COE Communications Engineering Document
Author:
Keith Gilbertson
Rev:
A
Date:
3/08/02
Pag
e:
26
of
33
GE
TRANSPORTATION
SYSTEMS
GLOBAL SIGNALING DIVISION
Title:
12RII FCC TECHNICAL MANUAL
10.2.11. Receive Signal Strength Indication (RSSI)
The RSSI output of the TXRX module is applied to A/D 2 input of U3. This input is processed by software and used during
scan operations to provide quick channel busy detection. The RSSI signal level is also be displayed as a graphic during
channel occupancy for information to the user.
10.2.12. MSK Digital Data
The MX829 device provides from 1200/2400 MSK data encoding and decoding. This will be used for radio ANI broadcast or
data transmission at a future time. MSK modulation encoding for 1200 baud signal will generate one cycle of 1200 Hz. for a
logic “1” and one and a half cycles of 1800 Hz. for a logic “0”. MSK modulation encoding for a 2400 baud signal will
generate a half cycle of 1200 Hz for a logic “1” and one cycle of 2400 Hz for a logic “0”. Data modulation is not routed
through the pre-emphasis circuit. It does pass through the standard audio BPF and limiter circuitry in the MX829 device.
10.2.13. Radio TX Signal Paths
Voice Transmission
Voice transmission is initiated by activation of the HS-PTT, REM-PTT or a serial PTT command from a control head. The
appropriate audio map is loaded which will select the assigned input to U10-CH1 (HSMIC, MIC1 or MIC2). They standard
voice transmission audio map will set the gain of U10-CH1 and enable output B. The voice signal will be applied to the pre-
emphasis circuit and be routed through U10-CH2 to be applied to the MIC input of U13 (MX829). Internal to the MX829 the
signal will be filtered. The radio channel programming will determine if the filtering is for wide or narrow band channel
operation. The signal will then pass through a limiter typically set for 80% of maximum system deviation. The output of the
limiter stage is applied to a summing amplifier. The summing amplifier output is routed through two stages of digital
attenuators which are used to set the maximum deviation level. The modulation signal output is applied to inputs of the two
digital amplifiers of U3. Amplifier 1 drives the MB (TCXO) modulation signal and amplifier 2 drives the MO (VCO)
modulation signal.
10.2.14. Radio RX Signal Paths
In Receive mode the output of the TXRX module is applied to analog channel 1 input of U6. The output of this amplifier is
applied to the RX input of U14 (MX828). The input amplifier of U14 amplifies the signal. The output is coupled internally to
the input of the audio Band Pass Filter and to the sub-audible and tone decoders for decoding if necessary. The external output
of the amplifier is connected to the Demodulation Input of U13. This input is coupled to the MSK decoder in data applications.
The output of the BPF in U14 is routed to the input of Mod. 1 Attenuator of U14. If the squelch criteria has been satisfied, the
Mod 1 output is enabled and the receive signal passes through the de-emphasis network, amplified
By CH3 of U9 and applied to U9-CH1, U9 CH2, U17-CH1, and U17-CH2 input #2. Depending on the Audio Map loaded for
receive mode, the outputs of U9 and U17 are enabled with the proper gains to drive the appropriate audio speaker or device.
10.3. Audio
Drivers
10.3.1. Low Level Audio Drivers
Amplifiers U7-A, U7-B and U12-A are used as low level drivers. Each of the amplifiers is connected an output of U9 or U17.
The amplifiers are non-inverting DC amplifiers with a gains of 3. The outputs are cap coupled through a series impedance of
604 ohms before connection the their respective load devices. The series impedance is used to buffer the low impedance of
handset elements to the driver amplifier. HSAUD signal drives the radio handset element at connector J2. AUD1 signal is used
as low-level audio to control head 1. AUD2 signal is used to drive a remote handset connected to J6 or control head 2 audio.
10.3.2. Speaker Drivers
Two independent speaker drivers, U15 and U16 are used for different applications. Each speaker amplifier has a standby
input that is used to mute the speaker when the radio is not receiving. This signal is controlled by the radio processor module.
The amplifiers also contain a diagnostic signal output which indicates when the amplifier is clipping or has been shut down due
to an output fault condition. These diagnostic signals are monitored by the RPM.
