SECTION 4 CIRCUIT DESCRIPTION

4-1 RECEIVER CIRCUITS

4-1-1 RF FILTER CIRCUIT (PLL AND MAIN UNITS)

Received signals from the antenna connector are applied to the transmit/receive switching and protection relay (PLL unit; RL7301) which is controlled by the CPU via the “TRXS” line. The signals pass through the 30 MHz cut-off low-pass filter (PLL unit; L7321, C7321–C7323, C7325), and then applied to the MAIN unit via the J7321.

The signals pass through the 1.6 MHz cut off high-pass fil- ter (MAIN unit; L51–L54, L57, C54, C56, C57, C59, C61–C64, C69) and transmit/receive switch (D54), and are then applied to one of the bandpass filters (including one low-pass filter for below 2.0 MHz). These filters are selected by the filter control signals (B0–B8) as described in the table below.

4-1-2 1ST MIXER AND IF CIRCUITS (MAIN UNIT)

The 1st mixer circuit converts the received signals into a fixed frequency, 64.445 kHz 1st IF signal using PLL output frequency. By changing the PLL frequency, only the desired frequency is picked up at the 64.445 kHz bandpass filter (FI201) at the next stage.

The IF amplifier (Q203) and resonator circuits are designed between the filter pair. The PLL output signal (1LO) enters the MAIN unit via the J601 and is amplified at the 1st LO amplifier (Q601). The amplified signal is passed through the

100MHz cut-off low-pass filter (L602–L604, C602, C604–C607, C609) to suppress harmonic components, and then applied to the 1st mixer circuit (Q201, Q202, Q204, Q205).

4-1-3 2ND MIXER AND IF CIRCUITS (MAIN UNIT)

The filtered signals are applied to the pre-amplifier (MAIN unit; Q102, Q103), and then passed through the 33 MHz cut-off low-pass filter (L202, L203, C202–C206). The filtered

Frequency (MHz)

LPF ctrl signal

BPF ctrl signal

0.5–1.9999

L1M

B0

2.0–2.4999

B1

 

2.5–2.9999

L2M

 

3.0–3.4999

B2

 

3.5–4.9999

L4M

 

5.0–5.4999

B3

 

5.5–6.9999

L6M

 

7.0–8.9999

B4

 

9.0–9.9999

 

 

 

10.0–13.9999

L12M

B5

14.0–14.9999

 

B6

15.0–17.9999

 

 

 

18.0–23.9999

L22M

B7

24.0–29.9999

 

B8

 

 

 

signals pass through the low-pass filter (MAIN unit; L202, L203, C202–C206), and are then applied to the 1st mixer circuit (Q201, Q202, Q204, Q205).

The 1st IF signal from the bandpass filter (FI201) is convert- ed again into a 455 kHz 2nd IF signal at the 2nd mixer cir- cuit (D303, L303, L304). The 2nd LO signal (2LO) from the PLL unit enters the MAIN unit via the J301 to be applied to the 2nd mixer circuit.

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

The 2nd IF signal passes through the low-pass filter (L305, C308, C310, C311, C315), and is then applied to the IF amplifier (Q401) via the ceramic bandpass filter (FI401). The amplified signal passes through the other ceramic bandpass filter (FI402), and then applied to the 3rd mixer circuit via the IF amplifier (Q501). The 2nd IF signal is converted into a 12 kHz 3rd IF signal at the 3rd mixer circuit (IC501). The 3rd LO signal (3LO) from the PLL unit enters the MAIN unit via the J3601 to be applied to the 3rd mixer circuit.

• RECEIVER CONSTRUCTION

Antenna

 

 

 

 

 

 

 

 

 

 

1st LO

 

2nd LO

 

 

 

64

 

 

94 MHz

 

64 MHz

 

 

 

 

 

 

 

 

 

 

 

 

0.5 1.9999 MHz

 

 

 

 

 

 

 

 

 

 

 

LPF

 

 

 

 

 

LPF

 

 

 

 

 

 

 

 

 

Crystal

 

 

Crystal

IF

 

 

 

 

 

 

 

1st mixer

BPF

2nd mixer

BPF

amp.

PLL unit

 

 

 

 

 

 

 

 

FI201

FI401

Q401

 

 

 

 

 

 

 

 

 

Q201

D303

 

 

 

 

 

 

BPF

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q202

 

 

 

 

 

 

 

 

 

 

2 30 MHz

Q203

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q204

 

3rd LO

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

443 kHz

 

 

to transmit/receiver switch

DSP

UNIT

IF

LPF

IF

Crystal

amp.

amp.

BPF

IC1002

 

3rd mixer Q501

FI402

 

 

IC501

 

4 - 1

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Icom IC-F7000 service manual Circuit Description, Receiver Circuits, DSP Unit