Capturing pulse periods by sampling accurately
The maximum frequency of a pulse may be defined by any criteria in time domain pulse width, jitter or spectral density cutoff point like -3dB or -20dB  depending on your requirements and the negative effects.

1) f-3dB= 0.35/tr for rise time 10 to 90%  where -3dB

2) 
Below on a Log scale are the time and frequency display of a pulse with a 1/16 ratio and showing that at 16x f repetition rate of approx. 1kHz there is a null.


Imagine if the duty cycle of the pulse changes from 10% to 90% PWM then the null spectrum occurs from 2f for 50% duty ( square waves have a null at 2nd harmonic and all higher  even harmonics , then at 10% and 90% the null only occurs at 9x f , 18x f etc so criteria 1) above based on rise time is the effective BW defined by -3dB

The problem in most filters that attenuate sharply is the phase shifts rapidly and causes group delay distortion near the breakpoint, except for Bessel aka Cauer Filters which use the lowest Q for smooth elliptical filtering. 

For Data pulses ( e.g. NRZ, RZ, Bi-Phase etc)  there is another class of filters called called Raised Cosine filters with resonance that rings exactly at the pulse zero crossings which is important for analog comparators to restore pulse width or data transitions to reduce jitter from Intersymbol interference (ISI) and thus preserve error rate.