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AD9361 Tx Spurious at Data Rate

Question asked by msal on Apr 12, 2018
Latest reply on Apr 22, 2018 by srimoyi

Hi all,

 

We are trying to use the AD9361 for our narrow-band application and are seeing some issues with spurious signals on in the transmit path.

 

Here's an example:

Fs (data_rate) = 800kHz

Fcarrier = 2GHz

Tx Attenuation = 10dB

We've set our data rate to an integer factor of the frequency of our reference clock. Then we transmit a constant DC value -6dBFs (1023 + 1j*1023). When I look at the transmit spectrum on a spectrum analyzer, I see spurs at +-800kHz. As you can see they are only about 40dB below the carrier.

 

This is not something I would expect from looking at the Filter Wizard generated frequency response - there we expected to see ~100dB of rejection at +-800kHz:

 

Furthermore, if I send a modulated signal through the transmit chain, I observe the +-800kHz spurs being modulated as well. Here is the transmitted samples followed by the resulting transmit spectrum:

 

Tx Samples

Input to AD9361 Transmit

 

AD9361 Tx Spectrum

 

Can anyone help address what is causing these spurs, as well as how to reduce them? It does not appear that the analog LPF filters can sufficiently reject these spurs.

 

Thank you,

-M

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