FM of square wave

I have to generate frequency modulated square wave and feed it to DAQ card. I could generate FM of sine wave and figured out how to convert that to square wave. The issue is that ouput of modulator is 3 element type and can be only fed to waveform generator. So i cant use that output for further calculations or sending to DAQ.

Please if anyone can suggest any procedure to carry out the task.

Thanks

Sid