I've never understood what these graphs or "plots" are for. Are there any guides that explain what they mean and how to read them? And how come it's called a "gnu" plot?
They are called "gnu" plots because a utility called 'gnuplot' is responsible for displaying them
Their purpose is to display different aspects of the digital signal processing to aid in setup, debugging and performance analysis. Max knows way more about this stuff than I do, but from my perspective I use the plots as follows:
#1 [fft] - shows the frequency domain across the full section of spectrum being sampled. i.e. Whatever slice of RF spectrum you have told your SDR device to receive will be shown in the plot window. Ideally you expect to see the desired signal peak at the middle of the tuning line, but you may see other nearby signals as well. You can use the FFT plot to broadly set up the PPM error correction (-q parameter) by adjusting correction value until your signal peak is centered.
#2 [constellation] - shows an X-Y plot of the complex IQ signal. In practice this plot is used to examine PSK signals, which in the case of P25 usually means CQPSK (quadrature phase shift keying) with 4 possible positions (the four "corners") along with whatever noise and distortion may also be present. An ideal signal has just 4 corners tightly clustered together. When the signal quality degrades, the clusters become broader and you start seeing an 'X' pattern or even just a randomly distributed noise.
#3 [symbol] - shows the 4 levels of decoded symbol data. Should be nice straight lines at +3, +1, -1, -3. May be displaced vertically by tuning errors and will show out of position plot points when the decode isn't clean (i.e. noisy signal).
#4 [datascope] - can be used to differentiate between c4fm (4 level frequency shift keying) and cqpsk (quadrature phase shift keying) by observing the size/number of "eyes" (openings) in the plot. This is one of those topics that Max knows inside-out whereas my head starts hurting... best left for him to explain!
#5 [mixer] - this plot is in the boatbod fork only and is a visual representation (fft plot) of the mixer output prior to c4fm/cqpsk decoding. It's purpose is to allow simplified adjustment of ppm (-q) and fine tuning (-d) by centering of the plot either side of the tuning line. The numeric "balance" value is an instantaneous summation (integral) of the two sides of the plot and should ideally be as close to 0 as possible.
