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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

Parents
• HI Umesh,

Thanks for your reply. Please see my reply in line. I don't see option to attach file here on EZone. Can you provide your mail ID so I can send you raw data captured as you requested.

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

[Dwijen] See the snapshot in below

Full BW Spectrum with Filter Inline

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

[Dwijen]See spectrum in below. Observe that Spur is still present at 48MHz

DDC Spectrum with RF Turned Off

[Umesh] I am afraid I dont fully understand the 48MHz = 3*375MHz - 700MHz math

[Dwijen] Umesh, My apologies for the typo in the above math. See in below updated math

48MHz = 2*375MHz (Second Harmonic of the Fout) - 702MHz (LO)

Here Fout is Fs/8. Second harmonic of the Fout is 750MHz. This beats with NCO set at 702MHz

To prove this I have provided another spectrum in below

I have changed the Fs to 2.991GHz. Fout = 373.88 MHz (Fs/8)

2*373.88 = 747.76MHz, NCO is set to 702MHz

Spur is seen at 45.76MHz = 747.76MHz - 702MHz

• HI Umesh,

Thanks for your reply. Please see my reply in line. I don't see option to attach file here on EZone. Can you provide your mail ID so I can send you raw data captured as you requested.

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

[Dwijen] See the snapshot in below

Full BW Spectrum with Filter Inline

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

[Dwijen]See spectrum in below. Observe that Spur is still present at 48MHz

DDC Spectrum with RF Turned Off

[Umesh] I am afraid I dont fully understand the 48MHz = 3*375MHz - 700MHz math

[Dwijen] Umesh, My apologies for the typo in the above math. See in below updated math

48MHz = 2*375MHz (Second Harmonic of the Fout) - 702MHz (LO)

Here Fout is Fs/8. Second harmonic of the Fout is 750MHz. This beats with NCO set at 702MHz

To prove this I have provided another spectrum in below

I have changed the Fs to 2.991GHz. Fout = 373.88 MHz (Fs/8)

2*373.88 = 747.76MHz, NCO is set to 702MHz

Spur is seen at 45.76MHz = 747.76MHz - 702MHz

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