Using OCXO as an External Oscillator for STM32H533

[modecs]

Dear,

I am planning to use an OCXO (Taitien NI-10M-2503) as the external oscillator (HSE) for synchronizing the MCU to a highly accurate clock source. The OCXO provides an RF output with a sine wave swinging between -2V and 2V.

According to the STM32H533 datasheet, it appears that the input signal on the OSC_IN pin must not exceed Vdd or fall below Vss. In my case, Vdd is 3.3V, and Vss is ground.

I would like to seek your guidance regarding the following:

1. Based on the datasheet, my understanding is that directly connecting the OCXO output to the OSC_IN pin would not work. Could you confirm if this is correct?
2. Would it be necessary to use a circuit such as a bias-T to condition the signal before feeding it into the OSC_IN pin?
3. Are there any example circuits or application notes available for this type of setup?
4. If there are alternative or better approaches, I would greatly appreciate your recommendations.

Thank you very much for your time and assistance. I look forward to your advice.

Best regards,

[Billy OWEN]

Hi @modecs 

This post has been escalated to the ST Online Support Team for additional assistance.  We'll contact you directly.

Regards,

Billy

[Uwe Bonnes · Accepted solution]

10dbm is 2 V pp, not 4 Vpp. Add a capatictor between OXCO out and STM32 Osc_In and 100 Ohm each from OSC_IN to GND and VCC and that should be do the job.

[modecs]

In my simulation setup, I have set the OSC IN resistance at 200 kΩ due to the R_feedback value being 200 kΩ. For the voltage divider resistor, I initially used 100 Ω but found it did not achieve a 3.3 Vpp ; hence, I adjusted it to 560 Ω. Regarding the capacitor for DC blocking, setting it to 1 nF seems appropriate to achieve a 3.3 Vpp signal level.