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AD9690 - Unwanted Signals at DDC Output

Question asked by castle on Apr 26, 2017
Latest reply on May 2, 2017 by UmeshJ



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.