AD9208: Harmonic of Decimated FS mixes with NCO

Hi Dwijen,

Thank you for your kind words. I appreciate it.

In some cases where the frequency plan and the decimation ratio is such that the bandwidths are pretty wide, the DC spur can rotate around the decimated data and show up in the spectrum. This rotation is due to the fact that the NCO shifts the frequency and then the decimation filter filters it. If the shifted DC spur falls out of the filter band, the spur appears attenuated. If the shifted DC is in the filter band, then you wont notice any attenuation. My suggestion is that the spur you see is the former (DC spur outside of filter band).

The reason I suspect this is because of the DDC spectrum screenshot you sent with no RF. The spectrum should be clean but it still seems to have a -90dB spur at 48MHz. This could either be coming in from the clock, or the analog input, due to the frequency planning. I doubt this is the clock or analog since you have the analog input RF turned off, and as it it, you are using high quality signal sources. I will set this up in our lab when we return from our holiday shutdown.

Here is some additional information on how our DDCs work

What’s Up With Digital Downconverters—Part 1 | Analog Devices 

What’s Up With Digital Downconverters Part 2 | Analog Devices 

Please let me know if you have any questions.

Thanks

Umesh

Hi Dwijen,

Thank you for your kind words. I appreciate it.

In some cases where the frequency plan and the decimation ratio is such that the bandwidths are pretty wide, the DC spur can rotate around the decimated data and show up in the spectrum. This rotation is due to the fact that the NCO shifts the frequency and then the decimation filter filters it. If the shifted DC spur falls out of the filter band, the spur appears attenuated. If the shifted DC is in the filter band, then you wont notice any attenuation. My suggestion is that the spur you see is the former (DC spur outside of filter band).

The reason I suspect this is because of the DDC spectrum screenshot you sent with no RF. The spectrum should be clean but it still seems to have a -90dB spur at 48MHz. This could either be coming in from the clock, or the analog input, due to the frequency planning. I doubt this is the clock or analog since you have the analog input RF turned off, and as it it, you are using high quality signal sources. I will set this up in our lab when we return from our holiday shutdown.

Here is some additional information on how our DDCs work

What’s Up With Digital Downconverters—Part 1 | Analog Devices 

What’s Up With Digital Downconverters Part 2 | Analog Devices 

Please let me know if you have any questions.

Thanks

Umesh