OPTION 006 Avionics
Annex-A-30
Accuracy of SDM (Sum Depth of Modulation) signal path
Specification
±2% of setting for carrier frequencies up to 400 MHz
The signal generator has a 1 dB AM bandwidth from DC to at least 30 kHz and consequently the
difference in AM response between 90 Hz and 150 Hz will be small and can be neglected.
Because of this, it is possible to test the SDM and the DDM accuracy of the instrument by the use
of a single tone modulation at 124 Hz rate (the geometric mean of 90 Hz and 150 Hz).
The 0% DDM signal is generated using the AM 1 channel. The phase shifted signal is generated
on the AM 2 channel. The design of the instrument ensures that the AM accuracy of each channel
at a fixed frequency is equal to the accuracy of the ILS waveform generated by the instrument.
Test equipment
Description Minimum
specification Example
Modulation meter*
50 kHz to 1 GHz
AM accuracy
±0.5% of reading at 40% depth
±0.7% of reading at 80% depth
Selectable ILS filter
IFR 2305
*The SDM and DDM accuracy of the Avionics Option can not be verified with a modulation meter calibrated to its
published performance specification. A specially calibrated modulation meter is necessary. If more than an approximate
verification is required, consult your local IFR agent or the Service Division for further details.
C0332
RF
OUTPUT
UUT
Modulation Meter
RF
INPUT
➤
Fig. A-28 Accuracy of SDM signal path test set-up
(1)
Connect the test equipment as shown in Fig. A-28.
(2)
Set up the test equipment as follows:-
Unit under test
[UTIL] [
Mod'n Mode] [Composite]
[SIG GEN]
Carrier freq
108.1 MHz
RF level
7 dBm
[AM]
AM 1 depth
40 %
[Select Source] [Internal F1]
120 Hz sinewave
[SIG GEN]
AM 1
Off
AM 2 depth
0 %
[Select Source] [Select Internal] [Internal F2]
124 Hz sinewave
[SIG GEN]
AM 2
Off