Receive a Sine Wave and transmit it back changing the phase

[MaurizioAndaloro]

Good Day everyone.

I am trying to make a serial to synchro converter and would like to use an external 400hz as a reference.
As an output I should have a sine wave in phase with the input one and others 2 out of phase of 120 and 240 degrees.
If the input frequency changes, the frequency of the three output sine waves must also change.

I have already managed to create a project that generates 3 sinusoids at 400hz and out of phase by 120 degrees from each other.
The amplitude of the three sinusoids will change according to the serial input data.
But it is necessary to synchronize the "0ϕ" of my sine wave with the "0ϕ" of a sine wave generated by another device.
Can you give me some suggestions? Should i use ESP32 internal adc or an external one?

Regards
Maurizio