5-2-3 TRANSMIT AMPLIFIERS (PA UNIT)

The transmit signal from the VCO is amplifi ed to the transmit output level by the transmit amplifi ers.

The transmit signal from the TX/RX switches (MAIN UNIT; D16 is ON, D17 is OFF) is amplifi ed by the pre-drive amplifi er (Q704), drive amplifi er (Q702) and power amplifi er (Q701) in sequence to obtain 5 W (approx.) of transmit output power.

The power-amplified transmit signal is passed through the antenna switch (D701), then applied to the antenna via the LPF (ANT UNIT; L801, L802, C802, C803, C807).

5-2-4 APC CIRCUIT (PA UNIT)

The APC (Automatic Power Control) circuit prevents the transition of the transmit output power level which is caused by load mismatching or heat effect, etc.

A portion of transmit signal is detected by the transmit power detectors (D702, D703) to produce DC voltage corresponding to the transmit output power level. The detected voltage is applied to the APC amplifi er (IC701, pin 3). The transmit power setting voltage “APC (T1)” from the D/A converter (MAIN UNIT; IC23, pin 1) is applied to another input terminal (pin 1) as the reference voltage.

The APC amplifier compares the detected voltage and reference voltage, and the difference of the voltage “APCV” is output from pin 4. The voltage “APCV” controls the bias of the pre-drive (Q704), drive (Q702) and power (Q701) amplifi ers to reduce/increase the gain of these amplifi ers for stable transmit output power.

The transmit power muting is carried out by the TX mute switch (MAIN UNIT; Q46), using the “TMUT” signal from the CPU (MAIN UNIT; IC22, pin 35).

5-2-5 OVER CURRENT DETECTION CIRCUIT (PA UNIT)

The driving current of the drive (Q702) and power (Q701) amplifiers is detected at the current detector (Q705, Q706) by detecting the difference of voltage between both terminals of R714. The detected voltage “ISENS” is applied to the CPU (MAIN UNIT; IC22, pin 47).

In case of the over current is detected, the CPU outputs “TMUT” signal from pin 35 to TX mute switch (MAIN UNIT; Q46) to stop the transmitting to protect the transmit amplifi ers (Q701, Q702, Q704).

• APC CIRCUIT

Q705, Q706

VCC

ISENS

T5V

Current detector

to the receiver circuits

D701

 

 

 

 

 

Pre-drive

 

Drive

 

Power

 

 

 

LPF

 

 

ANT

Transmit signal from the

 

 

 

 

amp.

amp.

 

amp.

 

 

 

 

 

SW

 

 

 

 

 

 

 

 

 

 

 

TX/RX switches

 

 

 

 

Q704

Q702

 

Q701

 

 

 

 

 

 

 

(D16, D17)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T1

1

+

IC701

 

 

 

 

D702

 

 

 

APC

 

APCV

 

 

 

 

TX power

Q46

APC

 

 

 

 

 

 

 

 

3

amp.

4

 

 

 

 

 

 

 

 

detectors

TMUT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LPF to the antenna

ANT UNIT

D703

P_DET

MAIN UNIT

PA UNIT

5 - 3

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Icom IC-F3021T, IC-F3023S Transmit Amplifiers PA Unit, APC Circuit PA Unit, Over Current Detection Circuit PA Unit

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