EFJohnson 7780 VCO and Tcxo Modulation, Synthesizer Chip IC1, Charge Pump Q1, Q2, Loop Filter

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CIRCUIT DESCRIPTION

the VCO from the receiver and transmitter and the counters in synthesizer IC1.

A switching circuit on the output of Q10 routes the VCO signal to the receiver in the receive mode and the transmitter in the transmit mode. In the receive mode, the R5V receive supply is applied to R12 and current flows through R12, D7A, and R51. Since D7A is forward biased, it presents a low impedance path to first mixer Q2 in the receiver. D7C is then reverse biased, so there is a high impedance path to the transmitter.

The 5V receive supply is also applied to R78 and current flows through R78, D7B, and R79. This results in less reverse bias and therefore charge on D7C to permit faster switching. In the transmit mode, the 5V receive supply is no longer present, so both D7A and D7B are reverse biased. D7C is forward biased by the current path through R50 and R51.

4.2.3 VCO AND TCXO MODULATION

Both the VCO and TCXO are modulated in order to achieve the required frequency response. If only the VCO was modulated, the phase detector in IC1 would sense the frequency change and increase or decrease the VCO control voltage to counteract the change (especially at the lower audio frequencies). Conversely, if only the TCXO was modulated, the VCO frequency would not change fast enough (espe- cially at the higher audio frequencies).

However, by modulating both the VCO and

TCXO, the two phase detector inputs remain in phase and no frequency shift is sensed. This produces a rela- tively flat audio response. Potentiometer B4 in IC209 is used to balance these signals and potentiometers B1 and B3 are used to set the data and audio deviation level. IC4 is an amplifier, and R73 sets the frequency of the TCXO by adjusting the DC bias of IC4.

4.2.4 SYNTHESIZER CHIP (IC1)

Synthesizer chip IC1 contains a prescaler, A, N, and reference counters, and a phase detector. Chan- nels are selected by programming the A, N, and refer- ence counters to divide by a certain number. This programming is performed by the microprocessor via the serial data bus which consists of lines PLLSK, PLLSO, and PLLST (pins 9-11 of IC1).

The divide numbers are chosen so that when the VCO is operating at the correct frequency, the VCO- derived input to the phase detector is the same frequency as the TCXO-derived input. The TCXO- derived input is produced by the reference counter in IC1. This counter divides the 14.925 MHz TCXO frequency by 2388 to produce a 6.25 kHz input to the phase detector. Using a 6.25 kHz reference frequency allows channels to be changed in 6.25 kHz steps.

The VCO-derived input is produced by dividing the VCO frequency using the prescaler and N counter in IC1. The divide number of the prescaler is controlled by the output signal from the A counter. The A and N counters function as follows:

Both the A and N counters begin counting down from the programmed number. When the A counter reaches zero, it halts until the N counter reaches zero. Both counters then reset and the cycle repeats. The A counter is always programmed with a smaller number than the N counter. When the A counter is counting down, the control output to the prescaler causes the prescaler to divide by 129. Then when the A counter is halted, the control output changes states and the pres- caler divides by 128.

4.2.5 LOCK DETECT (Q13)

When the synthesizer is locked on frequency, the LD (Lock Detect) output of IC1 (pin 7) is a high signal. This turns lock detect transistor Q13 off and the ULOCK signal fed to the microprocessor is low. When an unlocked condition is indicated, low-going pulses appear on pin 7. Q13 then turns on and the ULOCK signal goes high. The transceiver is then disabled by the logic and an error condition is indicated in the display.

4.2.6 CHARGE PUMP (Q1, Q2), LOOP FILTER

The charge pump circuit consisting of Q1 and Q2 charges and discharges C4 and C5 in the loop filter to produce the VCO control voltage. Supply voltage to Q1 is provided by CPV regulator IC2 on the IF board.

Pulses which control the charge pump are output by synthesizer IC1 on pins 15 and 16. When the VCO is on the correct frequency, both phase detector outputs are in phase. Pin 15 is then high and pin 16 is low which turns both Q1 and Q2 off. Then if the VCO

November 1999

 

Part No. 001-7780-500

4-2

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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 DescriptionPart Number Breakdown AccessoriesFactory Customer Service Transceiver IdentificationToll-Free 800 AccessoriesAccessory Factory ReturnsReplacement Parts Internet Home Intrinsically Safe InformationDefinitions Intrinsically Safe and Nonincendive Ratings Temperature Code T3CPossible Ignition Sources Classification of Hazardous Areas and AtmospheresTypical Hazard Group Class Accessing PC BoardsMaterial Classification Area ClassificationRemoving RF Unit Removing Logic UnitSpecifications GeneralFeatures Transceiver OperationControls and Display TX Indicator Red Indicates when the transmitter is keyed Low-Battery Indicator AmberDisplay Front Panel Keys Full Keypad ModelDisplay General Operation Turning Power on and Setting VolumeOption Switches BacklightOption Switch Functions Function ConvSee Tone ENABLE/DISABLE ScanningConventional and SMARTNET/ Smartzone Operation Conventional Features Display Mode SelectionMonitoring Before Transmitting Monitor Mode Busy Channel LockoutCall Guard Squelch Power Output Select Penalty TimerConversation Timer Conventional Mode ScanningPriority Channel Sampling DTMF/ANI Signaling Standard Conventional CallsStandard Group Calls SMARTNET/SMARTZONE FeaturesViewing Unit ID Enhanced Private Conversation CallsTransceiver Operation Private Conversation II Calls Telephone Calls Call Alert Emergency Alarm and Call MessagingSending Status Conditions Failsoft Operation SMARTNET/SMARTZONE ScanningDynamic Regrouping Supervisory Tones Keypad Programming Menu DescriptionZone Change Parameter Channel Change Parameter System ParametersChannel Parameters Transceiver Programming Programming SetupPctrunk Software Installation Computer DescriptionProgramming File Types Connecting RPI to Computer and TransceiverStarting and Exiting Help FilesFile Size Indication Screen Pop-Up Buttons and File Size IndicatorProgramming Procedure Creating and Displaying SystemsMenu Commands Menu BarSystems Menu Radio Type MenuDownload Menu Upload Menu Window MenuBand DefaultsRADIO-WIDE Parameter Screens ZonesModify List Screen Radio Wide Scan ListRadio-Wide Scan List Screen Assign Function ButtonsFull Spectrum CC Scan RADIO-WIDE Conventional ScreenRADIO-WIDE SMARTNET/SMARTZONE Screen Hot DtmfProgramming Conventional Systems and Channels Adjustable ParametersRADIO-WIDE Portable Options Screen Conventional System General ScreenKeypad Editing TimersConventional System Modify Scan List Screen Scan List ButtonScan Timers Setting UP Conventional ChannelsPriority Channel Selection Conventional Channel Screen Parameters Selected ChannelChannel Type Transmit PowerSignaling Tx Time-OutISW Delay Time Restricted AccessTime-Out Timer Splinter ChannelsPhone Interconnect SMARTNET/SMARTZONE System Other ID’S ScreenSMARTNET/SMARTZONE System Phone Interconnect Screen Private CallSMARTNET/SMARTZONE System Talk Groups Screen SMARTNET/SMARTZONE System Emergency Settings ScreenEmergency Alarm Failsoft ChannelSMARTNET/SMARTZONE System Lists Screens Trunking Phone List ScreenMessage Aliasing Screen Announcement Groups Screen Control Channels ScreenTrunking Call List Screen Setting UP SMARTNET/SMART- Zone Channels Status Aliasing ScreenSMARTNET/SMARTZONE Channel Screen Parameters 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 Circuit Description Power Switching and RegulationPower Switching Synthesizer DescriptionVCO and Tcxo Modulation Synthesizer Chip IC1Charge Pump Q1, Q2, Loop Filter Receiver Circuit Description RF Amplifier Q1, First Mixer Q2If Amplifier Q2, LIMITER/DETECTOR IC3 Transmitter Description Antenna Switch and LOW-PASS FilterSquelch Circuit IC4A/B, IC5A Amplifier Q9, Driver Q8Control Logic and Display Receive Audio ProcessingMicroprocessor IC306 Pin Descriptions Pin Port Name Input or Description Active High OutputHvfulswlrq Audio Amplifier IC104-IC106 Receive and Transmit Data ProcessingReceive Data FILTER/DETECTOR IC102A/B, IC107A/B Transmit Data Filter IC206B/ IC206A Transmit Audio ProcessingLOW-PASS Filter IC205A/B Smartnet Data ProcessingPower Setting Operating Time Battery Pack and Charger InformationBattery Pack 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 ScreenHigh Power Level = 3.0 watts Low Power Level = 1.0 watt Transmit Frequency and PowerTransmit Power Adjustment Screen Modulation BalanceAudio Deviation Squelch AdjustRssi Adjust Data DeviationPerformance Tests Receiver Performance TestsTransmit Frequency Power OutputTransmitter Performance Tests Transmit ModulationThis page intentionally left blank Chassis Parts Parts ListDisplay Assembly Ref No DescriptionKEY Assembly Display AssemblyIf Assembly KEY AssemblyIf 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 Schematic Diagrams and Component Layouts PLL Board TOP ViewPLL Board Bottom 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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