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AFE7950: NCO clock source?

Yoonsun Kim
Expert 1675 points
Part Number: AFE7950

Hello,

According to datasheet, NCO is specified as N x sample rate x 1/2^32.

Does sample rate mean ADC or DAC sample rate? Can I set N value by SPI?

When I use external clock for 4GSPS-DAC and need to make X-band RF signals, dose external clock should be 12GHz or just 4GHz is possible?

Regards,

  • +1
    TI__Expert 5090 points

    Hi Yoonsun,

    In the equation you mention above the sample rate parameter depends on if the NCO is part of the ADC or DAC path. If the NCO is part of the Rx chain then you use the ADC sampling rate. The N value is calculated based on the NCO value that is chosen and cannot be set externally by SPI. Once you update the NCO frequency the N value will also be updated.

    To operate in X-band the DAC will need a 12GHz clock. The AFE7950 DACs only operates in the 1st and 2nd Nyquist zones, so if you were to use a 4GHz clock then you would be limited to 4GHz on the outputs.

    Regards,

    David Chaparro

  • 0 in reply to David Chaparro
    Expert 1675 points

    Hi David,

    When DAC operates in 2nd Nyquist zone, do I need a external BPF? Or does AFE7950 have a filtering function for 2nd Nyquist zone?

    Regards,

  • +1 in reply to Yoonsun Kim
    TI__Expert 5090 points

    Hi Yoonsun,

    The AFE7950 transmitter chain supports two different reconstruction filters that are used to select the output in a certain nyquist zone. For outputs in the 1st nyquist zone a zero-order hold reconstruction filter is used, the filter response for this filter can be seen on page 13 of the PowerPoint given below. For outputs in the 2nd nyquist zone a return-to-complement, or mixed mode, reconstruction filter is used, the filter response for this filter can be found on page 17 of the PowerPoint given below.

    https://training.ti.com/sites/default/files/docs/TIPL4705%20-%20DAC%20Output%20Response.pdf 

    Please note that these reconstruction filters help attenuate the images in other nyquist zones but they do not fully suppress them. If further attenuation is required a BPF can be used. 

    Regards,

    David Chaparro