  We demonstrate that conventional modulated spectroscopy apparatus, used for
laser frequency stabilization in many atomic physics laboratories, can be
enhanced to provide a wideband lock delivering deep suppression of frequency
noise across the acoustic range. Using an acousto-optic modulator driven with
an agile oscillator, we show that wideband frequency modulation of the pump
laser in modulation transfer spectroscopy produces the unique single lock-point
spectrum previously demonstrated with electro-optic phase modulation. We
achieve a laser lock with 100 kHz feedback bandwidth, limited by our laser
control electronics. This bandwidth is sufficient to reduce frequency noise by
30 dB across the acoustic range and narrows the imputed linewidth by a factor
of five.
