def human_hz(v):
    """Returns a number of Hz autoselected for Hz, kHz, MHz, and GHz
    """
    if v < 1e3:
        return (v, 'Hz')
    if v < 1e6:
        return (v/1.0e3, 'kHz')
    if v < 1e9:
        return (v/1.0e6, 'MHz')
    return (v/1.0e9, 'GHz')