EFJohnson 7780 service manual Transmit Data Filter IC206B/ IC206A, Transmit Audio Processing

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CIRCUIT DESCRIPTION

cut-off frequency is approximately 190 Hz. Q103 lowers the cut-off frequency of the filter by switching additional capacitance into the filter.

From the filter the data signal is applied to a DC restoration circuit formed by IC107B and IC107A. This circuit converts the data signal from AC floating at half supply to a digital signal at 0- and 5-volt levels that can be read by the microprocessor. IC107B is a standard noninverting amplifier with a gain of approx- imately 5.6 set by R139 and R140 (R140 is AC grounded by C129). Diodes D101 and D102 charge and discharge C129 to establish a DC reference on pins 2 and 6 of IC107 that is the average of the posi- tive and negative going alternations of the data signal.

The amplified data signal is applied to pin 3 of IC107A. When this level rises above the reference level on pin 2, the output on pin 1 goes high (5 volts). Conversely, when the data signal falls below the refer- ence level, the output goes low (0 volts). In the transmit mode, Q104 is turned on which effectively grounds pin 3 of IC107A and forces the output on pin 1 low.

4.7.2TRANSMIT DATA FILTER (IC206B/ IC206A)

The transmit data signal is produced by the TXDATA and TXPREC outputs of the micropro- cessor (pins 27 and 34, see Table 4-1). A pseudo sine wave is produced by the four voltage levels that can be produced by these outputs. This signal is fed to a transmit data filter formed by IC206B and IC206A. This filter is the same as receive data filter IC102B and IC102A described in the preceding section. This filter attenuates harmonics present in the waveform produced by the microprocessor.

The output and input of the data filter is fed to gate IC203. This gate is used to bypass the transmit data circuit during alignment so that a wide range of harmonic frequencies are passed for use in setting modulation balance. The control signal to this gate is the TEST signal from pin 35 of the microprocessor. This signal is low when the transmit data filter is bypassed.

The signal is then routed to pin 7 or gate IC210 which selects either it or the SMARTNET data signal

from IC113B. The data output signal on IC210, pin 1 is then applied to pin 24 of IC209 which is the input to one of the digitally controlled potentiometers in that device. The data modulation level is set during align- ment and controlled by the logic. The output on pins 22 and 23 is then fed to IC201 where it is combined with the transmit audio signal (see Section 4.8.3).

4.8 TRANSMIT AUDIO PROCESSING

4.8.1GATE (IC204), HIGH-PASS FILTER (IC202)

The microphone audio signal is applied to pin 2 of one of the gates in IC204. This signal may come from the internal microphone or an external micro- phone connected to pin 8 of the accessory connector. This gate is controlled by the MMUTE signal from pin 33 or the microprocessor. When this signal is low, the microphone audio signal is muted.

The transmit DTMF tone is applied to pin 6 of IC204 which is the input of the other gate in that device. This gate is controlled by the DTMFMT signal from pin 82 of the microprocessor, and when that signal is low, the DTMF tone is muted.

The transmit audio and DTMF signals are then combined and applied to pin 3 of compandor IC208. When companding is enabled by a low input on the PT input of IC208, this stage provides 2:1 compression of the audio signal. This compression and then re-expan- sion when it is received reduces the amount of noise present in the audio signal. When the PT input of IC208 is high, companding is disabled and the signal is fed through IC208 without compression. NOTE: Companding is not currently used with this radio.

The transmit audio/DTMF signal is fed out of IC208 on pin 2 and applied to a high-pass filter formed by IC202A. This filter has a –3 dB cutoff frequency of approximately 300 Hz to attenuate frequencies that could cause interference with the Call Guard signals.*

4.8.2 LIMITER (IC202B)

Pre-emphasis of the DTMF tone or microphone audio signals is provided by C205/ R211. IC202B is an amplifier functioning as a limiter (limiting occurs when it saturates). Limiting prevents overmodulation

 

November 1999

4-9

Part No. 001-7780-500

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Contents 7780 Page Page Table of Contents Table of Contents CONT’D List of Figures List of TablesScope of Manual Equipment Description General InformationFactory Customer Service AccessoriesPart Number Breakdown Transceiver IdentificationAccessory AccessoriesToll-Free 800 Factory ReturnsInternet Home Intrinsically Safe Information Replacement PartsDefinitions Possible Ignition Sources Temperature Code T3CIntrinsically Safe and Nonincendive Ratings Classification of Hazardous Areas and AtmospheresMaterial Classification Accessing PC BoardsTypical Hazard Group Class Area ClassificationRemoving Logic Unit Removing RF UnitGeneral SpecificationsTransceiver Operation FeaturesControls and Display Low-Battery Indicator Amber TX Indicator Red Indicates when the transmitter is keyedFront Panel Keys Full Keypad Model DisplayDisplay Option Switches Turning Power on and Setting VolumeGeneral Operation BacklightFunction Conv Option Switch FunctionsSee Scanning Tone ENABLE/DISABLEConventional and SMARTNET/ Smartzone Operation Display Mode Selection Conventional FeaturesMonitoring Before Transmitting Busy Channel Lockout Monitor ModeCall Guard Squelch Conversation Timer Penalty TimerPower Output Select Conventional Mode ScanningPriority Channel Sampling Standard Conventional Calls DTMF/ANI SignalingViewing Unit ID SMARTNET/SMARTZONE FeaturesStandard Group Calls Enhanced Private Conversation CallsTransceiver Operation Private Conversation II Calls Telephone Calls Call Alert Messaging Emergency Alarm and CallSending Status Conditions SMARTNET/SMARTZONE Scanning Failsoft OperationDynamic Regrouping Supervisory Tones Menu Description Keypad ProgrammingZone Change Parameter System Parameters Channel Change ParameterChannel Parameters Pctrunk Software Installation Programming SetupTransceiver Programming Computer DescriptionStarting and Exiting Connecting RPI to Computer and TransceiverProgramming File Types Help FilesProgramming Procedure Screen Pop-Up Buttons and File Size IndicatorFile Size Indication Creating and Displaying SystemsMenu Bar Menu CommandsDownload Menu Upload Menu Radio Type MenuSystems Menu Window MenuRADIO-WIDE Parameter Screens DefaultsBand ZonesRadio-Wide Scan List Screen Radio Wide Scan ListModify List Screen Assign Function ButtonsRADIO-WIDE SMARTNET/SMARTZONE Screen RADIO-WIDE Conventional ScreenFull Spectrum CC Scan Hot DtmfRADIO-WIDE Portable Options Screen Adjustable ParametersProgramming Conventional Systems and Channels Conventional System General ScreenConventional System Modify Scan List Screen TimersKeypad Editing Scan List ButtonSetting UP Conventional Channels Scan TimersPriority Channel Selection Selected Channel Conventional Channel Screen ParametersSignaling Transmit PowerChannel Type Tx Time-OutTime-Out Timer Restricted AccessISW Delay Time Splinter ChannelsSMARTNET/SMARTZONE System Phone Interconnect Screen SMARTNET/SMARTZONE System Other ID’S ScreenPhone Interconnect Private CallEmergency Alarm SMARTNET/SMARTZONE System Emergency Settings ScreenSMARTNET/SMARTZONE System Talk Groups Screen Failsoft ChannelTrunking Phone List Screen SMARTNET/SMARTZONE System Lists ScreensMessage Aliasing Screen Control Channels Screen Announcement Groups ScreenTrunking Call List Screen Status Aliasing Screen Setting UP SMARTNET/SMART- Zone ChannelsSMARTNET/SMARTZONE Channel Screen Parameters Other Screen Parameters SMARTNET/SmartZone Channel ScreenRecommended Tone Call Guard Codes Freq Call Guard CTCSS/DCS Codes and TonesMHz Channels MHz Channels MHz Channels MHz Channels MHz Channels MHz Channels Power Switching Power Switching and RegulationCircuit Description Synthesizer DescriptionSynthesizer Chip IC1 VCO and Tcxo ModulationCharge Pump Q1, Q2, Loop Filter RF Amplifier Q1, First Mixer Q2 Receiver Circuit DescriptionIf Amplifier Q2, LIMITER/DETECTOR IC3 Squelch Circuit IC4A/B, IC5A Antenna Switch and LOW-PASS FilterTransmitter Description Amplifier Q9, Driver Q8Receive Audio Processing Control Logic and DisplayPin Port Name Input or Description Active High Output Microprocessor IC306 Pin DescriptionsHvfulswlrq Receive and Transmit Data Processing Audio Amplifier IC104-IC106Receive Data FILTER/DETECTOR IC102A/B, IC107A/B Transmit Audio Processing Transmit Data Filter IC206B/ IC206ASmartnet Data Processing LOW-PASS Filter IC205A/BBattery Pack Battery Pack and Charger InformationPower Setting Operating Time Function Fahrenheit CelsiusBattery Pack and Charger Information Test Setup Alignment Procedure and Performance TestsMm/3/32 Mm/1/8 Phone Jack Tune Software HqhudoMain Pctune Screen Preliminary SetupTransmit Power Adjustment Screen Transmit Frequency and PowerHigh Power Level = 3.0 watts Low Power Level = 1.0 watt Modulation BalanceRssi Adjust Squelch AdjustAudio Deviation Data DeviationReceiver Performance Tests Performance TestsTransmitter Performance Tests Power OutputTransmit Frequency Transmit ModulationThis page intentionally left blank Display Assembly Parts ListChassis Parts Ref No DescriptionDisplay Assembly KEY AssemblyKEY Assembly If AssemblyIf Assembly RF Unit PLL AssemblyRF Unit NJM2904V-TE1 IC RF Unit Logic Unit Logic Unit TDA7233D IC Logic Unit 200K ERJ3GE MP35 MP18 MP25 EP2 KEY Front PanelEP1 MP32 MP33 MP27 MP34 MP28 MP29 MP23 MP22 MP30 MP24 MP26 MP18 PLL Board TOP View Schematic Diagrams and Component LayoutsPLL Board Bottom View If Board Bottom View If Board TOP ViewRF Unit Transceiver Block DiagramMIC RF Board TOP ViewIf AMP RF Board SchematicHD1 Logic Board TOP ViewReceive Audio Processing Logic Board SchematicGND KEYR3 KEYR2 KEYR1 KEYR0 KEYS3 KEYS2 KEYS1 KEYS0 GND Display Board Bottom ViewDS3 DS4 DS5 Display Board SchematicKEY Board Back View KEY Board Front ViewS S R R R R KEY Board SchematicScan HC2 HC7 HC8 HC3 Schematic Diagrams and Component Layouts
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