results in degrading the inherent stability of the crystal in terms of its frequency vs temperature characteristic, its aging characteristic, and its short-term stability (and associated phase n oise) characteristic. Therefore, it is in the user's best interest not to specify a wider deviation than that absolutely required.
Phase Locking - When a VCXO is used in phase lock loop application, the deviation should always be at least as great as the combined instability of the VCXO itself and the reference or signal onto which it is being locked. Vectron produces a line of VCXOs especially intended for use in phase lock loop applications (described on the pages which follow). However, if the open loop stability requirements of a system are more stringent than available in this product line, a TC/VCXO may be required. For the highest stability open loop requirements, the appropriate oscillators may be those described in the TCXO or OCXO sections of this catalog, incorporating a narrow deviation VCXO option, rather than those described in the VCXO section.
Basic Oscillator Frequencies - Fundamental mode crystals (generally 10-25 MHz) permit the widest deviation, while 3rd overtone crystals (generally 20-70 MHz) allow deviation approximately 1/9th of that which applies to fundamentals. Therefore, all wide deviation VCXOs (greater than ±100 to ±200 ppm deviation) use fundamental crystals; narrower deviation VCXOs can use fundamental mode or 3rd overtone crystals, the selection of which often depends upon such specifications as linearity and stability. It is rare that higher overtone, and therefore higher frequency crystals find application in VCXOs. Thus, VCXOs with output frequencies higher or lower than available from the appropriate crystal frequencies include frequency multipliers or dividers.
General Note - While it is true of any type of crystal oscillator, it is especially important with VCXOs that the user not over-specify the product. The particular problem with VCXOs is that increased deviation results in degraded stability which can result in the need for still wider deviation, further degrading stability, resulting in a spiraling increase in the required deviation.