AnsweredAssumed Answered

Unexpected images in AD9789's QDUC mode

Question asked by Jianbing on Aug 3, 2017
Latest reply on Sep 1, 2017 by danf

Hi all,

I make use of AD9789 to design a DTMB transmitter.  I met a  problem and need your help.

The chip works in QDUC mode; all the 5 half-band interpolation filters are bypassed; and the rate converter is not used, P/Q=1;  Only the 16x interpolater and band-pass filter are used. Internal NCO is set to 0.

the frequency of DAC clock is 2.4G, the sample rate of data from FPGA is 150M (Fs keep high value).  

the center frequency of baseband is 0,  the center frequency of band-pass filter is 750M(BPF_Center_Freq= 0x5000), there are 2 bad images at 750-75M and 750+75M. 

Attached you can find the spectrum(Pic 1, Pic 4).


If a constant value siganl( I = 0x2000, Q = 0x0 ) is passed to AD9789, some spurs exists at 300M, 450M, 900M, 1050M (Pic 2) 

If a signal (frequency = 1M) is passed to AD9789, some spurs exists at 751M +- (n*75)M,  751M +- (n*150)M.  (Pic 3).


Attached you can find the value of AD9789's configuration regs. (AD9789 Config.txt,  in verilog).

Thanks for your help!