It mentions
"If a QPSK modulated signal undergoes filtering to reduce the spectral side lobes, the
resulting waveform will no longer have a constant envelop and in fact, the occasional
180o shifts in phase will cause the envelope to go to zero momentarily."
and with OQPSK
"When an OQPSK signal undergoes bandlimiting, the resulting intersymbol interference
causes the envelop to droop slightly to the region of ± 90o phase transition, but since
the phase transitions of 180 have been avoided in OQPSK, the envelop will never go
to zero as it does in QPSK."
I understand how the QPSK signal would undergo filtering to reduce the spectral side lobs but why is the envelope becoming zero momentarily bad?
The amount of phase variation represents how the value changes to a another value instantaneously. When the filter encounters the sudden change during this transition (a high frequency), its output goes to zero.
thats why amplitude fluctuations are smaller in OQPSK after filtering.