Icom IC-T90A manual PLL Circuits, APC Circuit RF Unit, General, MHz VCO Circuit

Page 13

4-2-5 APC CIRCUIT (RF UNIT)

The APC circuit protects the power amplifier from a mis- matched output load and stabilizes the output power. The APC circuit is designed to use VHF and UHF bands com- monly.

The APC sensor (R109) detects driving current from the drive voltage at the drive (Q52) and power (Q51) amplifiers. The detected current is converted into DC voltage at Q101, then applied to the APC control circuit (IC101, pin 2). The applied voltage is compared with a “PSET” voltage from the CPU via the D/A convertor (IC251), and the APC control circuit out- puts control voltage from pin 1 to control the drive and power amplifiers.

When the driving current is increased, input voltage of the dif- ferential amplifier (IC101, pin 2) will be increased. In such cases, the differential amplifier output voltage (IC101, pin 1) is decreased to reduce the driving current.

4-3 PLL CIRCUITS

4-3-1 GENERAL

A PLL circuit provides stable oscillation of the transmit fre- quency and the receive local frequency. The PLL circuit compares the phase of the divided VCO frequency to the ref- erence frequency. The PLL output frequency is controlled by the divided ratio (N-data) of a programmable divider.

4-3-2 144 MHz AND 430 MHz VCO CIRCUITS (VCO UNIT)

• 144 MHz VCO CIRCUIT

The oscillated signal at the 144 MHz VCO circuit (Q1–Q3, D1, D2) is amplified at two buffer amplifiers (Q7, Q11), and is then applied to the PLL IC (IC201, pin 19). The signal is divid- ed by serial data from the CPU (LOGIC unit; IC1) and phase detected with the divided reference frequency (5 kHz). The phase difference is output from pin 5 as pulses.

• APC CIRCUIT

 

 

 

 

 

 

144, 430 MHz

 

 

 

APC SENSOR CIRCUIT

 

RF transmit signal

 

 

 

 

 

 

 

VCC

 

 

R109

 

 

 

 

TXC

 

 

 

Drive

 

 

 

 

 

 

 

Power

to the antenna

 

 

 

 

 

 

 

Q102

 

 

 

 

amp.

amp.

 

 

 

 

 

Q51

 

 

 

 

 

Q52

 

 

Q101

 

 

 

 

 

 

from Tx/Rx

Buff.

Pre

 

 

 

 

 

 

amp.

drive

 

 

 

50 MHz

 

 

switch

 

 

 

 

 

IC51

Q54

 

 

 

 

 

 

 

 

 

RF transmit signal

 

 

 

 

 

 

 

 

 

D/A CONVERTER

 

2

1

VGGC

 

 

 

IC251

6

PSET

3 IC101

 

 

+3

(RF unit)

 

+

 

 

 

 

 

 

14 DSET

5 IC101

7

 

 

 

 

 

 

 

Differential

 

 

 

 

 

 

 

amplifier

 

 

 

 

4 - 5

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Contents IC-t90a Ordering Parts IntroductionRepair Notes Sample OrderTable of Contents General SpecificationsTransmitter Frequency MHz ReceiverWFM WFMLogic Unit Inside ViewsTop view Bottom viewRF Unit AF UnitTop view Bottom view Removing the rear panel Removing the AF unit Disassembly InstructionsReceiver Circuits Circuit Description 5 2ND if and Demodulator Circuits AF Unit 4 1ST Mixer and 1ST if Circuits RF, AF and VCO Units RF Circuit Squelch Circuitaf and Logic Units AF Amplifier Circuit AF Unit2ND if and Demodulator Circuit Transmitter Circuits APC Circuit RF Unit PLL CircuitsGeneral APC CircuitPLL Circuit VCO Divider Circuit VCO and RF Units2 R3V Switch IC AF Unit IC51 Power Supply CircuitsPort Allocations Voltage LineCPU Logic Unit IC1 Dtcs CtselLCD SEG35Operation on the Adjustment Mode Adjustment ProceduresOperation on the Output Power Adjustment PreparationJIG ConnectionAD-92SMA PLL Lock Trimmer AdjustmentVCO VoltageRF Unit VCO UnitL16 L456 L19Reference Adjustment Mode AdjustmentsDial FrequencyAdjustment Mode Adjustments PH ch 53.900 MHz Transmitting 8V 145 MHz Displayed frequency Connect an amme 45 a R325 Logic UnitR365 R326Ctcss Coil DetectorTracking CP0 AF UnitIC151, pin 12 SD line L151Displayed frequency Connect a DC volt Maximum voltage Meter Adjustment Adjustment Condition OperationOrder Description Section Parts ListCoil Eljre 3N9Z-F Ceramic ECJ0EB1H681K Connector AXK6S30445P Ceramic ECJ0EC1H060C Encoder Trimmer Tantalum ECST1EY474R Ceramic ECJ0EC1H2R5B Accessories Mechanical Parts and DisassemblyChassis Parts RF Unit Logic UnitChassis Parts AF Unit VCO UnitTransistors and Fets SEMI-CONDUCTOR InformationDiodes NameTOP View Board LayoutsMC1 Microphone DUPBottom View Logic unitSP1 SQL PTTRF Unit RF unit VCO Unit AF UnitEXT.SP EXT.MICREMOTO S253S252 Logic Unit Block DiagramLoop LPF HPF Drive BPF ANT FIL AMP Chassis11 1 Logic Unit Voltage DiagramIC5 2SK3476 11 2 RF UnitBU4094BCFV 11 3 AF UnitURL 32, Kamiminami, Hirano-ku, Osaka 547-0003, Japan
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