Icom IC-F3GS Circuit Description, Receiver Circuits, Antenna Switching Circuit, Rf Circuit

SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT

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 λ⁄4 type diode switching system.

Received signals are passed through the low-pass filter (L1, L2, C3, C8, C666). The filtered signals are applied to the λ⁄4 type antenna switching circuit (D2, D8).

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 (L17, D10, C85, C86). The amplified signals are applied to the 1st mixer circuit (Q13) after out-of-band signals are suppressed at the 3 stages tunable bandpass fil- ter (D401, L18, C89, C406, D11, L402, C91, C92, C94, D12, L19, C97, 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 46.35 MHz 1st IF signal.

The 1st IF signal is applied to a pair of crystal filters (FI1) to suppress 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

⋅3

8

7

5

3

2

R87

R88

Noise

Noise

Active

detector

comp.

Mixer

filter

Limiter

X4

15.3 MHz

amp.

"SQLIN" signal to 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