No you don’t need an external one, it’s actually possible to sample parts of high bandwidth using a lower bandwidth because of the small buffer in most SDR. The problem is that you can’t freeze the buffer so have to gather multiple “sweeps” get parts of ADSB stitched together.

Maybe I post the Rust code after some cleanup. Now I am busy build a new database from scratch to compete with them all https://github.com/punnerud/mpedb