EFJohnson 764X, 761X service manual VHF 7610 Circuit Description, VHF Receiver Circuit

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VHF (7610) CIRCUIT DESCRIPTION

SECTION 5A VHF (7610) CIRCUIT DESCRIPTION

NOTE: A VHF transceiver block diagram is located on page 8-1.

5.1 VHF RECEIVER CIRCUIT

5.1.1 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- C3, C8-C10, and C11-C13. The filtered signal is then fed through a quarter-wave type antenna switching cir- cuit formed by D13, D14, and L13 and applied to the receiver RF circuit.

5.1.2 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 D13/D14. The attenuator circuit 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 Q35. The D13/D14 current then increases and D13/D14 act as an attenuator.

This signal is then applied to a two-stage tunable bandpass filter controlled by D16 and D17. The fil- tered signals are amplified by RF amplifier Q12 and applied to another two-stage bandpass filter controlled by D18-D21. These filters attenuate unwanted signals. The filtered signal is then applied to the first mixer Q13.

The tunable bandpass filters controlled by D16-

D21 employ varactor diodes to tune them to the center

frequency of the RF passband. This gives a wide band- width receiver good image response rejection. The diodes are controlled by the CPU IC1 via level controller IC5.

5.1.3 FIRST MIXER AND FIRST IF CIRCUITS

The first mixer circuit converts the received sig- nal to a fixed frequency first IF signal of 31.050 MHz. The RF signal from the bandpass filter is applied to the first mixer Q13. This signal is mixed with the first LO signal from VCO Q7/Q8 and buffers Q6-Q4 to pro- duce the IF signal.

The 31.050 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 Q14 and applied to sec- ond IF circuit IC3.

5.1.4SECOND IF AND DEMODULATOR CIRCUITS

The second mixer circuit converts the 31.050 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 IC3 contains second mixer, limiter amplifier, quadrature detector, active filter and noise amplifier circuits (see Figure 5-1). The first IF signal from IF amplifier Q14 is applied to IC3, pin 16 which is the input to the second mixer section. The 31.050 MHz first IF signal is mixed with the 30.600 MHz sec- ond LO signal to produce the 450 kHz second IF sig- nal. The PLL reference frequency of 15.300 MHz is doubled to produce the 30.600 MHz second LO signal.

The second IF signal is fed out of IC3 on pin 3 and applied to ceramic filters FI3 and FI4 with narrow band operation or only FI4 with wideband operation (bypassing FI3). It is then fed back into IC3 on pin 5 and applied to the limiter amplifier and quadrature detector. The quadrature detector demodulates the audio signal contained in the second IF signal. Com- ponents connected to pins 10 and 11 including X3

 

November 1998

5A-1

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 75xx Accessories AccessoryToll-Free 800 Factory ReturnsReplacement Parts Internet HomeThis page intentionally left blank VHF and 7640 UHF Mobile Specifications GeneralGeneral Information Installation and Disassembly Transceiver InstallationInstalling Mounting Bracket Installing Microphone HangerPower Cable Installation Cable InstallationPower Cable Fuses Changing 20-Ampere Power Cable FusesUsing AN External Speaker Accessory Cable InstallationRemove shield E Transceiver DisassemblyRemoving PC Board 2-TONE/5-TONE Module Installation Installation ProcedureSetting 5-Tone Deviation 2 2-TONE ProgrammingInstallation and Disassembly Features OperationBasic Operation Controls and DisplayTransceiver Back Panel Direct Selection The F1-F4 Receiving and Placing Calls TIME-OUT Timer Programmable FunctionsOption Key Programmable Functions Function Key Label Description Section NoMonitor Mode Select Output Power SelectCall Guard Tone Select Channel SelectChannel Scan Priority Channel Sampling Programming Setup Section ProgrammingProgramming Cables Programming SoftwareStarting the Program Special Keys and FunctionsMenu Flow File MenuScreen Menu Program MenuPrint Menu Model MenuParameter Description Bank Select Setup MenuMemory Channel Screen Description LMR Models Timer Ch AtrFrequency Rx/TxAuto Reset LockoutScan ToneKey and Display Assign Screen Description Key DescriptionKey Description Label RF Power Dtmf Autodial Screen DescriptionSelection Bank/FreeParameter Description Continuous Tone Screen DescriptionScan Function Screen Description Stop Timer 2-Tone Code Channel Screen DescriptionResume Auto CHEXO Timer Timer Auto TxAuto 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 CircuitPower Amplifier Module VHF Transmitter CircuitMicrophone Amplifier Circuit Modulation CircuitPLL Circuit APC CircuitVHF PLL Circuit Pin Port Description VHF IC PIN DescriptionsLine Description VHF I/O Expander IC5 Main Unit PinVHF Port Expander IC11 Main Unit 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 UnitAdjustment Procedure Test SetupSuggested Test Equipment Equipment Grade and RangeTest Channels Test CableUHF Test Frequencies COMPUTER-AIDED TuningVHF Test Frequencies VHF Adjustments VHF AdjustmentsPreliminary Setup PLL Adjustment VHF ModelsFM Deviation Adjust VHF Adjustments CONT’DOutput Power Adjust Dtcs Waveform Adjust Receiver Adjustments VHF ModelsBandpass Filter Adjust 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 Schematic Diagrams and Component Layouts VHF 7610 Block DiagramMain Unit 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