Oven Controlled Crystal Oscillator Basics
An OCXO (or Oven Controlled Crystal Oscillator) is near the top of the food-chain when it comes to quartz based frequency control devices, superseded only by the mighty DOCXO or Double Oven!
An OCXO is a temperature-controlled device that maintains a quartz crystal's constant operating temperature. This prevents changes in the specified frequency due to variations in ambient temperature.
The frequency output of a crystal oven is dependent on the temperature of the quartz crystal. You can see in the figure below that temperature can have a significant impact on frequency of the oscillator.
In most applications, a simpler TCXO or Temperature Compensated Crystal Oscillator can be used to reduce the negative effects of ambient temperature changes. But there are environments where a TCXO just can’t meet the stability requirements of the application. In these cases, RF engineers can take a step up and take advantage of the OCXOs improved stability.
This improved stability is achieved by maintaining a near constant crystal temperature regardless of the ambient temperature the oscillator is experiencing. Inside an OCXO, there is a heater circuit in addition to the oscillator circuit. This heater circuit can be configured in many different ways, but the most common is a proportionally controlled oven that uses a heating element to heat the crystal and a thermistor to sense the temperature of the crystal.
These elements are part of a bridge network that acts as a closed loop feedback network to hold the temperature of the crystal at a constant temperature. The figure below shows a block diagram of a common OCXO oven control circuit.
Pre-heat to 185ºF
Setting OCXO Oven Temperature
Keeping the crystal at a constant temperature minimizes the affects of changing ambient temperature and thereby drastically improves the frequency stability of the oscillator. But how do we determine the right set point for the oven? To do this we need to look at the relationship between the crystal’s frequency and it’s temperature a second time. In the graph below, you can see that there is an optimum point on the curve where equal changes in temperature will result in the minimum change in frequency. This point on the curve is so important we’ve given it a special name, this is referred to as the “turning point” of the crystal.
The turning point of the crystal can be ‘designed’ by a crystal engineer and is determined by many factors, including something as simple as the angle at which the crystal blank is cut from the bulk quartz material. That being said, each crystal will behave slightly different and the oven set point is something that is finely tuned for each device produced.
When designing the turning point of a crystal there is a common design trade-off that must be considered. The ideal design goal would be to set the turning point of the crystal above the highest operating temperature of the OCXO. This is because the oven can ‘heat’ the crystal to a set temperature, but if the ambient environment is much hotter than this set point, the crystal will end heat up beyond it’s set point. The downside of pushing the turning point higher is the frequency vs. temperature curve gets sharper. This effectively narrows the operating window of the oven and makes oven accuracy critical. The other major draw back of pushing the turning point higher is that crystal aging performance is reduced.
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AT vs. SC cut crystals
Another option oscillator designers have to increase the performance of their OCXO is to alter the type of crystal insides of the oscillator. Entry level OCXOs are typically built using AT cut crystals. These crystals offer good overall performance and are suitable for may applications; however, as the turning point of crystal needs to be pushed higher, the AT cut runs out of steam. At that point, an SC cut crystal can be used inside the OCXO to push the turning point higher without sacrificing frequency stability. The frequency versus temperature curves of AT cut and SC cut is shown in the figure below. You can see the frequency response is much more uniform over the oven’s operating temperature which improves oscillator stability.
In addition to improved frequency stability, SC cut crystals offer other benefits too. These benefits include:
- Improved Frequency Stability: You can see from the graph above the response of the SC cut is much flatter over a wider temperature range than the AT cut. This translates to tighter frequency stability over temperature.
- Higher Operating Temperature: The turning point of SC cut crystals is higher than AT cut crystals which make them ideal for ovenized applications. The drawback of this higher turning point is the sharp roll-off at lower temperatures. Because of this trait, the SC cut crystals are only suitable for ovenized oscillators and will not work well in other oscillator configurations (TCXOs, VCXOs, etc.)
- Improved Aging: SC cut crystals (which stands for “stress compensated”) offer improved aging performance that is 2 to 3 times better than AT cut crystals.
- Warm-up time: An SC cut OCXO will settle in at it’s final frequency much faster than a comparable AT cut OCXO.
- Phase Noise: The SC cut crystal offers much better Q (quality) factors than AT cut crystals. As such, the close-in or close to the carrier phase noise of the OCXO is improved.
- G-sensitivity: The stress compensation of the SC cut crystal also reduces the g-sensitivity of the oscillator. Quartz is a electro-mechanical devices… if you apply a voltage across the crystal, it will oscillate. The downside is that if you apply a vibration across the crystal it will induce a voltage. This voltage shows up as acceleration or vibration induced phase noise.
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