I am trying to figure out the mechanics of plotting the frequency response of a FIR filter. For example, I have used an algorithm (Parks-McClellan), to generate a low pass FIR filter with an even number of coefficients. I've got 34 tap values. Now I want to show the frequency response of this configuration in a normalized frequency 0 ... 1.0 (Fs) graph. The literature says, "The frequency response of the filter is computed by passing the array of coefficients through the discrete Fourier transform (DFT)." The text goes on to show this nice smooth graph, plotting magnitude vs frequency from 0.0 to 1.0. I would like to reproduce that graph.
So I sent it through my DFT code, well a 34 point DFT generates a very blocky DFT and it looks nothing like the graphic. I wanted more points of the DFT (between 0 and 34) and tried adding fractional points 0.0, 0.1, 0.2, .. 34.0) and got a very "loopy" DFT with lots of humps (and also not at all like the graphic), I tried "upsampling" adding zeros between the values, still no joy. I tried allowing the tap sequence to repeat, no joy. So I'm really curious how its done. Sending it to the MATLAB plot function produces the expected graph, so I have some confidence the data is good. Just my understanding of the mechanics of how to get the higher resolution graph are incomplete.