SECTION 3 CIRCUIT DESCRIPTION

3-1 RECEIVER CIRCUITS

3-1-1 RF FILTER CIRCUIT (FILTER AND MAIN UNITS)

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

The signals pass through the transmit/receive switch (D53) and 1.6 MHz cut off high-pass filter (L51–L54, C54, C56, C57, C59, C61–C64), 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.

The filtered signals pass through the 33 MHz cut-off low- pass filter (L202, L203, C202–C206), and are then applied to the 1st mixer circuit (Q201, Q202).

Frequency (MHz)

LPF ctrl signal

BPF ctrl signal

0.5–1.999

L0

B0

2–2.999

L1

B1

3–4.999

L2

B2

5–6.999

L3

B3

7–9.999

L4

B4

10–13.999

L5

B5

14–17.999

L6

B6

18–19.999

 

 

 

20–21.999

 

B7

22–23.999

L7

 

24–29.999

 

B8

 

 

 

3-1-2 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 pair of crystal filters (FI301a, FI301b) via 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 (L604, L605, C604,

C606–C610) to suppress harmonics components, and then applied to the 1st mixer circuit (Q201, Q202).

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

The 1st IF signal from the crystal filter (FI301b) is converted again into a 455 kHz 2nd IF signal at the 2nd mixer circuit (D302, 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.

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

The 2nd IF signal passes through the low-pass filter (L305, L306, C307–C311), and then applied to the IF amplifier (Q401) via the ceramic bandpass filter (FI401). The amplified signal passes through the 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.

3-1-5 DSP RECEIVER CIRCUIT (MAIN AND DSP UNITS)

The DSP (Digital Signal Processor) circuit enables digital IF filter, digital noise reduction, digital PSN (Pulse Shift Network), phase demodulation, digital automatic notich, and etc.

The 3rd IF signal is applied to the IF amplifier (MAIN unit; IC1002, pin 5) after being passed through the low-pass filter (MAIN unit; IC1002, pins 3, 1). The amplified 12 kHz 3rd IF signal is amplified at the differential amplifiers (IC651a/b), and is then applied to the A/D convertor section in the CODEC IC (IC501) on the DSP board (EX-2432). At the same time, the converted signal is level-shifted 5 V to 3.3 V in the IC (IC501).

•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.

FILTER unit

 

 

 

 

 

 

 

 

 

FI301A

FI401

Q401

 

 

 

 

 

 

 

 

 

Q201

Q201

 

 

 

 

 

 

BPF

 

 

FI301B

 

 

 

 

 

 

 

 

 

 

 

 

Q202

 

Q202

 

 

2—30 MHz

3rd LO

443 kHz

to transmit/receiver switch

DSPUNIT

IF

LPF

IF

Crystal

amp.

amp.

BPF

IC1002

 

3rd mixer Q501

FI402

 

 

IC501

 

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