Objective
Examine the temperature compensation of an Oven-Controlled Crystal Oscillator (OCXO) by observing how its frequency stability is maintained under different temperature conditions.
Materials Needed
- OCXO module (e.g., 10MHz OCXO)
- Frequency counter or oscilloscope
- Temperature-controlled chamber or heat source
- Thermometer
- Breadboard or PCB for mounting the OCXO
- Power supply
Theory
An Oven-Controlled Crystal Oscillator (OCXO) uses a temperature-controlled oven to keep the crystal at a constant temperature, thereby ensuring high frequency stability. By maintaining a stable temperature, the OCXO mitigates the frequency drift commonly seen in crystal oscillators due to ambient temperature variations.
This experiment will test the frequency stability of the OCXO under varying external temperature conditions and observe how the internal oven compensates to keep the frequency constant.
Steps
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Set Up the Circuit
Connect the OCXO module to the power supply according to the datasheet's voltage specifications. Mount the OCXO on a breadboard or PCB.
Connect the output of the OCXO to a frequency counter or oscilloscope for real-time monitoring of the frequency.
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Record Baseline Frequency at Ambient Temperature
At room temperature (approximately 25°C), allow the OCXO oven to warm up. Once the internal oven reaches its set temperature (as indicated by stable frequency readings), record the output frequency as your baseline.
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Increase External Temperature
Apply heat to the external environment of the OCXO, using a heat source or temperature-controlled chamber. Slowly increase the external temperature in increments (e.g., 30°C, 40°C, 50°C), while monitoring the OCXO frequency.
Observe whether the frequency remains stable as the external temperature changes.
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Decrease External Temperature
Once the OCXO returns to room temperature, apply cold to the external environment (e.g., ice packs or refrigeration chamber). Record the frequency at lower temperatures (e.g., 20°C, 10°C, 0°C).
Observe the frequency stability as the external temperature decreases.
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Analyze Frequency Stability
After collecting frequency data at various external temperatures, plot the results against the temperature changes. Compare the measured stability to the OCXO’s specified stability range, typically in parts per billion (ppb).
Example Calculation
For an OCXO with a specified stability of ±5ppb/°C, the frequency drift for a 10MHz oscillator can be calculated as:
Frequency drift = (±5 ppb) × (Temperature change in °C) × (Nominal frequency in MHz)
For a 10MHz OCXO with a 20°C temperature change, the expected frequency drift would be:
Frequency drift = ±5 × 20 × 10 = ±1Hz
This gives an expected drift of ±1Hz over a 20°C change. Compare this with your measured frequency data to evaluate the OCXO's performance.
Conclusion
This experiment demonstrates how an OCXO achieves superior frequency stability compared to standard oscillators, thanks to its temperature-compensated oven. The ability to maintain a consistent frequency, even under varying external temperatures, makes OCXOs ideal for applications where precision timing is critical.