Hi,
 
Could someone point me in the right direction here please.
 
I am using a NI DAQ USB Series USB-6341.
 
I have a mechanical resonator, I am exciting the resonator via an amplifier and I want to vary the frequency to maintain a phase angle of 90 degrees. I am driving and measure a voltage signal.
 
Does anyone have any suggestions how best to go about this problem. 
 
Thanks in advance