CIRCUIT DESCRIPTION
■IF Circuit
The first IF signal (38.85 MHz) amplified by the IF amplifier (Q24) and the second IF signal (38.4 MHz) generated by tripling the 12.8 MHz reference oscillator frequency of the TCXO (X1) by Q23, are mixed in the FM IC to produce a second IF signal (450 kHz) (Lower heterodyne).
The second IF signal passes through a ceramic filter (CF1) to remove unwanted components.
The second IF signal passing through the ceramic filter (CF1) passes through the IF amplifier in the FM IC again and is detected to produced an audio signal.
■Squelch and Mute Circuit
A noise component is obtained by passing FM detection output (FM IC pin 9) through an operational amplifier in the
FM IC and
The squelch voltage enters the SQ terminal (pin 48) of the MPU (IC8) and is compared with the reference voltage preset in the MPU to control audio signal ON/OFF.
When "L" is output from the AFM terminal (pin 9) of the MPU, the AF mute switch (Q20) is activated to mute the audio signal.
■S Meter Circuit
The S meter voltage is output from the RSSI terminal (pin
12)of the FM IC (IC3) and input to the SM terminal (pin 49) of the MPU. Then the voltage is converted from analog to digital in the MPU to control the S meter display on the LCD.
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| VOL IN |
| AFM | 9 |
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| Q20 | |
IC8 |
| 49 | SMV |
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SM |
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| 5R | AF MUTE | |||
MPU |
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| 48 | SQV |
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| SWITCH | |
| SQ |
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| CD1 |
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| AF OUT |
1st IF input |
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| Q21 | |
(38.85MHz) |
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| AF FILTER | |
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| MIX |
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| RSSI |
| LPF | |
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| Noise |
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| Quadrature | |
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| comp |
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| detector | |
| IC3 |
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| Rectifier |
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| FM IC |
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| Buffer |
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| AMP |
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| Local |
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| IF AMP | Inverter | |
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| OSC |
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| AMP |
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| R135 | |
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| C173 | C174 |
| 2nd local OSC |
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| R126 | ||
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| R125 | ||
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| CF1 |
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| 450kHz |
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| 5R | |
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Fig. 3 IF, Squelch, Mute and S meter circuit
■AF Amplifier
The detected audio signal passes through an AF filter (Q21) and enters an AF mute switch (Q20).
After passing through the AF mute switch (Q20), the signal passes through an AF volume (VR201) and is amplified to a specified output level with an AF amplifier (IC6).
The audio signal amplified with the AF amplifier (IC6) is output through an internal speaker or an external speaker jack (J4).
The beep tone and the DTMF signal are output from the DTMF/BEEP terminal (pin 40) of the MPU (IC8), enters the AF amplifier (IC6) and is output as a monitor tone.
IC3 |
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FM IC |
| Q21 |
| Q20 |
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| 9 |
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AF OUT |
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| FILTER |
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| SWITCH |
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| VR201 |
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| SPG |
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AF VOLUME |
| IC6 |
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| REM/TXD | J4 | ||||||||||||||
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| AF AMP |
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| AFO |
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40
SP
DTMF/BEEP
IC8 REM 46
MPU
Fig. 4 AF amplifier
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