BARRETT 900 SERIES TRANSCEIVERS
4.2
RF / Audio PCB
The RF section of the RF / Audio PCB provides the complete exciter and receiver for the transceiver. It
includes:-
First mixer
Roofing filter at 45 MHz
45 MHz IF amplifier with AGC
45 MHz Noise blanker receiver
Second mixer
Noise blanker gate
SSB filter and narrow filter for AFSK
Envelope detector for AM
Product detector for SSB,FSK and CW signals
AGC detector
The RF section has a common RF input and output port and separate audio input for Tx and output for Rx. A
double superheterodyne system is used, with a first IF of 45MHz and second IF at 455 kHz. Three oscillator
injection frequencies are taken from the synthesiser, LO1 at (Carrier fre 45 MHz), LO2 at 45.455
MHz and LO3 or the Carrier Insertion Oscillator at 453.3kHz on USB and 456.6kHz on LSB. There is a
separate 45 MHz receiver to generate gating pulses for the noise blanker.
The audio section of the RF/Audio PCB handles the routing of audio signals in the transceiver and
implements audio processing and frequency tailoring. It also handles the extensive I/O with the outside
world such as Auxiliary 600 input and output (950, 980 only), computer Interface (950,980 only), ATU
Interface and external alarm. The audio power amplifiers for both the local and remote head speaker
outputs are also fitted on the RF/Audio PCB.
The 30 MHz low pass filter, L16, L20 and associated capacitors, minimises the image response of the
receiver. It also serves to control LO1 breakthrough on both Rx and Tx. The first mixer U29 and post mixer
amplifier Q1 is switched as a block between receive and transmit by relay RL1. The local oscillator level for
this mixer is +13dBm. The amplifier achieves the best possible broadband termination for the mixer IF port,
important to maintain low intermodulation distortion. The post mixer amplifier then feeds into the noise
blanker receiver via an isolation amplifier Q28, and also into the main receiver IF section via a dual section
45 MHz roofing filter giving medium selectivity of approxi/- 3 kHz @ -3dB and +/-25 kHz @ 60dB.
The roofing filter feeds or is fed by the dual gate FET 45 MHz IF amplifier consisting of Q4 and Q5. The input
of this IF amplifier is matched to 50 by L1 and C21. The output is matched to 50 by the L network L9, C37.
AGC is applied to the 45 MHz IF amplifier via the second gates on Q4 and Q5. RL2 and RL3 control the Tx/Rx
turn around of this IF amplifier.
In transmit the 45MHz IF amplifier is reversed in its direction by relay RL2 and RL3, the output from the
second mixer is amplified by this 45MHz amplifier which in the transmit direction is gain controlled by a
voltage switched in place of the AGC signal. This voltage is adjusted by VR2 which sets the drive of the
exciter. The output of the 45MHz IF amplifier is then passed through the 45MHz roofing filter consisting of F2
and F2A and into the first mixer U29 and its associated post mixer pad and amplifier. The whole mixer, pad
and amplifier is reversed in the direction used in receive by relay RL1. The output of the mixer is fed into low
pass filter consisting of L16, L20 and associated capacitors.
4.2.1
RF section
4.2.1.1 30 MHz Low pass filter, first mixer and 45 MHz IF amplifier.
Receive
Transmit
PAGE 19
Содержание 900 Series
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Страница 89: ...BARRETT 900 SERIES TRANSCEIVERS PA schematic See File PA SCHEMATIC pdf PAGE 89 ...
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Страница 99: ...BARRETT 900 SERIES TRANSCEIVERS Front panel PCB overlay PAGE 99 ...
Страница 100: ...BARRETT 900 SERIES TRANSCEIVERS PAGE 100 Front Panel Schematic 900 Front Panel Schematic Drawing No 900516A1 ...
Страница 103: ...BARRETT 900 SERIES TRANSCEIVERS Remote Control Head PCB overlay PAGE 103 ...
Страница 108: ...BARRETT 900 SERIES TRANSCEIVERS Scrambler PCB overlay PAGE 108 ...
Страница 109: ...BARRETT 900 SERIES TRANSCEIVERS Scrambler Schematic PAGE 109 900 Scrambler Module Schematic 900D54A5 ...
Страница 111: ...BARRETT 900 SERIES TRANSCEIVERS GPS ALE Motherboard Overlay PAGE 111 ...
Страница 112: ...BARRETT 900 SERIES TRANSCEIVERS GPS ALE Motherboard Schematic PAGE 112 900 GPS ALE Schematic ...
Страница 114: ...BARRETT 900 SERIES TRANSCEIVERS 950MR Low Pass Filter Power PCB overlay PAGE 114 ...
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Страница 121: ...BARRETT 900 SERIES TRANSCEIVERS 980PSU DC DC Switch Mode Power Supply PCB Overlay PAGE 121 ...
Страница 133: ...BARRETT 900 SERIES TRANSCEIVERS 900 Hand Microphone Schematic PAGE 133 930 940 950 980 Hand Microphone Schematic ...
Страница 134: ...BARRETT 900 SERIES TRANSCEIVERS 900 Cloning Cable Schematic PAGE 134 930 950 980 Cloning Cable Schematic ...
Страница 139: ...940 Front Metalwork Connection PCB Overlay BARRETT 900 SERIES TRANSCEIVERS PAGE 139 ...
Страница 142: ...BARRETT 900 SERIES TRANSCEIVERS 940 Automatic Antenna Tuner PCB Overlay Barrett P N BCA94009 PAGE 142 ...
Страница 148: ...940 Tuner Front Panel PCB Overlay BARRETT 900 SERIES TRANSCEIVERS PAGE 148 ...
Страница 149: ...940 Tuner Front Panel PCB Schematic BARRETT 900 SERIES TRANSCEIVERS PAGE 149 ...
Страница 153: ...BARRETT 900 SERIES TRANSCEIVERS 940 Charge reg 13Ah 12V DC input PCB overlay Barrett P N BCA94021 PAGE 153 ...
Страница 163: ...BARRETT 900 SERIES TRANSCEIVERS PAGE 163 940 Programming Cable Schematic 940 Transceiver Programming Cable Schematic ...