ACCEPTANCE TESTING
5-1-28
AVIONICS TESTING
The NAV-750C uses a method of Direct Digital Synthesis (DDS) to generate the
required avionics waveforms. The accuracy of the generated waveform is therefore determined
by stored digital data and the AM performance of the signal generator. These tests check the
waveform generation and analogue and RF signal paths in the signal generator. Additional tests
can be undertaken using specialized ILS and VOR receivers if they are available.
Functional testing of ILS waveform generation
TEST EQUIPMENT
Description Minimum
specification
Example
Spectrum analyzer
100 Hz to 400 MHz
Aeroflex AN920
Oscilloscope
100 MHz bandwidth
Tektronix 2235
Aeroflex uses a method of Direct Digital Synthesis (DDS) to generate the ILS
waveforms. One DDS generated waveform contains both the 90 Hz tone and the 150 Hz tone
with 0% DDM (Difference Depth of Modulation).
A second waveform is generated with 0% DDM but with the relative phase of the 150 Hz
tone reversed compared to that of the 90 Hz tone.
When small levels of the second waveform are added to the first, the resultant waveform
has a constant SDM and adding a proportion of the second waveform will result in the DDM of
the resultant signal changing.
Fig. 5-1-15 ILS functional test set-up
(1)
Connect the test equipment as shown in Fig. 5-1-15.
(2)
Set up the test equipment as follows:-
Unit under test
ILS mode
Carrier freq
108.1 MHz
RF level
7 dBm
SDM 40
%
ILS rate
30 Hz
DDM 0
%
90 Hz dominant
Fly RT
