I am using the ADL5375 IQ modulator with the AD9122 DAC IQ outputs. The DAC outputs are lowpass filtered and the LO drive is single ended. The LO generator is not an ADI part, but generates the LO at required levels with good phase noise. I get very good sideband/ image-rejection (> -40dBc) and LO rejection (> -40dBm) out of the ADL5375 output when operating in 2-4 GHz range (10MHz IF typically). I do not use any IQ phase/amplitude correction out from the AD9122 DAC. But when i am operating in the 400MHz to 1.5 GHz LO ranges, i am observing bad sideband rejection (roughly -25dBc). The LO rejection is still good (>-45 dBm). I am not quite sure how this can happen. I do not think the IQ imbalance suddenly increases for the same IF frequency. Right now i digitally correct for IQ phase imbalance and get better sideband rejection in the 500MHz to 1.5GHz range (close to -40dBc, staying within the allowable DAC phase adjust range).
Is there any section of the circuit that i need to pay close attention to ? Is the LO drive something to look at (higher order harmonic levels maybe) ? Please let me know if this behavior is typical.