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 band- width, and are factory tuned for receiving in the 150-162 MHz bandwidth. 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 monolithic crystal filters hav- ing 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 external 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 micro- phone is passed through a 3 kHz low pass filter, preempha- sized 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 at- tenuates frequencies above 20 kHz by more than 54 dB be- fore 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 attenu- ates 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 deliv- ers 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 fil- ter 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 frequen- cies is filtered through a 6 kHz high pass filter; amplified and then limited. A DC detector generates a DC voltage rep-
resenting the amplitude of the noise. This DC voltage is ap- plied to a comparator. The out-put from the comparator is fed to the microprocessor The microprocessor squelches the receiver under conditions of high noise and absence of car- rier.
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 de- tector 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 (N29/85154000950) 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 re- cessed 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 numbers 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 description of the indicators, controls and operation of the radio.
The microphone connector is an RJ-11 type, female con- nector 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 magnetic sensing is provided with the radio to be mounted externally on the dashboard of the vehicle. Re- moving 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 (N29/85154000930) con- tains the Power Amplifier Board mounted to an aluminum die- cast heatsink. The PA Board contains the RF Power Amplifier, 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 ex- citer 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 148- 174 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 an- tenna jack. The IGN_A + External 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 mi- crophone connector on the front of the radio. Refer to TQ-3363 for a complete set of instructions on how to program the MDS Conventional radio.
Tracking Data
The test handset is used to make operational circuit adjust- ments in the radio using digitally controlled potentiometers. The resulting "Tracking Data" is stored in the EEPROM per- sonality of the radio. A utility in the PC Programming software can be used to read the Tracking Data. All programming 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 re- ceiving a call and muting the receiver audio path. This is com- monly referred to as reducing squelch tails. The STE burst is transmitted when the microphone PTT is released. The receiv- ing radio de codes 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 99 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 pro- gramming, 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 OP- ERATION AND PROGRAMMING, on page 11, for more detailed information on Type 99 Decode.
Automatic Number Identification (A.N.I.)
As an option, one ID number per radio is programmed on a per channel basis. The microprocessor generates the special 1600 Hz A.N.I signal which is coded with a 4-bit phase revers- ing code. It is sent at a 400 baud rate as part of the handshak- ing preamble to a repeater just after PTT has been pressed, or alternatively just after release of the PTT key. A.N.I. is re-
