Design Strategy for Stabilizing 50MHz Crystal Oscillator Drift at -150dbc/Hz, 1MHz Offset

bmr13 wrote:

I want to discipline my onboard crystal oscillator drifting due to temperature and process variations. My reference frequency is at 25MHz and my output is at 50 MHz. The requirements for my 50MHz phase noise at 1 MHz offset is -150dbc/Hz. Suggest suitable design strategy for this.

Achieving a low phase noise of -150 dBc/Hz at a 1 MHz offset from a 50 MHz signal can be challenging, especially when you also have to deal with temperature and process variations that affect crystal oscillator stability. Here are some design strategies that you could consider:

1. OCXO (Oven-Controlled Crystal Oscillator): If your application can accommodate the size, power, and cost, an OCXO can provide excellent frequency stability over temperature variations. The oven maintains the crystal at a constant temperature to minimize frequency drift.

2. TCXO (Temperature-Compensated Crystal Oscillator): A TCXO uses a temperature-sensitive network, such as a thermistor, to adjust the frequency with temperature changes. These are typically smaller and consume less power than OCXOs.

3. External Reference: Consider using an external high-stability reference like a GPS-disciplined oscillator, which can provide very accurate frequency references and can be used to calibrate your onboard oscillator periodically.

4. PLL (Phase-Locked Loop): Implementing a PLL can help you achieve a higher frequency output from your reference frequency and maintain phase noise performance. A PLL can also filter out some of the phase noise from the reference oscillator. For your case:
- Use a low phase noise reference oscillator.
- Choose a PLL IC with low phase noise characteristics.
- Design the loop filter carefully to optimize the noise performance at your required offset frequencies.

5. Crystal Quality: Use a high-quality crystal with low phase noise characteristics and specify tight tolerances for frequency versus temperature characteristics.

6. Circuit Design and PCB Layout: Ensure that your oscillator circuit and PCB layout minimize noise. Keep the oscillator and PLL loop filter away from noisy components and traces. Use proper grounding and shielding techniques.

7. Calibration: Implement a calibration routine where you measure the frequency output and adjust the control voltage to the VCXO (Voltage-Controlled Crystal Oscillator) as part of the PLL, to compensate for process variations.

8. Software Compensation: Utilize software algorithms to predict and compensate for temperature-induced frequency drifts based on temperature sensor data.

9. Environmental Control: If possible, control the environment around the oscillator to reduce temperature swings.

10. Harmonic Generation: Since you need a frequency output that is double your reference frequency, consider using a frequency doubler and ensure its phase noise contribution is minimized. This is especially important if you opt for a solution that doesn't inherently produce the desired output frequency.

Each of these strategies has its own pros and cons regarding complexity, cost, power consumption, and physical size. The best approach would depend on the specific constraints and priorities of your application. Can you provide more details about the context in which the oscillator will be used and the constraints you face (size, power consumption, cost, etc.)? This will help to narrow down the best solution for your specific needs.