Features, Front Panel Assembly | Motorola LBI-38653 guide

LBI-38653

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.