User's Manual

1.3.2.4.2 CCA, Receiver RF Block Diagram Theory
This section describes the details of the Receiver RF CCA. Throughout this section refer to Figure 1-9 and
012181-9001 schematic.
Figure 1-9 Receiver RF Block Diagram
The Receiver RF CCA uses a dual heterodyne receiver with a first intermediate frequency (IF) of 125MHz
and second IF of 10.7 MHz. The receiver has a calculated noise figure of approximately 4dB which, when
coupled with the external fixed-loss components such as the circulator, preselector, and power splitter
provides a maximum receiver sensitivity of minus 94dBm.
The usable receiver sensitivity is configurable via the PMDT interface from -94dBm to -72dBm. This
allows the installation to match the receiver range to the transmitter range. By default the receiver
sensitivity is set to -82dBm for 100W stations and -87dBm for 1000W stations. These long-established
settings ensure that the DME station only replies to aircraft within the working range of the transmitter
operating power. For instance a 100W DME transmitter reply can only reach aircraft within a 40NMI
radius; therefore the receiver would be unnecessarily overloaded with aircraft interrogations within
200NMI if the receiver sensitivity was maximized to -94dBm.