AD9957 works in QDUC mode with 5 MHz modulation rate.
CFR1 = 00 20 00 02
CFR2 = 00 40 E8 00
CFR3 = 35 38 C1 28
PROF0 = C9 FA 00 00 1C 2F 83 7B
Input frequency (clk) = 50 MHz
Output frequency = 110.1 MHz
Pins 52, 53, 54 are electrically grounded (PROF0).
I[17:0] = sin(10 kHz)
Q[17:0] = cos(10kHz)
This type of modulation means that we shift radiofrequency to 10 kHz.
The problem is in sign of shifting during initialize of AD9957. I mean sometimes output frequency is 110.11 MHz and sometimes 110.09.
It happens if swap I and Q. Also it's possible to change output frequency by inverting Spectral Invert bit in Profile register.
Any ideas why DDS invert spectrum randomly after initialize?