In our project, we use AD9690 ADC for data acquisition. Sampling frequency of the ADC is 333.3 MSPS. We have a carrier at 111.1 MHz (Fs/3). We use the DDC in the ADC with fs/3 NCO frequency for downconvertion the signal to baseband (I/Q). Decimation rate is set to 2. So, only HB1 is enabled.
Our problem is, at the output of the DDC, we see a component of the input signal at fs/6. Input spectrum is very clean, there are not any components around the signal. According the datasheet, HB1 is always enabled, and it should be filter out the component at fs/3 after the NCO+Mixer. However, as we understand, the component at fs/3 is not filtered out and after the decimation stage it appears at fs/6. This component is filtered out If we use 4x decimation ratio.
You can see the ADC settings in the attachment.