Let's start with arbitrary transfer function as an example, such as:
obj = tf([1 2],[1 0 2 5]); nyquist(obj); grid on; axis([-2 2 -2 2]); axis equal
As far as I do understand what is gain (expressed in dB), I am unable to figure out, nor find any information about circles that appear after we use
grid. For some reason there is vertical line at
Re=-0.5 associated with
0 dB value. On the left and right of this line we see non coincident circles in the shape of peacock's eye with decreasing or increasing values. Circles converge to points
We can see that system's curve starts at point
[-0.4,0] which corresponds to
-7.9588 dB gain, crosses
-10 dB circle twice, where on both occurrences system's gain doesn't match
-10 dB value.
Moreover, just from the fact that that we have infinite line denoted with
0 dB value, one can infer that it has absolutely nothing to do with actual gain, which on Nyquist plot should be circular around point
[0,0]. In this case my question is simple: what does those dB values and circles mean?