1.1 The Modulation Meter Type 409 is a wide range, highly sensitive instrument designed to measure percentage amplitude modulation (a.m.) of carriers from 3 - 1200MHz, and frequency deviation (f.m.) on carriers from 3 - 1500MHz. The meter scales are 0 - 10%, 0 - 30% and 0 - 100% a.m. and 0 - 3kHz, 0 - 10kHz, 0 - 30kHz, 0 - 100kHz, 0 - 300kHz and 0 - 600kHz deviation. Outputs at the intermediate frequency (nominally 1MHz) and at l.f. are available at front panel terminals and an in-built, switched noise filter permits measurement of very narrow f.m. deviations using a sensitive external l.f. millivoltmeter. 1.2 Changes in mean carrier level when a.m. is applied can be measured to better than 1% accuracy and effective i.f. limiting permits measurement of spurious f.m. on a.m. 1.3 An f.m. calibrator circuit, which can be checked directly on an electronic counter, is provided within the instrument to enable periodical calibration to be carried out. Tuning the instrument at very high frequencies has been simplified by the provision of a fine tuning control and a SET I.F. position on the meter switch. In this position the tuning control is simply adjusted until the meter reads to the SET I.F. mark, thus eliminating tedious peaking procedures. 1.4 The instrument is fully stabilised against changes in mains supply voltage of ±10% from the value set on the mains voltage selector. The power consumption is approximately 120 VA. The instrument is supplied in a metal case with quickly detachable top, base and sides. A rack mounting kit is available.