# Calculating low-pass filter prototype specifications

So I'm just wondering what I need to figure out before I can actually calculate anything here, (only considering the a-part now)? I understand that 2.5 GHz should be 3dB and 3.75 GHz should have 25 dB of gain, and if it's overshooting the specifications, but I'm not at all sure how I can calculate this.

I'm guessing that I'll need to figure out an actual equation for the filter, but I'm also clueless on what principles I should try to use here. Any pointers on what direction I should be headed would be cool.

Should I apply normalized low-pass prototype element values even though I've already been given the component values, or do I want to use that to compare if the components are overkill (assuming they work)?

• Not gain; you want 3dB and 25 dB of attenuation. And two points give you a slope; since slope in logarithmic scale is related directly to the filter order, the question is asking if the filter is of high enough order to possibly meet that requirement regardless of component values. – Hearth Feb 5 at 23:09
• @Hearth I'll nitpick a bit and say that pole-zero filters (even pole only) will make it impossible to judge the slope, unless there is a large enough difference in bandwidth. OP's case is more lenient, though. – a concerned citizen Feb 6 at 7:14
• @Grak First de-normalize the elements and then use whichever method suits you better to create the transfer function (without omitting the I/O 50 Ohm terminations). Then it's as simple as evaluating the t.f. at two frequencies. – a concerned citizen Feb 6 at 7:16