The evalfr command allows you to evaluate the frequency response of a system at any point in the complex plane. However, bode evaluates points only on the:
- Imaginary axis (s = jw) for continuous-time systems.
- Unit circle (z = e^jwT) for discrete-time systems.
Therefore, the reason you are getting inconsistent results between bode and evalfr is that you are using the s-domain complex variable with your discrete-time system rather than the z-domain variable.
The following code shows how to get the same response at a given frequency using bode and evalfr:
%%Create discrete-time system.
T = 1/1000;
sys = tf([4700 4393 3.245e08],[1 7.574 1.202e5 0 0],T);
%%Frequency = 50 Hz
w = 50*2*pi;
%%bode result
[magB,phaseB] = bode(sys,w)
%%evalfr result for z = e^jwT
resultE = evalfr(sys,exp(j*w*T));
magE = abs(resultE)
phaseE = angle(resultE)*180/pi
This information is not clearly conveyed in the documentation. I've updated the enhancement request in our tracking system, and we will update the documentation in a future release.
