Icom IC-F50, IC-F51 Receiver Circuits, Antenna Switching Circuit Main Unit, RF Circuit Main Unit

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SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN UNIT)

The antenna switching circuit functions as a low-pass filter while receiving and a resonator circuit while transmitting. This circuit does not allow transmit signals to enter the receiver circuits.

Received signals enter the antenna connector (CHASSIS; J1) and pass through the low-pass filter (L1, L2, C1–C5). The filtered signals are passed through the λ4 type antenna switching circuit (D5, D6, L5, L6) and then applied to the RF circuit.

4-1-2 RF CIRCUIT (MAIN UNIT)

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 pass through the two-stage tunable bandpass filters (D4, D8, L7, L8). The filtered signals are amplified at the RF amplifier (Q2) and then passed through the another two-stage tunable band- pass filters (D9, D10, L9, L11) to suppress unwanted sig- nals. The filtered signals are applied to the 1st mixer circuit.

D4, D8–D10 employ varactor diodes, that are controlled by the CPU via the D/A converter (IC6), to track the bandpass filter. These varactor diodes tune the center frequency of an RF pass band for wide bandwidth receiving and good image response rejection.

4-1-3 1ST MIXER AND 1ST IF CIRCUITS (MAIN UNIT)

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

• 2ND IF AND DEMODULATOR CIRCUITS

The RF signals from the bandpass filter are mixed with the 1st LO signals, where come from the RX VCO circuit via the attenuator (R26–R28), at the 1st mixer circuit (Q3) to pro- duce a 46.35 MHz 1st IF signal. The 1st IF signal is passed through a monolithic filter (FI1) in order to obtain selection capability and to pass only the desired signals. The filtered signal is applied to the 2nd IF circuit after being amplified at the 1st IF amplifier (Q4).

4-1-4 2ND IF AND DEMODULATOR CIRCUITS (MAIN UNIT)

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

The 1st IF signal from the IF amplifier (Q4) is applied to the 2nd mixer section of the FM IF IC (IC1, pin 16), and is mixed with the 2nd LO signal to be converted into a 450 kHz 2nd IF signal.

The FM IF IC (IC1) contains the 2nd mixer, 2nd local oscil- lator, limiter amplifier, quadrature detector, active filter and noise amplifier circuits. A 2nd LO signal (45.9 MHz) is pro- duced at the PLL circuit by tripling it’s reference frequency (15.3 MHz).

The 2nd IF signal from the 2nd mixer (IC1, pin 3) passes through the ceramic filter (FI2) to remove unwanted hetero- dyned frequencies. It is then amplified at the limiter amplifi- er section (IC1, pin 5) and applied to the quadrature detec- tor section (IC1, pins 10, 11) to demodulate the 2nd IF sig- nal into AF signals.

The demodulated AF signals are output from pin 9 (IC1) and applied to the AF circuit via the receiver mute circuit.

"SQIN" signal from the D/A converter IC (IC6, pin 2)

AF signal "DET"

8

Active filter

FM

detector

9

 

2nd IF filter

 

 

 

 

 

 

 

 

 

 

X2

 

450 kHz

 

 

 

 

Q19

 

 

 

 

 

 

 

 

 

 

 

 

 

15.3 MHz

 

 

 

 

 

45.9

MHz

 

 

PLL IC

 

 

FI2

 

 

3

2

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

7

5

 

 

3

 

2

 

 

 

 

IC4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Noise

Noise

 

 

 

detector

comp.

 

Mixer

 

Limiter

 

 

 

 

 

 

 

amp.

 

 

 

 

 

RSSI

 

IC1 TA31136FN

 

 

 

 

10

11

12

13

16 1st IF from the IF amplifier (Q4)

 

 

 

"NOIS" signal to the CPU (FRONT unit; IC401, pin 41)

 

R5V

"RSSI" signal to the CPU (FRONT unit; IC401, pin 50)

X2

4 - 1

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Contents VHF Transceiver Ordering Parts IntroductionRepair Notes Table of Contents ‘ Receiver Specifications‘ General ‘ TransmitterMain Unit Inside ViewsFront Unit Removing the Main Unit Disassembly InstructionsRemoving the Chassis Unit Removing the Front Unit3 1ST Mixer and 1ST if Circuits Main Unit Antenna Switching Circuit Main UnitReceiver Circuits RF Circuit Main UnitMicrophone Amplifier Circuit Front and Main Units Transmitter CircuitsAF Amplifier Circuit Main and Front Units Receive Mute Circuits Main and Front UnitsAPC Circuit Main Unit DRIVE/POWER Amplifier CircuitsPLL Circuits Modulation Circuit Main UnitFront Unit Voltage Line Power Supply CircuitVCO Circuit Main Unit Main Unit Voltage LineExpander IC Front Unit IC410 Other CircuitsPort Allocations Compounder Circuit Main UnitMsck SEG1SCK MdioExpander IC Main Unit IC12 4 D/A Converter IC Main Unit IC6Preparation Ctcss Adjustment Frequency ListFrequency Adjustment Item Screen Display ExampleSP+ ConnectionJIG Cable Ptte PTT MIC MiceSoftware Adjustments Transmitting Convenient Software Adjustments ReceivingVR Board Connector BoardFront Unit IC9 Main UnitVaricap HVC375BTRF Themistor Ertjoep 473J Ceramic ECJ0EC1H0R5B Mechanical Parts and Disassembly Unitmm inch Transistor and FET’S SEMI-CONDUCTOR InformationDiodes Main Unit Board LayoutsTOP View Q23 Bottom ViewJ402 Front UnitMC401 VR Board VHF B6034BMP701 Connector BoardCPU Section Block Diagram2SC4116BL Voltage DiagramTC7W66FK 11-3 VR / Connector Boards VR Board Icom America Inc 32, Kamiminami, Hirano-ku, Osaka, 547-0003, Japan