The ASI-413-2 provides the stable, precision 90 and 150 Hz tone signals required to modulate RF signal generators used for testing and alignment of airborne ILS receivers. The design of the unit allows flexibility for interfacing with existing RF signal generators. The 90 and 150 Hz localizer and glide slope modulation signals, generated by the ASI-413-2, have a high degree of frequency and amplitude stability, which is maintained by an automatic calibration loop. Front panel switching is provided for selection of various differences in the depth of modulation (DDM) test signals for both localizer and glide slope functions.