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Application Note
OCXO: Oven Controlled Crystal Oscillator
Introduction
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.
Note
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.
B+
T
Differential
Amplifer
Power
Amplifier
Figure 1.
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.
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Application Note
Employing a proportionally controlled oven can improve oscillator temperature stability relative to the crys-
tal’s inherent stability by more than 5000 times (from ±1x10
-5
to ±1x10
-9
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 tem-
perature changes can “pull” the frequency. Therefore, a change in ambient temperature will result in small
changes in oven temperature.
Setting Oven Temperature
Figure 2.
X
Frequency
(1)
(2)
Y
Temperature
A
B
C
D
As shown above, the actual temperature to which the oven is set is critical in minimizing the effect of ambi-
ent temperature change.
Referring to Figure 2, if the oven temperature were set to the point designated as (1), and a change in am-
bient 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 tempera-
ture 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 ac-
complished by adjusting the potentiometer shown as one leg of the bridge in Figure 1.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com
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Application Note
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 30x10
-6
. This is simply due to the fact that the frequen-
cy 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 balanc-
es in 10-15 minutes 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 designs.
Figure 3.
3
Frequency (x10-6)
2
1
0
-1
10
20
30
Time (Minutes)
Turnover Temperature
The oven operating temperature (crystal turnover temperature) must be several degrees higher than the high-
est 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 itself ).
However, there are disadvantages associated with high oven temperature operation. First, the crystal’s frequen-
cy vs. temperature characteristic is sharper with higher turnover crystals resulting in more sensitivity to minute
changes in oven temperature as shown in Figure 4.
Figure 4.
Frequency
Crystal with
60°C
Turnover
Crystal with
80°C
Turnover
Crystal with
100°C
Turnover
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com
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Application Note
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 determin-
ing 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 operating temperature.
Stability
A. Aging - Aging refers to the continuous change in crystal oscillator frequency with time, all other param-
eters 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 10
-8
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 am-
bient temperature causes a small change output frequency. The frequency shift is an offset from the oscil-
lator’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
, not 5x10-
9
/day. This characteristic is shown below.
Figure 5.
Frequency Offset
Frequency
Change in
ambient temp.
Return to initial
ambient temp.
Aging characteristic
at constant ambient
Frequency curve with
temp. change
Time
Ambient temperature changes do not produce hysteresis effects; that is, if there is a change in ambient tem-
perature 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 change.
When the required temperature stability is beyond that which can be achieved with a standard proportion-
ally 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 con-
tain the oscillator circuit.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com
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Application Note
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 pre-aged.
In most applications, oven-controlled crystal oscillators are continuously energized. This being the case, ag-
ing 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-ener-
gized (a practice which should be avoided whenever possible). When applications require frequent turn-off,
an additional series of characteristics should be considered.
Figure 6.
Frequency
f1
f1
f2
f3
Turn On
T1
Turn Off
T2
T3
Time
T4
T5
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. Three characteristics may then be of significance:
1.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 T4 = f1 - f3.
2.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 T5 = (f3 - f2) / (T5 - T4)
3.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 com-
ponent 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 continu-
ously.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com
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