EFJohnson 7780 Squelch Circuit IC4A/B, IC5A, Transmitter Description, Amplifier Q9, Driver Q8

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CIRCUIT DESCRIPTION

input signal to these transistors is high for narrow band channels. Therefore, with narrow band chan- nels, D14/D16 are forward biased and D13/D15 reverse biased and vice versa for wideband channels.

The 450 kHz signal from these filters is then fed back into IC3 on pin 5 and applied to a limiter- amplifier stage. From the limiter the signal is fed to the quadrature detector. An external phase-shift network connected to pins 10 and 11 shifts the phase of one of the detector inputs by 90° at 450 kHz (the other inputs are unshifted in phase). This network consists of C17 and ceramic discriminator X1. The detector is adjusted for wide and narrow band chan- nels by Q3, R8, and C16. The detected audio signal is then amplified and fed out of IC3 on pin 9.

4.3.3 SQUELCH CIRCUIT (IC4A/B, IC5A)

The audio output signal on pin 9 of IC3 contains audio and also high-frequency noise. This signal is fed to the receive audio and data processing circuits on the logic board (Section 4.6.1) and also to the squelch circuit. The amount of noise in this signal tends to be inversely proportional to signal strength (the higher the noise level, the lower the signal strength and vice versa). Therefore, it can be used for squelch control.

A bandpass filter that passes only noise frequen- cies occurring above the audio band is formed by an op amp in IC3 and external components. The input to this filter is pin 8 and frequency determining compo- nents (R17-R19, C20, C21) are connected to pin 7. Other components on the input such as Q4 provide frequency and wide/narrow band compensation, and thermistors R14/R26 provide temperature compensa- tion. The input audio/noise level is adjusted by R13. The detected noise output on pin 13 of IC3 is applied to pin 32 of the microprocessor in the logic unit.

4.4 TRANSMITTER DESCRIPTION

NOTE: A block diagram of the RF and logic units is located on page 8-3.

4.4.1 AMPLIFIER (Q9), DRIVER (Q8)

In the transmit mode, the VCO output signal is the transmit frequency. It is switched to the transmitter

by the switching circuit consisting of D7 and other components (see Section 4.2.2) and applied to ampli- fier Q9. Power to this stage and also Q8 is the 7.5-volt battery supply switched by Q16 and Q17. This switch is controlled by the switched T5V supply. Therefore, when the transmit mode is selected, the TX5V supply goes high which causes the output of Q17 to go low and turn switch Q16 on.

The output signal from Q9 is fed to driver Q8 which provides additional amplification. Impedance matching with IC1 is provided by L15, C62, and a resistive pad consisting of R41-R43. Power amplifier module IC1 provides up to 3 watts of RF power at the antenna jack. It contains three internal amplifier stages, each of which has a separate supply voltage (VCC1-VCC3). The supply voltage to the first stage (VCC1) is from the power control circuit, and the supply voltage to the last two stages (VCC2/VCC3) is the unswitched battery supply.

4.4.2ANTENNA SWITCH AND LOW-PASS FILTER

The antenna is switched between the receiver and transmitter by an antenna switch consisting of D11, D1-D3, and several other components. In the transmit mode, the TMUT input on pin 2 of Q7 is high. The T5V supply on pin 4 then appears on pin 3 and current flows through R20, L14, D11, D3, and L1/D1 and L2/D2. Since D11 and D3 are forward biased, the transmit signal has a low impedance path to the antenna.

L14/C31, L1/C2/D1, and L2/D2/C17 each form discrete quarter-wave line that is AC grounded at one end. When one end of a quarter-wave line is AC grounded, the other end presents a high impedance to the quarter-wave frequency. Therefore, L14/C31 isolate the 5-volt battery supply from transmitter RF, and the other two circuits isolate the receiver. In the receive mode, D11 and D1-D3 are no longer forward biased. Therefore, the receive signal has a high- impedance path into the transmitter and a low-imped- ance path into the receiver. From the antenna switch the transmit signal is fed to low-pass filter FI7. This filter attenuates harmonic frequencies occurring above the transmit band.

November 1999

 

Part No. 001-7780-500

4-4

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Contents 7780 Page Page Table of Contents Table of Contents CONT’D List of Tables List of FiguresGeneral Information Scope of Manual Equipment DescriptionAccessories Factory Customer ServicePart Number Breakdown Transceiver IdentificationAccessories AccessoryToll-Free 800 Factory ReturnsDefinitions Replacement PartsInternet Home Intrinsically Safe Information Temperature Code T3C Possible Ignition SourcesIntrinsically Safe and Nonincendive Ratings Classification of Hazardous Areas and AtmospheresAccessing PC Boards Material ClassificationTypical Hazard Group Class Area ClassificationRemoving RF Unit Removing Logic UnitSpecifications GeneralControls and Display FeaturesTransceiver Operation TX Indicator Red Indicates when the transmitter is keyed Low-Battery Indicator AmberDisplay DisplayFront Panel Keys Full Keypad Model Turning Power on and Setting Volume Option SwitchesGeneral Operation BacklightSee Option Switch FunctionsFunction Conv Tone ENABLE/DISABLE ScanningConventional and SMARTNET/ Smartzone Operation Monitoring Before Transmitting Conventional FeaturesDisplay Mode Selection Call Guard Squelch Monitor ModeBusy Channel Lockout Penalty Timer Conversation TimerPower Output Select Conventional Mode ScanningPriority Channel Sampling DTMF/ANI Signaling Standard Conventional CallsSMARTNET/SMARTZONE Features Viewing Unit IDStandard Group Calls Enhanced Private Conversation CallsTransceiver Operation Private Conversation II Calls Telephone Calls Call Alert Sending Status Conditions Emergency Alarm and CallMessaging Dynamic Regrouping Failsoft OperationSMARTNET/SMARTZONE Scanning Supervisory Tones Zone Change Parameter Keypad ProgrammingMenu Description Channel Parameters Channel Change ParameterSystem Parameters Programming Setup Pctrunk Software InstallationTransceiver Programming Computer DescriptionConnecting RPI to Computer and Transceiver Starting and ExitingProgramming File Types Help FilesScreen Pop-Up Buttons and File Size Indicator Programming ProcedureFile Size Indication Creating and Displaying SystemsMenu Commands Menu BarRadio Type Menu Download Menu Upload MenuSystems Menu Window MenuDefaults RADIO-WIDE Parameter ScreensBand ZonesRadio Wide Scan List Radio-Wide Scan List ScreenModify List Screen Assign Function ButtonsRADIO-WIDE Conventional Screen RADIO-WIDE SMARTNET/SMARTZONE ScreenFull Spectrum CC Scan Hot DtmfAdjustable Parameters RADIO-WIDE Portable Options ScreenProgramming Conventional Systems and Channels Conventional System General ScreenTimers Conventional System Modify Scan List ScreenKeypad Editing Scan List ButtonPriority Channel Selection Scan TimersSetting UP Conventional Channels Conventional Channel Screen Parameters Selected ChannelTransmit Power SignalingChannel Type Tx Time-OutRestricted Access Time-Out TimerISW Delay Time Splinter ChannelsSMARTNET/SMARTZONE System Other ID’S Screen SMARTNET/SMARTZONE System Phone Interconnect ScreenPhone Interconnect Private CallSMARTNET/SMARTZONE System Emergency Settings Screen Emergency AlarmSMARTNET/SMARTZONE System Talk Groups Screen Failsoft ChannelMessage Aliasing Screen SMARTNET/SMARTZONE System Lists ScreensTrunking Phone List Screen Announcement Groups Screen Control Channels ScreenTrunking Call List Screen SMARTNET/SMARTZONE Channel Screen Parameters Setting UP SMARTNET/SMART- Zone ChannelsStatus Aliasing Screen SMARTNET/SmartZone Channel Screen Other Screen ParametersCall Guard CTCSS/DCS Codes and Tones Recommended Tone Call Guard Codes FreqMHz Channels MHz Channels MHz Channels MHz Channels MHz Channels MHz Channels Power Switching and Regulation Power SwitchingCircuit Description Synthesizer DescriptionCharge Pump Q1, Q2, Loop Filter VCO and Tcxo ModulationSynthesizer Chip IC1 If Amplifier Q2, LIMITER/DETECTOR IC3 Receiver Circuit DescriptionRF Amplifier Q1, First Mixer Q2 Antenna Switch and LOW-PASS Filter Squelch Circuit IC4A/B, IC5ATransmitter Description Amplifier Q9, Driver Q8Control Logic and Display Receive Audio ProcessingMicroprocessor IC306 Pin Descriptions Pin Port Name Input or Description Active High OutputHvfulswlrq Receive Data FILTER/DETECTOR IC102A/B, IC107A/B Audio Amplifier IC104-IC106Receive and Transmit Data Processing Transmit Data Filter IC206B/ IC206A Transmit Audio ProcessingLOW-PASS Filter IC205A/B Smartnet Data ProcessingBattery Pack and Charger Information Battery PackPower Setting Operating Time Function Fahrenheit CelsiusBattery Pack and Charger Information Alignment Procedure and Performance Tests Test SetupTune Software Hqhudo Mm/3/32 Mm/1/8 Phone JackPreliminary Setup Main Pctune ScreenTransmit Frequency and Power Transmit Power Adjustment ScreenHigh Power Level = 3.0 watts Low Power Level = 1.0 watt Modulation BalanceSquelch Adjust Rssi AdjustAudio Deviation Data DeviationPerformance Tests Receiver Performance TestsPower Output Transmitter Performance TestsTransmit Frequency Transmit ModulationThis page intentionally left blank Parts List Display AssemblyChassis Parts Ref No DescriptionKEY Assembly Display AssemblyIf Assembly If AssemblyKEY Assembly PLL Assembly RF UnitRF Unit NJM2904V-TE1 IC RF Unit Logic Unit Logic Unit TDA7233D IC Logic Unit 200K ERJ3GE MP35 MP18 MP25 KEY Front Panel EP2EP1 MP32 MP33 MP27 MP34 MP28 MP29 MP23 MP22 MP30 MP24 MP26 MP18 PLL Board Bottom View Schematic Diagrams and Component LayoutsPLL Board TOP View If Board TOP View If Board Bottom ViewTransceiver Block Diagram RF UnitRF Board TOP View MICRF Board Schematic If AMPLogic Board TOP View HD1Logic Board Schematic Receive Audio ProcessingDisplay Board Bottom View GND KEYR3 KEYR2 KEYR1 KEYR0 KEYS3 KEYS2 KEYS1 KEYS0 GNDDisplay Board Schematic DS3 DS4 DS5KEY Board Front View KEY Board Back ViewKEY Board Schematic S S R R R RScan HC2 HC7 HC8 HC3 Schematic Diagrams and Component Layouts
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