# AD9361 Tx BB LPF and maximal bandwidth

Question asked by Sebri on Sep 5, 2014
Latest reply on Sep 9, 2014 by dpu
Branched to a new discussion

Hey,

I am planing to realize a proprietary wide band signal with an RF bandwidth (RF BB) of approximately 30 MHz.

Now I am a bit confused about the explanations that are given in the "AD9361 Reference Manual" in the Section "Filter Guide". It is mentioned there that "The Tx BB LPF is a third-order Butterworth low-pass filter with a programmable 3 dB corner frequency. The Tx BB LPF corner frequency is programmable over the range of 625 kHz to 32 MHz. The Tx BB LPF is typically calibrated to 1.6 times the baseband channel bandwidth."

Which mathematics is correct:

a) The RF BB is the same as the baseband bandwidth (BB BW) and, thus, the 3 dB corner frequency (f_3dB) is half of BB BW. Under this assumption my 30 MHz would not be attenuated by the BB LPF filter.

b) The RF BB is double of the BB BW, which equals to 3 dB corner frequency (f_3dB). Then I would run into trouble since the high frequency components in my band will be attentuateed by the BB LPF filter. Thus, I have to perform a preemphasis.

I just want to get sure, since the above mentioned description in the "AD9361 Reference Manual" is not clearly defined for me!

Furthermore I would like to ask the following question as well: How can I combine the TX channel signal path filter with the maximum 56 MHz bandwidth that is possible with the AD9361? I don't see the limiting effect of the analog filter to the bandwidth as mentioned in the "AD9361 Reference Manual". With my assumption a) I can see that the maximal RF BW would be 2*32 MHz = 64 MHz. Where is the limitation to 56 MHz?