EFJohnson 764X, 761X service manual UHF 7640 Circuit Description, UHF Receiver Circuit

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UHF (7640) CIRCUIT DESCRIPTION

SECTION 5B UHF (7640) CIRCUIT DESCRIPTION

NOTE: The UHF transceiver block diagram is located on page 8-6.

5.5 UHF RECEIVER CIRCUIT

5.5.1 ANTENNA SWITCHING CIRCUIT

The antenna switching circuit functions as a low- pass filter while receiving and a resonator circuit while transmitting. This prevents the transmit signal from entering the receiver in the transmit mode.

Received signals enter the antenna connector and pass through a low-pass filter consisting of L1-L3, C1, C2, and C9-C12. The filtered signal is then fed through a quarter-wave type antenna switching circuit formed by D4, D5, and L15 and applied to the receiver RF circuit.

5.5.2 RF CIRCUIT

The RF circuit amplifies the frequencies that are within the receive band range and attenuates out-of- band signals.

The signal from the antenna switching circuit passes through attenuator D4/D5. The attenuator cir- cuit functions only when the attenuator function is assigned to a programmable key. It is enabled to mini- mize RF amplifier distortion caused by excessively strong receive signals.

When the attenuator function is turned on, CPU IC1, pin 32 switches the voltage level of the “RF ATT” line from high to low and then controls the attenuator switch Q1. The D4/D5 current then increases and D4/D5 act as an attenuator.

This signal is then applied to a two-stage tunable bandpass filter controlled by D7 and D8. The filtered signals are amplified by RF amplifier Q2 and applied to another two-stage bandpass filter controlled by D9 and D10. These filters attenuate unwanted signals.

The filtered signal is then applied to the first mixer Q3.

The tunable bandpass filters controlled by D7-

D10 employ varactor diodes to tune them to the center frequency of the RF passband. This gives a wide bandwidth receiver good image response rejection. The diodes are controlled by the CPU IC1 via level controller IC12.

5.5.3 FIRST MIXER AND FIRST IF CIRCUITS

The first mixer circuit converts the received sig- nal to a fixed frequency first IF signal of 46.350 MHz. The RF signal from the bandpass filter is applied to the first mixer Q3. This signal is mixed with the first LO signal from VCO Q23 and buffers Q28 and Q30 to produce the IF signal.

The 46.350 MHz first IF signal then passes through a pair of crystal filters (FI1A/B) which sup- press out-of-band signals. The filtered signal is then amplified by first IF amplifier Q4 and applied to second IF system IC1.

5.5.4SECOND IF AND DEMODULATOR CIRCUITS

The second mixer circuit converts the 46.350 MHz first IF signal to a 450 kHz second IF signal. A double-conversion superheterodyne receiver like this improves the image rejection ratio and provides a stable receiver gain.

FM IF system IC1 contains second mixer, limiter amplifier, quadrature detector, active filter and noise amplifier circuits (see Figure 5-4). The first IF signal from IF amplifier Q4 is applied to IC1, pin 16 which is the input to the second mixer section. The 46.350 MHz first IF signal is mixed with the 45.900 MHz second LO signal to produce the 450 kHz second IF signal. The PLL reference frequency of 15.300 MHz is tripled to produce the 45.900 MHz second LO signal.

The second IF signal is fed out of IC1 on pin 3 and applied to ceramic filters FI2 and FI3 with narrow band operation or only FI2 with wideband operation (bypassing FI3). It is then fed back into IC1 on pin 5

 

November 1998

5B-7

Part No. 001-7600-001

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Contents 761x VHF 764x UHF Conventional Mobile Conventional FM TWO-WAY Radio Table of Contents Table of Contents CONT’D Schematic Diagrams and Component Layouts General Information Toll-Free 800 75xx AccessoriesAccessory Factory ReturnsReplacement Parts Internet HomeThis page intentionally left blank VHF and 7640 UHF Mobile Specifications GeneralGeneral Information Installation and Disassembly Transceiver InstallationPower Cable Installation Installing Mounting BracketInstalling Microphone Hanger Cable InstallationUsing AN External Speaker Power Cable FusesChanging 20-Ampere Power Cable Fuses Accessory Cable InstallationRemove shield E Transceiver DisassemblyRemoving PC Board Setting 5-Tone Deviation 2-TONE/5-TONE Module InstallationInstallation Procedure 2 2-TONE ProgrammingInstallation and Disassembly Features OperationBasic Operation Controls and DisplayTransceiver Back Panel Direct Selection The F1-F4 Receiving and Placing Calls Option Key Programmable Functions TIME-OUT TimerProgrammable Functions Function Key Label Description Section NoCall Guard Tone Select Monitor Mode SelectOutput Power Select Channel SelectChannel Scan Priority Channel Sampling Programming Cables Programming SetupSection Programming Programming SoftwareMenu Flow Starting the ProgramSpecial Keys and Functions File MenuPrint Menu Screen MenuProgram Menu Model MenuParameter Description Bank Select Setup MenuMemory Channel Screen Description LMR Models Frequency TimerCh Atr Rx/TxScan Auto ResetLockout ToneKey and Display Assign Screen Description Key DescriptionKey Description Label Selection RF PowerDtmf Autodial Screen Description Bank/FreeParameter Description Continuous Tone Screen DescriptionScan Function Screen Description Resume Stop Timer2-Tone Code Channel Screen Description Auto CHAuto Tx EXO TimerTimer Auto Tx StunCommon Screen Description Expert Screen Description Unique PMR European Screens Unique PMR ScreensCloning Programming ONE Transceiver with Another Memory Channel Screen PMR Models This page intentionally left blank VHF 7610 Circuit Description VHF Receiver CircuitAudio Amplifier Circuit Receiver Mute CircuitMicrophone Amplifier Circuit Power Amplifier ModuleVHF Transmitter Circuit Modulation CircuitPLL Circuit APC CircuitVHF PLL Circuit Pin Port Description VHF IC PIN DescriptionsLine Description VHF Port Expander IC11 Main Unit VHF I/O Expander IC5 Main UnitPin Pin Port Description NameUHF 7640 Circuit Description UHF Receiver CircuitSecond if System IC1 UHF UHF Transmitter Circuit PLL Circuit UHF UHF PLL CircuitUHF Power Supply UHF CPU IC1 Port AllocationUHF I/O Expander IC12 Main Unit UHF Port Expander IC13 Main UnitSuggested Test Equipment Adjustment ProcedureTest Setup Equipment Grade and RangeTest Channels Test CableUHF Test Frequencies COMPUTER-AIDED TuningVHF Test Frequencies Preliminary Setup VHF AdjustmentsVHF Adjustments PLL Adjustment VHF ModelsFM Deviation Adjust VHF Adjustments CONT’DOutput Power Adjust Bandpass Filter Adjust Dtcs Waveform AdjustReceiver Adjustments VHF Models Squelch AdjustBeep Tone Level Adjustment Beep Tone LevelTransmitter Adjustments UHF Models UHF AdjustmentsPLL Adjustment UHF Models UHF Adjustments CONT’D UHF Adjustment PointsUHF Adjustments CONT’D Receiver Adjustments UHF Models Parts List Æçìíýõ9+ôý0$,1ý81,7Íäç HUDPLFý&ìçíåý-%ýì+ýìíë.ð7ð$ Íëëðêäíçðíçí Íäæ Ìää HUDPLFý&ìçíåý-%ýì&ýìíé.7ð1 Íëëðêäíçðçìé Ëíí Ëäí HUDPLFý&ìçíåý-%ýìýìíê.ð7ð$ Íëëðêäíçðíçê Ëäì RLOý0/ìçíå$ýì5í.ð7 Íëëðêäíçðçèç 03ì Ýìäëëý9&2ýFDVH 5íéé 5HVý5-ê*-ýìííý9ýõìíýRKPô Íëëðêäíçðìêå 5íéè 5ìèé 5HVý5-ê*-ýìíéý9ýõìííýNýRKPô Æçéíýõ8+ôý0$,1ý81,7 Íêì HUDPLFý&ìçíåý-%ýì+ýìíë.ð7ð$ Íëëðêäíçðíçí Íêë Íäé HUDPLFý+0çí6-ý&+ýíêíýèíí9ý Õéäíðèìëý0+ô Ìèì 5HIý1Rï Êëç Íëëðêäíçðäèë 4éì 7UDQVLVWRUý7&êçê.ý7ìéæ 5íæé 5ìçå 5HVý5-ê*-ýìíéý9ýõìííýNýRKPô 5217ý81,7 Õ9+ý$1ý8+ý02/6ô =HQHUý0$åíèìð0ýõ7ô Æçìíýõ9+ôý&+$66,6ý3$576 VHF 7610 Exploded View Front UnitUHF 7640 Exploded View Main Unit Schematic Diagrams and Component LayoutsVHF 7610 Block Diagram Front UnitVHF 7610 Transistor Basing Diagrams Foldout → VHF 7610 Main Board TOP View VHF 7610 Main Board Schematic VHF Main UnitUHF 7640 Block Diagram UHF 7640 Main Board Bottom View UHF 7640 Main Board TOP View J4 T o O p tion al Un itUHF 7640 Main Board Schematic VHF/UHF Front Unit Board TOP View VHF/UHF Front Unit Board Bottom ViewVHF/UHF Front Unit Schematic UHF 7640 Transistor Basing Diagrams