Player jitter when jump with Fixedupdate

I used fixed update in my player movement script , every thing is good , except Jumping , my player jitter when jump because of Fixed Update , when i use update it become better ... but i dont want to use update I want to use fixed update , can someone help can i use update and fixed update together? or can someone tell me how to use fixed update without getting any problem? this is my script :

public class PlayerMovement : MonoBehaviour {

  public float _gravity = 1.5f;
  public float _yVelocity = 0.0f;
  public float _moveSpeed = 25.0f;
  public float _jumpSpeed = 40.0f;
  public CharacterController _controller;
 
  private bool _onGround;
  Animator anim;

void Start () {

      _controller = GetComponent<CharacterController>();
      anim = GetComponent<Animator>();
      rb = GetComponent<Rigidbody>();

} void FixedUpdate () {

        _onGround = _controller.isGrounded ; //Gets if the controller is grounded
      Vector3 direction = new Vector3(SimpleInput.GetAxis("Horizontal"), _yVelocity, SimpleInput.GetAxis("Vertical"));
      if (direction.sqrMagnitude > 1f) direction = direction.normalized;
      Vector3 velocity = direction * _moveSpeed; 

         if (_onGround )
      {
          //JUMPING
          if (CrossPlatformInputManager.GetButtonDown("Jump"))
          { 
              _yVelocity = _jumpSpeed; 
           //  rb.velocity += new Vector3(0 , _jumpSpeed , 0);
          
       
      }
      else
     
          _yVelocity -= _gravity;
          
       
        
      _controller.Move(velocity * Time.deltaTime);
      Vector3 facingrotation = Vector3.Normalize(new Vector3(SimpleInput.GetAxis("Horizontal"), 0f, SimpleInput.GetAxis("Vertical")));
      if (facingrotation != Vector3.zero)         //This condition prevents from spamming "Look rotation viewing vector is zero" when not moving.
          transform.forward = facingrotation;

} }