OPERATION
3-1-17
The sum of depth of modulation (SDM) is the arithmetic sum of depth of the modulating
90 Hz and 150 Hz tones. Using the
[SDM]
key the depth can be entered in %. The ILS rate is
normally set to 30 Hz and is the ILS waveform repetition rate. Its frequency can be modified
using the
[ILS Rate]
key. If the ILS rate is adjusted the frequency of the 90 Hz and 150 Hz alter
on a pro-rata basis (e.g. If
ILS Rate
= 29 Hz the tone frequencies are 87 Hz and 145 Hz).
DDM control
From the SDM Selection Menu (Fig. 3-1-10) DDM may be entered by pressing the
[DDM]
key to obtain the display shown in Fig. 3-1-11.
Fig. 3-1-11 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 log
10
⎭
⎬
⎫
⎩
⎨
⎧
−
+
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 localizer 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 localizer
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.
