OPTION 006 Avionics
Annex-B-8
DDM control
From the SDM Selection Menu (Fig. B-4) DDM may be entered by pressing the [DDM]
key to obtain the display shown in Fig. B-5.
Fig. B-5 DDM selection menu
The DDM can be entered in %, as a modulation index (%/100) or in microamps (μA) and
displayed on the Sig Gen menu in %, as a modulation index, microamps or as the attenuation
ratio between the 90 Hz and 150 Hz tones in dB, according to the formula:
R
dB
= 20 log
10
⎭
⎬
⎫
⎩
⎨
⎧
−
+
DDM)(SDM
DDM)(SDM
μA
If the setting to be entered is a modulation index terminate the entry with the [enter] key.
Alternatively, if the setting to be entered is in microamps terminate the entry with the [ms] key.
The microamp entry is converted to the nearest 0.01%. If the carrier frequency is below
200 MHz the signal is assumed to be for a localiser and if it is above 200 MHz the signal is
assumed to be for a glideslope. The following conversion factors are used:
150 μA ≡ 15.5% DDM on the localiser
150 μA ≡ 17.5% DDM on the glideslope
The units displayed can be changed without entering any data. Press the [enter] and [%]
keys to toggle between modulation index and percentage. Press the [ms] and [dB] keys to toggle
between microamps and attenuation ratio.
The [Fly/LT/RT] key can be used to set which tone has the greater depth of modulation
and the dominant tone is displayed under the DDM set. When 90 Hz is dominant the aircraft is
either to the left (localiser) or too high (glideslope). The corresponding action to be taken is
displayed as FLY RT and FLY DN. Similarly when 150 Hz is dominant the aircraft is either to
the right (localiser) or too low (glideslope). The corresponding action to be taken is displayed as
FL LT and FLY UP. If 0% DDM is set the additional text ON COURSE is displayed.
DDM values can be entered using the DDM presets which provide a fast method of
selecting commonly used DDM values. The DDM presets are selected using the [Presets
ON/OFF] key and Fig. B-6 is produced.
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