Convert FIR Type I lowpass to FIR Type 1 lowpass with inverse bandwidth
g = firlp2lp(b)
When b is a narrowband filter, g will be a wideband filter and vice versa. The passband and stopband ripples of g will be equal to the stopband and passband ripples of b.
Overlay the original narrowband lowpass and the resulting wideband lowpass
b = firgr(36,[0 .2 .25 1],[1 1 0 0],[1 5]); zerophase(b); hold on h = firlp2lp(b); zerophase(h);