Barrett 900 Series, Техническое руководство

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BARRETT 900 SERIES TRANSCEIVERS
4.3 PA PCB
The Power Amplifier raises the power from the exciter to specified output power. It also provides harmonic
filtering for the transmitter, and band filtering and a 8 dB RF pre-amp for the receiver.
The signal from the exciter at a level of approx. 35 mV P-P is fed to a gain controlled amplifier U1. This stage
gives a gain variation of 13 dB. Transistor Q1 is configured as an emitter follower to provide buffering, gain
stage Q2 raises the signal level suitable to drive the first of two push pull power amplifiers. This pre-driver
stage employs RF negative feedback via T1, R12 and C4 to achieve low distortion.
A push-pull drive stage Q3 and Q4 is employed to raise the power to a level suitable to drive the 100 W - 125
W power amplifier stage. Transistor Q19 provides the bias reference for the driver stage. Final amplification
to the output level is achieved using Q5 and Q6 in push-pull. This final amplifier stage has an active bias
circuit using U4:A and Q7 with Q21 providing a bias reference which thermally tracks the power devices due
to physical contact.
Sub-octave harmonic filtering is provided by seven Cauer-Chebyshev filters with bands of 1.6 to 2.48 MHz,
2.48 to 3.84 MHz, 3.84 to 6.0 MHz, 6.0 to 9.3 MHz, 9.3 to 14.5 MHz, 14.5 to 22.48 MHz and 22.48 to 30.0
MHz. These are selected by a 3 to 8 decoder U9 which drives a Darlington driver U8 to select the relevant
LPF relays.
Forward and reverse power sampling for the ALC system is implemented using current transformer T4 and
detector diodes D5 and D4. A dual time constant system is used in the ALC detector which feeds the main
ALC amplifier U3. This amplifier compares the incoming combination of forward and reverse power with a
reference voltage and generates a control voltage which is fed through the emitter follower buffer Q12 to the
gain controlled amplifier. Both forward and reverse power DC levels are buffered by U7 and fed to the Micro
PCB. When optional external linear amplifiers are used ,U3B buffers an external current sourced ALC signal
from these linear amplifiers to control the power output of the PA.
Output power is preset by a DC
voltage generated by the microprocessor PCB and fed to the ALC circuit via pin 19 of the 26 way connector
P2.
Overvoltage and reverse voltage protection is incorporated on the PA PCB (Q10, Q11, D2, D14 and SCR1)
and transorb. diode D3 absorbs any instantaneous voltage peaks over 18 volt.
A switching system on the PA PCB, configured using J2, enables an optional UHF connector to be fitted on
the rear of the transceiver. Once fitted the two UHF connectors are configured as described in section 7 of
this manual. The second antenna socket position in the chassis can also be used to locate the GPS BNC
antenna socket when the internal GPS receiver option is fitted.
The receive RF pre-amplifier consisting of Q22, T5 and associated components provide 8 dB gain at the
front end of the receiver. This amplifier is preceded by a high pass filter L12, L13, and associated capacitors
to prevent strong broadcast band transmissions from interfering with the wanted signal.
Regulator U5 provides the +10 V power rail for use in all sections of the transceiver. Q8 switches the +10 V
power rail to p10TX on the PA during transmit mode. U2 provides the +5 V rail, again used in many
sections of the transceiver. Q16 switches the +5 V power rail to provide 5VTX used on the PA during transmit
mode
4.3.1 Gain control amp
4.3.2 Low pass filters
4.3.3 ALC control
4.3.4 Over-voltage and reverse voltage protection
4.3.5 Auxiliary antenna socket
4.3.6 Receiver RF amplifier
4.3.7 Power supply regulators
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