OPTION 006 Avionics
Annex-A-8
DDM control
From the SDM Selection Menu (Fig. A-4) DDM may be entered by pressing the
[DDM]
key to
obtain the display shown in Fig. A-5.
Fig. A-5 DDM selection menu
The DDM can be entered in %, as a modulation index (%/100) or in microamps (
μ
A) 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 log10
⎭
⎬
⎫
⎩
⎨
⎧
−
+
DDM
SDM
DDM
SDM
μ
A
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
μ
A
≡
15.5% DDM on the localiser
150
μ
A
≡
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. A-6 is produced.