For R5550/R5750/R5500 products, ThinkRF spec at 25us for pulse width signal detection (POI). Anything less than 25us may not be detected. 


Certain span and RBW values affect the 25us pulse width signal detection, and it is required to do it under SH mode. 

Please refer to this YouTube video as an example.  

https://youtu.be/1D3qbiqrutM