4-2-5 APC CIRCUIT (RF UNIT)
The APC circuit protects the power amplifier from a mis- matched output load and stabilizes the output power. The APC circuit is designed to use VHF and UHF bands com- monly.
The APC sensor (R109) detects driving current from the drive voltage at the drive (Q52) and power (Q51) amplifiers. The detected current is converted into DC voltage at Q101, then applied to the APC control circuit (IC101, pin 2). The applied voltage is compared with a “PSET” voltage from the CPU via the D/A convertor (IC251), and the APC control circuit out- puts control voltage from pin 1 to control the drive and power amplifiers.
When the driving current is increased, input voltage of the dif- ferential amplifier (IC101, pin 2) will be increased. In such cases, the differential amplifier output voltage (IC101, pin 1) is decreased to reduce the driving current.
4-3 PLL CIRCUITS
4-3-1 GENERAL
A PLL circuit provides stable oscillation of the transmit fre- quency and the receive local frequency. The PLL circuit compares the phase of the divided VCO frequency to the ref- erence frequency. The PLL output frequency is controlled by the divided ratio
4-3-2 144 MHz AND 430 MHz VCO CIRCUITS (VCO UNIT)
• 144 MHz VCO CIRCUIT
The oscillated signal at the 144 MHz VCO circuit
• APC CIRCUIT
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| APC SENSOR CIRCUIT |
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VCC |
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| R109 |
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TXC |
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| Q102 |
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| Q101 |
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from Tx/Rx | Buff. | Pre |
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amp. | drive |
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switch |
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IC51 | Q54 |
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| D/A CONVERTER |
| 2 | 1 | VGGC |
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| IC251 | 6 | PSET | 3 IC101 |
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+3 | (RF unit) |
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| 14 DSET | 5 IC101 | 7 |
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| Differential |
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| amplifier |
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