I am now using AD9959 evaluation board as a frequency source. It seems to me that when I set a channel to a certain type of modulation (Frequency/Amplitude/Phase), the amplitude scale factor on all the other channels will be bypassed. Here is the detail: I am using the Single-bit serial 3-wire mode to communicate with the device. The device is set to 4-level modulation mode in Channel 0 and Channel 1 (FR1[9:8]=01) (FR1[14:12]=000). At first for each of the four channels, modulation is disabled (CFR[23:22]=00), Linear Sweep is disabled (CFR=0), amplitude multiplier enable bit(ACR) is set to 1. The output frequency (CTW0 0x04) of Channel 0 and Channel 1 are set to 25 MHz. Both the outputs are monitored by oscilloscope, and they both have the amplitude of 400mVpp. Now change the amplitude scale factor (ACR 0x06) for CH0 and CH1 from 1 to 0.5 and notice the corresponding change in the scope (Vpp change to 200mV). Now if I switch CH0 to Frequency modulation mode(CFR[23:22]=10), the amplitude of the waveform from both CH0 and CH1 will change back to original value (back to 400mV). I checked that the amplitude word on (ACR 0x06) still indicates that the amplitude scale factor is 0.5 (but this 0.5 is no longer effective). Does this mean that the scale factor in CH0 and CH1 is bypassed? Thank you very much.