Hi @vikas_kumar.
I work with PX4 and QGroundControl, and there is no need to enable PPM for this setup.
I am quite sure that the range is not the only difference between FS-iA6 and FS-IA6B. I believe the FS-iA6 has no PPM output.
On QGroundControl you can check the RC signals when you connecting the PIXHAWK (via USB or radio) to the computer running QGroundControl.
I am quite sure Mission Planner has also this feature.
Follow below a figure to illustrate it on QGroundControl.
If connected, the small white dots will move as you move the radio sticks.
Let me know if I can help you with anything else.
Paulo Alves