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:
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.