Icom IC-F3GS, IC-F3GT service manual Circuit Description, Receiver Circuits

Page 8

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT

Received signals are passed through the low-pass filter (L1, L2, C1–C5, C8). The filtered signals are applied to the λ4 type antenna switching circuit (D2, D8).

The antenna switching circuit functions as a low-pass filter while receiving. However, its impedance becomes very high while D2 and D8 are turned ON. Thus transmit signals are blocked from entering the receiver circuits. The antenna switching circuit employs a λ4 type diode switching system. The passed signals are then applied to the RF amplifier cir- cuit.

4-1-2 RF CIRCUIT

The RF circuit amplifies signals within the range of frequen- cy coverage and filters out-of-band signals.

The signals from the antenna switching circuit are amplified at the RF amplifier (Q12) after passing through the tunable bandpass filter (D9, D10, L16, L17, C78–C80, C83, C86, C277). The amplified signals are applied to the 1st mixer cir- cuit (Q13) after out-of-band signals are suppressed at the tunable bandpass filter (D11, D12, L18, L19, C91, C92, C94, C96–C98).

Varactor diodes are employed at the bandpass filters that track the filters and are controlled by the CPU (IC8) via the expander IC (IC10) using T1–T4 signals. These diodes tune the center frequency of an RF passband for wide bandwidth receiving and good image response rejection.

The signals from the RF circuit are mixed at the 1st mixer (Q13) with a 1st LO signal coming from the VCO circuit to produce a 31.05 MHz 1st IF signal.

The 1st IF signal is applied to two crystal filters (FI1) to sup- press out-of-band signals. The filtered 1st IF signal is applied to the IF amplifier (Q14), then applied to the 2nd mixer circuit (IC2, pin 16).

4-1-4 2ND IF AND DEMODULATOR CIRCUITS

The 2nd mixer circuit converts the 1st IF signal into a 2nd IF signal. A double conversion superheterodyne system (which converts receive signals twice) improves the image rejection ratio and obtains stable receiver gain.

The 1st IF signal from the IF amplifier is applied to the 2nd mixer section of the FM IF IC (IC2, pin 16), and is mixed with the 2nd LO signal to be converted into a 450 kHz 2nd IF sig- nal.

The FM IF IC contains the 2nd mixer, limiter amplifier, quad- rature detector and active filter circuits. A 2nd LO signal (30.6 MHz) is produced at the PLL circuit by doubling it’s ref- erence frequency.

The 2nd IF signal from the 2nd mixer (IC2, pin 3) passes through a ceramic filter (FI2) to remove unwanted hetero- dyned frequencies. It is then amplified at the limiter amplifi- er (IC2, pin 5) and applied to the quadrature detector (IC2, pins 10, 11) to demodulate the 2nd IF signal into AF signals.

4-1-3 1ST MIXER AND 1ST IF CIRCUITS

The 1st mixer circuit converts the received signal into a fixed frequency of the 1st IF signal with a PLL output frequency. By changing the PLL frequency, only the desired frequency will pass through a crystal filter at the next stage of the 1st mixer.

• 2nd IF and demodulator circuits

 

 

 

 

 

 

2nd IF filter

 

PLL IC

 

 

 

 

 

 

 

450 kHz

 

 

 

 

C122

 

C121

 

 

IC1

 

 

 

R86

 

 

17

16

 

 

 

FI2

2

 

 

 

 

 

 

 

 

8

 

7

5

3

2

 

R87

R88

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Noise

 

Noise

 

 

 

 

 

Active

 

detector

 

comp.

 

 

 

 

 

filter

 

Limiter

 

Mixer

 

X1

 

 

 

 

 

 

 

 

15.3 MHz

 

 

 

 

 

amp.

 

 

 

 

 

 

 

 

 

 

 

 

 

"SQLIN" signal from the

 

FM

 

 

 

 

 

 

 

D/A convertor (IC10, pin 23)

 

 

 

 

 

 

 

 

 

detector

 

 

 

 

RSSI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

IC2 TA31136F

 

 

9

10

C116 11

 

12

13

16 1st IF from the IF amplifier (Q14)

AF signal "DET"

R83

 

R82

 

 

"NOIS" signal to the CPU pin 19

 

 

 

 

 

C112

C113

 

 

 

R5

"SD" signal to the CPU pin 98

 

 

 

 

 

R84

 

 

 

 

 

 

X3

 

 

 

 

 

 

 

 

 

 

 

 

4 - 2

Image 8
Contents VHF FM Transceivers Repair Notes IntroductionOrdering Parts Table of Contents Section Specifications EIA/TIABottom View Inside ViewsMain Unit TOP View Disassembly and Option Instructions Disassembly InstructionRemoving the Chassis Panel Removing the Main UnitOptional Unit Installations Circuit Description Receiver CircuitsTransmitter Circuits Power Supply Circuits Voltage Line PLL CircuitPort Allocations Output Expander IC IC10Adjustment Procedures Preparation¥ JIG cable Top view PLL Adjustment Software Adjustment TXFSoftware Adjustment Parts List Main UnitVariable Thermistor NTCCM20124AG473J-T Tantalum ECST0JY106R Chassis Unit Chassis Parts Mechanical Parts and DisassemblyAccessories IC-F3GS parts Diodes SEMI-CONDUCTOR InformationTransistor and FET’S Board Layouts General and EIA/TIA Versions Except W/N-TYPEMP6 Main Unit Bottom ViewIC1 ETS/CEPT VersionMP5 VHF FI1 Parts List Disassembly InformationBC-137 Optional Desktop Charger Informasion ConvenientVoltage Diagram Board LayoutBlock Diagram Option UnitVoltage Diagram BU4066BCFVBand of ETS/CEPT version N-type DTA144EU Section Voltage Diagram Avref Vtref Toned Tonem AvccPage 32, Kamiminami, Hirano-ku, Osaka, 547-0003, Japan UHF FM Transceivers IC-F4GTIC-F4GS Preparation PLL Adjustment Software Adjustment Power supply requirement APC IC3A NJM3403AV Disassembly and Option Instructions Optional Unit Installations 2ND if and Demodulator Circuits RF signal From PLL Driver PLL Circuit Power Supply Circuits Voltage Line Resistor R132PTT Adjustment Procedures Screen Display Example DC Power Cable Connections L405 L11 Powerlo BPF T1 NJM3403AV-TE1 IC4 Resistor ERJ2GE Thermistor NTCCM1608 4LH 223KC Ceramic ECUE1H470JCQ ECEV1CA470SP Other Ceramic ECJ0EB1E471K Band MHz band MHz and H-band IC-F4GS parts DTA144 EU DTC144 EE DTC144 EU DTC144 TU LOW Band DS1FI1 440-470 MHz Band Except W/N-TYPE Icom 470-500 MHz and High Bands B5542B C308 Q19 B5688B BC-137 Optional Desktop Charger Informasion Voltage Diagram Type Type of GEN Type of GEN and EIA/TIA version NJM3403AV M1-band of GEN and EIA/TIA version G W-type of GEN Type 2SC3585 R44 Q403 2SC5107 O Band of ETS/CEPT version N-type M1 440-470 MHz-band Page 13713IZ-C1U
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