OCXO Application Notes
If stability requirements are too stringent to be met by a basic crystal oscillator or TCXO, the
crystal and critical circuits may be temperature controlled by an oven. The block diagram for
a Vectron oven controlled crystal oscillator is similar to that for a Vectron TCXO except that
the varactor diode and associated thermistor compensation network are deleted and the oscillator
is instead temperature controlled by a proportionally controlled oven.
Proportional Oven Controlled
A proportional control is an electronic servo system which continuously supplies power to the
oven; it varies the amount of oven power, continuously compensating for the ambient temperature
changes. In many Vectron oven controlled oscillators, a thermistor is heat sunk to the oven's
metal shell to sense temperature. The thermistor is one leg of a resistance bridge, as
shown in the following diagram.
The bridge operates such that if the temperature at the oven decreases due to an ambient
temperature change, the change in thermistor resistance causes the bridge to unbalance, developing
an increase in bridge output voltage. This voltage is amplified in a high-gain differential
amplifier. The output of the differential amplifier is further amplified in a power amplifier
which drives directly into the oven winding. Thus, the small voltage increase resulting from
bridge unbalance generates a large voltage increase across the oven winding. This increase in
power to the oven generates more heat, compensating for the temperature decrease which was
initially sensed by the thermistor. Similarly, an increase in temperature at the oven causes a
reduction in bridge output voltage, which results in reduced power into the oven and a
compensating temperature decrease.
An alternative to this design, used in some Vectron OCXOs, has the power amplifier(s) heat sunk to the oven shell as the heat transfer mechanism,
in lieu of having a heater winding. The concept is the same, the only difference being the
vehicle by which the heat is applied to the oven.
Employing a proportionally controlled oven can improve oscillator temperature stability relative to
the crystal's inherent stability by more than 5000 times (from ±1x10-5
over 0-50°C, for example). However, the oven control system is not
perfect because (a) the open loop gain is not infinite, (b) there are internal temperature
gradients within the oven and (c) circuitry outside the oven which is subjected to ambient
temperature changes can "pull" the frequency. Therefore, a change in ambient temperature
will result in small changes in oven temperature.
Setting Oven Temperature
As shown above, the actual temperature to which the oven is set is critical in minimizing the
effect of ambient temperature change.
Referring to Figure 2, if the oven temperature were set to the point designated as (1), and a
change in ambient temperature caused a change in oven temperature from A to B, a frequency
change of magnitude X would result. However, if the oven temperature were set to the upper turnover
point (2), an equal temperature change (C to D) would result in a significantly reduced change
in frequency (magnitude Y). Therefore, each Vectron oven is individually set to the turnover
temperature of the crystal which it houses. This is accomplished by adjusting the potentiometer
shown as one leg of the bridge in Figure 1.
Warmup with AT cut crystals
When an oscillator is initially turned on at room temperature the frequency is extremely high
relative to the output frequency after the oven stabilizes, typically by 30xl0-6
This is simply due to the fact that the frequency of an AT cut crystal is considerably higher
at room temperature than at its upper turnover temperature. As the oven warms up, the crystal
frequency rapidly decreases. In standard Vectron oscillators, the oven balances in 10-15
but the crystal displays a rubberband effect and overshoots its final frequency per Figure 3,
prior to stabilizing. Typically, relatively high degree of stability is achieved within 30
minutes after turn-on; this time can be reduced to less than 5 minutes in special fast warm-up
The oven operating temperature (crystal turnover temperature) must be several degrees higher
than the highest ambient temperature in which the oscillator is to operate in order that the
oven may maintain good control (considering the internal heat rise generated by the oscillator
However, there are disadvantages associated with high oven temperature operation. First, the
crystal's frequency vs. temperature characteristic is sharper with higher turnover crystals
resulting in more sensitivity to minute changes in oven temperature as shown in Figure 4.
Second, and more important, crystal aging (discussed below) degrades with an increasing
temperature. Therefore, in designing an oven controlled crystal oscillator, one is faced with
a compromise in determining the desired oven operating temperature; it should be low as
practicable, but it must be high enough to provide good control at the maximum ambient
- Aging refers to the continuous change in crystal oscillator frequency with
time, all other parameters held constant. Prior to delivery, each Vectron oven controlled
oscillator is pre-aged until it achieve its specified aging rate. Aging rate is often used
synonymously with the word stability; thus, an oscillator with an aging rate of one part in
per day (1x10-8
/day) is sometimes referred to as one part in 108
oscillator. This is incorrect terminology, as aging rate (long term stability) must be referred
to time, and represents only one facet of oscillator stability.
B. Temperature Stability
- As previously noted, because no oven control system is perfect,
a change in ambient temperature causes a small change output frequency. The frequency shift
is an offset from the oscillator's aging curve. This deviation from the normal aging characteristic
is not related to time, but is a fixed offset. Thus, the frequency offset vs. temperature
(temperature stability), for a given temperature change is, for example, 5x10-9
/day. This characteristic is shown below.
Ambient temperature changes do not produce hysteresis effects; that is, if there is a change
in ambient temperature followed by a return to the original temperature, the final frequency
will be essentially that which would have resulted had there been no ambient temperature
When the required temperature stability is beyond that which can be achieved with a standard
proportionally controlled oven, a double oven system can be employed in which the standard oven
is housed within a second oven. The outer oven then buffers the ambient temperature changes to
the inner oven, which contain the oscillator circuit.
C. Restabilization And Retrace
- When a crystal oscillator is turned off for a period of
time and then turned on again (as occurs when the unit is shipped), the crystal requires a
restabilization period. The characteristic is similar to the initial factory aging characteristic,
but high stability is achieved significantly more quickly because the crystal has been factory
In most applications, oven-controlled crystal oscillators are continuously energized. This
being the case, aging is the critical characteristic with turn-off/turn-on characteristic being
of little or no significance. However, certain applications require that oven controlled
crystal oscillators be frequently deenergized and re-energized (a practice which should
be avoided whenever possible). When applications require frequent turn-off, an additional
series of characteristics should be considered.
In Figure 6, assume that an oscillator is energized until time T2
when it is
turned off for a period of time and then turned on again at time T3
characteristics may then be of significance:
- How close does the oscillator return to the output frequency at turn-off, a specified
time after turnon. This is called the retrace characteristic. Retrace error at
- How much will the frequency change over moderate periods of time (hours) after the oven
has stabilized. This is called the restabilization, or warmup, characteristic. Restabilization
rate from T4 to
- How long does it take the oscillator to achieve its specified aging rate following a
specified off period (This is called "reaging").
Many factors affect retrace, restabilization and reaging characteristics. Proper circuit design and
component selection minimize their effects, leaving (1) the crystal and, (2) the length of
off-period prior to oscillator turn-on as the prime factors. There is significant variation in
these characteristics from crystal to crystal and they should only be specified when absolutely
required and then only to the degree needed, as "tight" specifications in this area
can have a major impact upon oscillator cost due to low yield. These characteristics are of
little consequence in oscillators which are energized continuously.
Advantage of SC Crystals:
Double Rotated (SC and IT Cut) Crystals
While most high stability crystal oscillators use AT Cut Crystals, SC and IT Cut Crystals are
often used in the highest stability models.
An SC Cut Crystal is one of a family of double rotated crystals (quartz crystals cut on an
angle relative to two of the three crystallographic axes). Others in the family include the
IT Cut and FC Cut. The SC Cut represents the optimum double rotated design as its particular
angle provides maximum stress compensation, but similar performance is achieved with the IT
Following is a comparison between double rotated (referred to simply as SC for convenience) and
AT Cut crystals.
Disadvantages of SC Crystals:
- Improved Aging. For a given frequency and overtone (e.g. 10 MHz, third overtone), the
SC crystal provides 2:1 to 3:1 aging improvement relative to AT crystals.
- Warm-up. In oven controlled oscillators with a given oven design and turn-on power, the
SC crystal achieves its "final frequency" in considerably less time than does the
- Phase Noise. For a given oscillator design, crystal frequency and overtone, the SC
crystal provides higher Q and associated improved phase noise characteristics. This
improvement applies primarily close to the carrier as the noise floor is determined by
circuit design rather than the crystal.
- High Operating Ambient Temperature. Figure 7 shows the relative frequency-temperature
characteristics of AT, IT and SC crystals. The upper temperature turnover point of the AT
crystal ("A" in Figure 7) and lower temperature turnover point of the SC crystal
("B" in Figure 7) are optimally in
the 70°C to 90°C temperature range. Based upon (a) the desired 10°C difference
between the highest operating ambient temperature and the crystal turnover temperature, and
(b) the manufacturing tolerance of crystal turnover temperatures, these crystals are best
suited for maximum operating ambient temperatures of 50°C to 75°C. However, the
upper temperature turnover point of the IT crystal ("C" in Figure 7) is well
suited to higher temperature operation and thus the IT crystal is a logical choice for high
stability oven controlled oscillators having a maximum operating temperature in the 85°C
to 95°C range. Note that while SC and IT crystal curves are relatively flat at elevated
temperatures, their frequency falls off rapidly at low temperatures. Thus, while they serve
well in high stability HIF oven controlled oscillators, they are generally not well suited
for other types of stable crystal oscillators.
- Orientation Sensitivity (tip-over). When the physical orientation of an
oscillator is changed, there is a small frequency change (typically not more than several
parts in 10-9 for any 90 degree rotation), due to the change in stress on the
crystal blank resulting from the gravitational affect upon the crystal supports. Tip-over is
expressed in 10-9/g where one g represents one half of a 180° orientation
change. The SC crystal is less frequency sensitive to orientation change than is the AT.
However, the tip-over difference between AT and SC crystals is not consequential for most
applications and this characteristic is usually not a specification consideration.
- Spurious Under Vibration. When a crystal oscillator is subjected to vibration,
spurious frequencies are generated, offset from the frequency oscillation by the frequency
of vibration. The amplitude of these spurious outputs is related to the amplitude of
vibration, the mechanical design of the crystal support, and the mechanical design of the
oscillator. The SC crystal produces lower amplitude spurious ouput under vibration than does the
AT; however, this characteristic is determined more by the mechanical designs of the
crystal and oscillator than by crystal cut.
- Cost. Because of difficulties associated with tightly-controlled angle rotations around
two axes in the manufacture of SC crystals vs one axis for the AT, the SC crystal is
significantly higher in cost than that of an AT of the same frequency and overtone.
- Pullability. The motional capacitance of an SC crystal is several times less
than that of an AT of the same frequency and overtone, thus reducing the ability to
"pull" the crystal frequency. This restricts the SC crystal from being used
in conventional TCXOs and VCXOs, or even in oven controlled oscillators requiring the
ability to deviate the frequency of oscillation by any significant degree.
In summary, the suitability of double rotated crystals for use in crystal oscillators is
essentially restricted to those oven controlled applications where the improved aging, warm-up,
and close-in phase noise characteristics justify a significant cost increase.