You will not be able to filter the second harmonic of 125MHz if the -3dB
bandwidth of the LPF is 300MHz.
If your max frequency is 125MHz, then the -3dB cutoff
frequency should be no higher than say 140MHz, in order to ensure that at the second
harmonic (250MHz) the level is well attenuated. There may be other products generated
by the mixer, depending on your system topology. Thus you should ensure that the expected
products in all conditions are above the low frequency -3dB cut-off frequency. What
are all the frequencies present at the mixer ports?
At bottom, life is all about
Sucking in and blowing out.