How can I get 'sync' signals for sine wave out? [FPGA USB 7856R]

[SeanAhn]

Hello,

I'm Sean Ahn using NI 7856R, USB FPGA with LabVIEW 2014.

I'm trying to make 'sync' signals for sine wave of adjustable frequency, like general function generator's sync out.

I use sine waves whose frequency is feedback-congtrolled between 25~30 kHz and need sync signals of square wave.

Like [1] below, when I try resettng two wavefom-generators, sine and sync signals are not perfectly synchronized.

And everytime I adjust sine's frequency, the phase difference between sine and sync changes arbitrarily 

In case of [2] using 'zero-crossing detector', I have severe jitter of sync signals not suitable as 'sync' signals for other parts.

Please, give me some help.

Thanks in advance

Sean Ahn

[1] Reseting two waveform generators

[2] Zero-crossing detect