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AD9957 spectral invert

Question asked by vdpavel on Sep 1, 2011
Latest reply on Sep 1, 2011 by vdpavel

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?

 

Thanks.

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