Honeywell
COMPONENT MAINTENANCE MANUAL
PART NUMBER 964-0452
1I.B.1516A Page 12
Mar 30/01
23-12-01
(4) The HFDR then tunes to the first frequency and listens for a periodic uplink (called a
squitter) from the ground station. If no squitter is detected within 32 seconds, the HFDR
tunes to the next frequency on its list.
(5) The HFDR attempts to send a "log-on" downlink when a valid squitter is received. If the
HF Ground Station correctly decodes the downlink, it indicates this in subsequent squitter
uplinks. When this is received by the HFDR, it considers itself logged-on and indicates to
the ACARS MU that data mode is available. The indication on the ACARS display unit
changes from unavailable to available. Transmission of ACARS downlinks and reception
of uplinks is now possible. Note that the HFDR never transmits on an HF frequency
unless the frequency is in its database and a valid squitter is received on that frequency.
(6) If the "log-on" handshake is unsuccessful, the HFDR selects the next frequency from its
list. The HFDR reprioritizes its frequency list when it receives a squitter, since a squitter
contains a list of frequencies that is used by all HF Ground Stations at that moment. The
HFDR indicates that data mode is unavailable after it has exhausted all the frequencies
on its list, but continues to search for a valid connection.
(7) The HFDR begins a search for a new frequency if it fails to detect the periodic squitter or
fails to get a squitter acknowledgement for a downlink.
(8) The existing HF radio system typically shares a common antenna on the aircraft between
the 2 transceivers. If the HFDR detects a Push-To-Talk (PTT) on the other radio operat-
ing in voice mode, it delays the transmission of a pending downlink for about 1 minute
(database parameter) to allow the flight crew to complete their voice conversation.
(9) The flight crew may use the HFDR operating in data mode for voice operation at any time
by placing the unit in voice mode via the cockpit control function.
4. Theory of Operation
A. General
The Transceiver performs reception and transmission of voice and data in the frequency
range 2.0MHz to 29.999MHz. The nominal output power of the transmitter is 400W PEP with
a maximum average power of 125W.
A simplified block diagram of the Transceiver is shown in Fig. 5, and a Transceiver intercon-
nect diagram is shown in Fig. 6.
The drive signal for the RF Amplifier, A3, is generated in Receiver/Exciter, A1. The input audio
signal (voice audio from microphone input MP-1A/1B, external audio from AF data input
MP-1H/1J, or internal HFDL data audio from Modem, A10), is digitized in this module and
converted to the necessary modulation signals (SSB or AM(E)) in a signal processor with the
aid of digital signal processing algorithms at a 25kHz IF. This signal undergoes analog con-
version again and is up-converted to the final RF level via 2 mixers.
The frequencies of the up-conversion oscillators are generated in a synthesizer according to
the method of digital direct signal synthesis (DDS). This synthesizer also generates the fre-
quencies of the down-conversion oscillators for the receive direction.
