SEA 157S, Предварительное руководство по обслуживанию

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TRANSMIT/RECEIVE ANTENNA FILTERING: The 7-section low-pass filter
comprised of C4, L1, C5, C7, L2, C126, C8, L3, C127, C9, L4, and
C10 provides VHF and UHF attenuation of the transmitter harmonics
and receiver images.
5.6 AUTOMATIC RF POWER CONTROL (APC) AND TX LOGIC:
In transmit mode a negative feedback control system continuously
monitors and, if necessary, corrects the output power level at the
antenna terminals. C119 samples the RF voltage at the RF power
amplifier module output terminal (4). Diode D34 converts this RF
signal to a DC level representing the output power level. This DC
power level signal is fed to one channel of the internal A/D
converter of CPU IC, U1, located on the Mezzanine board. The
digitized signal is processed by a power control routine, which,
through the D/A converter U17, drives the DC amplifier consisting
of Q5 and Q13 to provide the correct DC supply voltage to Q9 and
bias to the power amplifier module U16. This RF power level
closed-loop system thus maintains the RF output power at the
proper level. Two references are used in the control routine which
correspond to 1 watt or 25 watt output levels. When adjusted
according to the alignment instructions, the APC system will
closely maintain the 1W or 25W output level (as selected) over a
wide range of power supply voltage and ambient temperatures. In
the unlikely event that the automatic power control system should
fail, the power amplifier cannot produce much more 30 watts.
5.7 TX LOGIC DETECTOR: The DC level from rectifier D34 which is
used by the above APC circuit is also used by the CPU to determine
when the output power level exceeds 1/2 watt and turns on the TX
annunciator on the front panel liquid crystal display.
5.8 POWER and AUDIO CIRCUITS:
All receiver volume, squelch, beep and muting processes are
performed within the DSP U13. The processed audio is sent through
the CODEC U8 to U11 where it is amplified up to a maximum of 4
watts.
Most of the SEA 157S internal circuitry is powered from the 13
volt switched (13VSW) bus. This is derived directly from the fused
and filtered input voltage from the main power source via Q1-A.
The 13V_TX bus is also powered by the 13VSW bus by way FET switch
Q-B.
The audio power amplifier and a number of control switches and
voltage regulators are powered by the +13VSW bus. The +5Vdc
regulator U9 operates from this bus while the +3.3Vdc regulator U2
and the +1.6Vdc regulator U5 derive their power from the +5Vdc
bus.
Both the Receiver and the VCO modules operate from the 13VSW bus
and contain their own regulators, U4 and U1 respectively.
5.6