Error fit: Complex value computed by model function

Hi I want to fit my data through the equation: but it appears the error: Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.

[Pietro Scapolo]

The equation I put in the fitting tool is:

5.*e.^(-a.*x)./(sqrt(b.^2 - a.^2)) .*(a.*sin(x.*sqrt(b.^2-a.^2))+sqrt(b.^2-a.^2).*cos(x.*sqrt(b.^2-a.^2)))

[Alex Sha]

There are two parameters (a and b) only, the result will not be good:

Sum Squared Error (SSE): 779.783298373557
Root of Mean Square Error (RMSE): 0.279246002365935
Correlation Coef. (R): 0.843000031198818
R-Square: 0.710649052601208

Parameter	Best Estimate
----------	-------------
a	3714.9148653265
b	-572016.731340392

Answer 1

[Torsten]

Fit under the constraint

b^2-a^2 >= 0