creating sine wave with variable amp and freq using matlab function block goes wrong !!

i'm trying to create sine wave with variable freq and amp decaying with time but every thing goes wrong in the middle as u can see and i can't figure it out till now can any one help me with this problem !? thanks in advanceis

my code is

function [Va,Vb,Vc,] = BACK_EMF_ph(clk,f,amp)

Va=amp*(sin(f*2*pi*clk));

Vb=amp*(sin((f*2*pi*clk)-((2*pi)/3)));

Vc=amp*(sin((f*2*pi*clk)+((2*pi)/3)));

[Sindar]

include your code

[mostafa amr]

done

[Sindar]

How are you calling it, and what are f and clk (why are they separate parameters?)?

[mostafa amr]

f is the frequency and it is decaying and clk is the clock to get the real time of the simulation

[mostafa amr]

and this is the shape of freq variation with time