This is why all of your t_start values are the same.  You are changing the t_tart value in-place, which means all analogsignals get the same t_start variable.  That variable changes each cycle of the loop.

You should do something like this:

```
t_start = t_start + length*dt
```

That will create a new t_start variable each cycle of the loop, so each analogsignal will have an independent value.