I designed a precision phase detector using the AD630.The measurements signals (sine wave) in the range of 10kHz -500kHz with maximum Vp= 5.0V. The device is versatile and within my budget. For this purpose I follow the design in circuits diagram 9(b) gain of two balanced modulator from the AD630 datasheet. I found the signal between 10kHz - 150kHz the output which is full wave rectifier (as the measurement signal is in phase with reference signal) is good. However when the signal greater than 150kHz the signal is bad because the peak output is not at the same level alternately. Can anyone explain why this happen?