OPTION 006 Avionics
Annex-A-31
Modulation meter
tune Autotune
Function AM
Second function 28
On (see note)
Second function 28 selects an ILS filter. This is a 15 kHz low-pass filter selected separately from,
and without any accompanying high-pass section. This provides sufficient high frequency
bandwidth for the AM ILS signal without admitting an excessive amount of noise. In the left-
hand window a 1 selects the filter and a 0 deselects it.
(3)
With AM channels 1 and 2 turned off, allow the modulation meter reading to settle, then
measure the residual AM noise and note the reading.
Residual AM noise depth
____________%
(4)
On the UUT turn the AM 1 channel on. Allow the reading on the modulation meter to
settle and note it. (This is equivalent to an SDM of 40 %.)
Measured
AM
depth
____________%
Actual AM depth = Measured AM depth - Residual AM noise
Set
AM Minimum
Actual
Maximum
40% 39.2%
______%
40.8%
(5)
Repeat (4) for an AM 1 depth of 80% (this is equivalent to an SDM of 80%) with the
carrier frequency set to 330 MHz.
Set AM
Minimum
Actual
Maximum
80% 78.4%
______%
81.6%
Accuracy of DDM (Difference Depth of Modulation) waveform
Specification
±
0.03% of depth
±
0.02 of setting
This specification is checked by measuring the modulation channel balance at 19.9% and 40%
SDM (with and without final divide-by-5 internal attenuator).
Test equipment
Description Minimum
specification Example
Spectrum analyzer
or
FFT analyzer
DC to 25 kHz
3 Hz filter
IFR 2382
(1)
Connect the test equipment as shown in Fig. A-29.
In order to select second function 28 it is first necessary to unlock the 2305 to its 1st level of
protection (this is detailed in the operating manual).