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# AD9208: Harmonic of Decimated FS mixes with NCO

Hi,

In my setup Fs is 3GHz

Decimation Rate is 8

Decimated Fout (Decimated Fs) is 375MHz (3GHz/8)

NCO is 702MHz

RF In to the ADC is 800MHz

I have observed that there is spur in the output spectrum that is arising from mixing of the harmonic of Fout with NCO.

For the above setup there is spur at 48MHz.

2*375MHz - 702MHz = 48MHz

The snapshot of the ACE setup and the output spectrum is attached.

Is this spur expected? What can be done to minimize this spur?

Regards,

Dwijen

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• Hi Dwijen,

What does the full BW plot look like with the filter in line?

Also, what does the DDC spectrum look like when you remove the input signal (turn RF Off)?

I am afraid I dont fully understand the 48MHz = 3*375MHz - 700MHz math. Are you implying that this 425MHz tone aliases to 50MHz? I am also curious as to how you came to the 3*375MHz conclusion.

If you check the FFT spectrum, the 48MHz is tagged as "DC Frequency" in ACE. I am not sure if this is correct. Would it be possible for you to

- take an FFT (with DDCs on) with the 5% BPF between the MXG and analog input

- export the raw data and send it to me

Thanks

Umesh

• Hi Dwijen,

What does the full BW plot look like with the filter in line?

Also, what does the DDC spectrum look like when you remove the input signal (turn RF Off)?

I am afraid I dont fully understand the 48MHz = 3*375MHz - 700MHz math. Are you implying that this 425MHz tone aliases to 50MHz? I am also curious as to how you came to the 3*375MHz conclusion.

If you check the FFT spectrum, the 48MHz is tagged as "DC Frequency" in ACE. I am not sure if this is correct. Would it be possible for you to

- take an FFT (with DDCs on) with the 5% BPF between the MXG and analog input

- export the raw data and send it to me

Thanks

Umesh

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