Motorola LBI-38653 - Manual (Page 4)

Receiver
The RF front end of the receiver includes the PIN diode T/R
switch, the tuned bandpass preselector filters, and the low noise
RF amplifier. The filters have a 12 MHz bandwidth, and are tuned
for the specific split in either of the two UHF Bands. The receiver
is factory tuned to 450-462 MHz or 470-482 MHZ. Refer to the
Service Section LBI-38759 for retuning to a different frequency.
The first mixer (with its low side injection from the VCO)
generates a 45.000 MHz IF signal which is filtered with mono-
lithic crystal filters having a 15 kHz bandwidth. A 455 kHz low
IF circuit module contains the second local oscillator, the second
mixer with low side injection, external ceramic filters, an exter-
nal 44.545 MHz crystal for the oscillator and the FM quadrature
detector circuit.
Transmit Audio Processing
In the transmit audio path, the audio from the microphone
is passed through a 3 kHz low pass filter, pre-emphasized and
hard limited for a maximum modulation peak deviation of 4.5
kHz. A MIC Mute switch is controlled by the logic as appropriate
for tone or voice modulation of the transmitter. Channel Guard
tones and the A.N.I. signal from the microprocessor are summed
into the audio path before the gain control. Next, a 3 kHz post
limiter low pass filter attenuates frequencies above 20 kHz by
more than 54 dB before passing the audio signal on to the
modulation input of the VCO.
Receive Audio Processing
Audio from the demodulator enters the voice path through
a 300 Hz high pass filter, followed by a 3 kHz low pass filter. A
"Receive Mute" switch is provided so the voice signal can be
muted by the microprocessor when it sums its Alert tones into
the path. The "Receive Mute" switch attenuates the voice audio
by 50 dB when the Alert tones are summed into the path. The
path is completed with a gain control, and a logic controlled
switch for speaker muting while transmitting. The audio power
amplifier which delivers 4 watts to the speaker is connected on
the Main Board, but is mounted on the aluminum chassis for heat
sinking.
Tone And Squelch Processing
The Channel Guard path includes a 220 Hz low pass filter
for passing the received Channel Guard tones without audio. A
tone limiter insures a 5 volt pp maximum input tone level to the
microprocessor.
In the Squelch path, noise above the audio voice frequencies
is filtered through a 6 kHz high pass filter; amplified and then
limited. A DC Detector generates a DC voltage representing the
amplitude of the noise. This DC voltage is applied to a compa-
rator. The output from the comparator is fed to the microproces-
sor. The microprocessor squelches the receiver under conditions
of high noise and absence of carrier.
Since the frequencies of Type 99 tones fall in the voice band,
they are passed through the 300 Hz high pass filter in the voice
path. Next, the tones are shunted through a closing switch and
passed into the tone path in front of the tone limiter. The tone
limiter passes the tones on to the tone detector port of the
microprocessor.
Voltage Regulator Circuitry
The battery voltage (IGN A+) enters the radio at J11 on the
Power Amplifier Board. From the PA Board, IGN A+ is routed
directly to the Main Board where it is filtered and passed through
a relay as A+_SW. A+_SW is fed to the regulator circuits which
provide a regulated 8, 5 and 2 volts to the appropriate circuits
throughout the radio. A+_SW is also fed to the Audio Power
Amplifier circuits.
FRONT PANEL ASSEMBLY
The Front Panel Assembly (N29/SMR18) houses the Front
Panel Board, the internal 4-watt speaker, and all control switches
and indicators for the radio. The panel is made of highly durable
plastic with rounded corners and recessed controls and indicators
for passenger safety. The Front Panel Board mounts on back of
the Front Panel. The following display elements are on the front
panel:
•
Channel Number Display, 7-segment LED with num-
bers 1-8
•
Transmit Light, lights when radio is transmitting
•
Busy Light, ON when channel is busy
See the Operator’s Manual LBI-38659 for a detailed de-
scription of the indicators, controls and operation of the radio.
The microphone connector is an RJ-11 type, female connec-
tor mounted to the Main Board but located on the front panel of
the radio. A strain relief is provided to secure and protect the
microphone connector. A microphone Hook Switch with mag-
netic sensing is provided with the radio to be mounted externally
on the dashboard of the vehicle. Removing the microphone from
the Hook Switch disables the Channel Guard, enabling the
operator to monitor the channel before sending a message.
POWER AMPLIFIER ASSEMBLY
The Power Amplifier Assembly (N29/SMR13 for the 440-
470 MHz band and N29/SMR12 for the 470-512 MHz band)
contains the Power Amplifier Board mounted to an aluminum
diecast heat sink. The PA Board contains the RF Power Am-
plifier, the RF output LP harmonic filter, the PIN diode T/R
switch, and a stripline directional coupler for power output
sensing.
The RF Power Amplifier is driven by the output of the
exciter on the Main Board, which is connected by coax to the
PA Board. The PA Board amplifies the output from the Main
Board to a level of 40 watts over the frequency range of both
band splits (440-470 MHz & 470-512 MHz). The output of
the PIN diode T/R switch is coaxially connected to the receiver
front end circuitry on the Main Board. The T/R switch has a
separate connection to the antenna jack. The IGN_A+, Exter-
nal Speaker and External Alarm lines connect to the Power
Amplifier Board at J11.
FEATURES
PC PROGRAMMABLE
The entire personality of the radio is programmed into the
radio using an IBM or IBM compatible personal computer and
the following equipment:
•
Serial Programming Interface Module TQ-3310
•
MDS Programming Cable TQ-3361
•
MDS Conventional Programming Software TQ-
3363
The interface module is connected between a serial port
on the computer and the RJ-11 microphone connector on
the Front Panel of the radio. An RS-232 cable connects the
serial port to the Interface Module. The Programming Cable
(TQ-3361) is connected from the Interface module to the RJ-
11 microphone connector on the front of the radio. Refer to
TQ-3363 for a complete set of instructions on how to pro-
gram the MDS Conventional radio.
Tracking Data
The test handset is used to make operational circuit ad-
justments in the radio using digitally controlled potentiome-
ters. The resulting "Tracking Data" is stored in the EEPROM
personality of the radio. A utility in the PC Programming
software can be used to read the Tracking Data. All program-
ming is done through the microphone connector on the Front
Panel of the radio, without the need of opening the radio
package. The Tracking Data adjustments made with the test
handset include the following:
•
Reference voltage for Transmit RF power output
level
•
Voltage for VCTCXO frequency adjustment
•
Modulation deviation adjustment for VCO
•
Channel Guard modulation deviation adjustment
•
Microphone gain adjustment, 2 pots in voice path
•
Other filter adjusting pots
Programmable Features/Options
Squelch Tail Elimination
Squelch Tail Elimination (STE) is used with both tone and
digital Channel Guard to shorten the noise burst between
receiving a call and muting the receiver audio path. This is
commonly referred to as reducing squelch tails. The STE burst
is transmitted when the microphone PTT is released. The
receiving radio decodes the burst and mutes the receiver audio
for 250 ms. The duration of this mute time includes time for
the end of transmission plus that of the squelch tail. The radio
looks for STE on the received signal when the microphone is
either ON or OFF-hook. The STE is enabled for transmit
and/or receive by PC programming the radio’s personality.
Carrier Control Timer
The Carrier Control Timer (CCT) turns off the transmitter
after the microphone push-to-talk (PTT) switch has been
keyed for a pre-programmed time period. A pulsing alert tone
will warn the operator to unkey and then rekey the PTT to
continue the transmission. The timer can be programmed to
time out for 15 to 225 seconds in 15 second increments. The
CCT will be the same for all channels in the personality.
Type 99 Decode
Type 88 2-tone sequential selective calling is programma-
ble on a per channel basis, with the radio enabled to decode
only. On a channel where Type 99 has not been selected in
programming, the Type 99 tone decoder is disabled. The Type
99 selective calling option operates in either Type 99 Select
Mode or Type 99 Monitor Mode. The default mode can be
designated in programming. See the section DETAILED
TYPE 99 OPERATION AND PROGRAMMING, on page
11, for more detailed information on Type 99 Decode.
LBI-38653 LBI-38653
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