We have a question regarding the Tx FIR filter and output power when the filter is enabled. We are using the FIR coefficients that are part of the reference software for the IIO Scope program, when the Auto Filter option is selected.
The filter works very well but the output power (both peak and channel power of the wanted spectrum) is attenuated compared to when it is not enabled, in the order of 4-6dB. I see in the reference manual that for the Tx FIR filter only 0dB or -6dB gain settings are available, so given we set it to 0, what else could be affecting the output? (I have tried the -6dB setting and it does indeed reduce the output level by a further 6dB).
The Tx output attenuation registers are also set to 0 so we can’t compensate for the loss in the output stage.
Is it a case of overflows in the filter stages? Do we need to tweak the digital samples somehow?
For reference, screenshots with FIR Off & FIR On are shown.