3-4 DSC CIRCUITS ([USA], [CAN] ONLY)

3-4-1 DSC BANDPASS FILTER AND RF AMPLIFIER CIRCUITS (MAIN UNIT)

The RF circuit filters out-of-band signals and amplifiers signals within the range of frequency coverage.

(1) 2–7 MHz signals

The signals from the antenna connector (J2101) pass through the bandpass filter (L2201, L2202, L2207, L2210, L2213, L2216, L2219, L2220, L2225, L2228, L2231, C2201, C2204, C2207, C2210, C2213, C2216, C2219, C2222, C2225, C2228, C2234, C2240) to obtain desired 2–6 MHz signals and suppress any undesired signals.

(2) 8 MHz signals

The signals from the antenna connector (J2101) pass through the bandpass filter (L2204, L2208, L2211, L2214, L2217, L2221, L2222, L2229, L2232, C2202, C2205, C2208, C2210, C2211, C2214, C2217, C2220, C2223, C2226, C2236, C2241) to obtain desired 8 MHz signals and suppress any undesired signals.

(3) 12 MHz signals

The signals from the antenna connector (J2101) pass through the bandpass filter (L2235–L2242, L2244, L2245, C2229, C2235, C2237, C2239, C2243–C2250, C2252–C2254, C2263, C2264) to obtain desired 12 MHz signals and suppress any undesired signals.

(4) 16 MHz signals

The signals from the antenna connector (J2101) pass through the bandpass filter (L2205, L2206, L2209, L2212, L2215, L2218, L2223, L2224, L2230, L2233, C2203, C2206, C2209, C2212, C2215, C2218, C2221, C2224, C2226, C2227, C2238, C2242) to obtain desired 16 MHz signals and suppress any undesired signals.

The filtered signals are amplified at the RF amplifiers (Q2301, Q2302), and then applied to the 1st mixer circuit (D2301).

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

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.

The signals from the RF circuit are mixed at the 1st mixer (D2301) with a 1st LO signal coming from the PLL circuit to produce a 10.7 MHz 1st IF signal.

The 1st IF signal is applied to the crystal bandpass filters (FI231) to suppress out-of-band signals. The filtered 1st IF signal is amplified at the 1st IF amplifiers (Q2401, Q2501), then applied to the 2nd mixer circuit (IC2701, pin 3).

3-4-3 DSC 2ND IF AND DSP CIRCUITS (MAIN AND DSP UNITS)

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

The 1st IF signal from the 1st IF amplifiers is applied to the 2nd mixer IC (IC2701, pin 3), and is mixed with the 2nd LO signal to the converted into a 12 kHz 2nd IF signal.

The 12 kHz 2nd IF signal from the 2nd mixer (IC2701, pin 5) is applied to the 2nd IF amplifier (IC2702, pin 3), and then passes through the low-pass filter (IC2702, pins 5, 7). The signal is applied to the DSP unit as the “DSPI2” signal.

The signal is amplified at the differential amplifier (DSP unit; IC601), and is then applied to the A/D converter secition in the CODEC IC (DSP unit; IC501). At the same time, the converted signal is level-shifted 5 V to 3.3 V in the IC (DSP unit; IC501).

The level-shifted signal is applied to the DSP IC (DSP unit; IC301) for the digital IF filter, demodulator and noise reduc- tion, etc.

The output signal from the DSP IC is applied to the D/A converter section in the CODEC IC (DSP unit; IC501) to convert into the analog audio signals. Also the signals are level-shifted 3.3 V to 5 V at the level converter section in the IC.

The level-shifted audio signals are passed through the active filter (IC701b), and then applied to the MAIN unit via the J1102 (pin 21) as the “DSPO2” signal.

3-4-4 DSC AGC CIRCUIT (DSP AND MAIN UNITS)

The receiver gain is determined by the voltage on the AGC2 line from the DSP unit. The D/A converter for the AGC (IC102) supplies control voltage to the AGC2 line and sets the DSC receiver gain.

The 3rd IF signal from the CODEC IC (IC501) is detected at the AGC detector section in the DSP IC (IC301). The output signal from the DSP IC is level-shifted at the level converter (IC101) and applied to the D/A converter (IC102). The AGC voltage is amplified at the buffer amplifier section in the IC102 and applied to the MAIN unit to control the AGC2 line.

When receiving strong signals, the detected voltage increases and the AGC2 voltage decreases. As the AGC2 voltage is used for the bias voltage of the IF amplifiers (MAIN unit; Q2401, Q2501), IF amplifier gain is decreased.

3-5 FRONT UNIT (RC-25)

3-5-1 LCD CIRCUIT (DISPLAY AND CONNECT BOARDS)

The LCD (CONNECT board; DS8220) is controlled by the LCD driver (CONNECT board; IC8240) via the drive signals (CL1, RES, CS, RS, WR, RD, DB0–DB7) from the display board. The LCD’s back light employs 24 LED (DS8300–DS8323). The LED are controlled by dimmer (Q8300, Q8301) and dimmer controller (IC8280) circuits.

3-5-2 MICROPHONE CIRCUIT (DISPLAY BOARD)

The AF signals from the microphone are amplified at the AF amplifier (IC8280) via the “MIC” signal. The signal is applied to the IC-M802’s main unit through the J8903 via the “MI” signal.

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