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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
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
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
∙Channel Number Display,
∙Transmit Light, lights when radio is transmitting
∙Busy Light, ON when channel is busy
See the Operator’s Manual
The microphone connector is an
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
∙MDS Programming Cable
∙MDS Conventional Programming Software
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
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
Carrier Control Timer
The Carrier Control Timer (CCT) turns off the transmitter after the microphone
Type 99 Decode
Type 99
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
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