def _order(tf):
    """Returns the number of coefficients in numerator and denominator

    Parameters
    ----------
    tf : TransferFunction
        The transfer function object

    Returns
    -------
    nnum : int
        Number of coefficients in numerator.
    nden : int
        Number of coefficients in denominator.
    """
    nnum = len(tf.minreal().num[0][0])
    nden = len(tf.minreal().den[0][0])
    return nnum, nden