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FX-440 CIRCUIT ·ovERVIEW
The FX-440 UHF FM Transceiver circuit theory is explained in
progressive stages and in some detail as part of our "Learn As You
Build" approach to electronic kits. Builders are encouraged to study and
- learn about a stage or section, build it and then test it before going to the
next ·stage. The circuit explanations are necessarily written for people
with all levels of experience, starting with and FAVORING beginners.
~
Following is a straight and "minimally chatty• synopsis or overview of
FX-series technical information provided in the building stages. However,
we' II still follow the same stage-by-stage designations of the building
process.
A: DC Power Input
- Much of the circuitry operates on the regulated 8 volts supplied by
voltage regulator VR1. • +SR" or • + ST" are points where the regulated
av
output is switched for Receive or Transmit by the PTT circuitry (012,
U4c, U4d, 013, 014).
The Receiver IC (U1) and the digital frequency synthesis circuit is
powered by + 5 volts regulated by VR2. The op amps used in the circuit
(U4 and US) operate from this single supply through the use of voltage
divider networks at the respective IC' s. The full 12-15 volt input is
supplied to the transmitter RF output section and to the receiver audio
amplifier (U2).
. .. Components L20 and C42 provide ignition noise filtering. The 5-pin QIN
Packet 1/0 Jack (J1) has pinouts corresponding to current conventions for
TNC's. Receiver audio to pin 4 may be taken from the amplified speaker
output, or from the FM discriminator output or from the true FSK data
_ output of U1. Selection is by a jumper wire on the PC-board.
B: Receiver Audio Amplifier
The LM380 is a self-contained general purpose audio amplifier capable of
over 2 watts audio output with a voltage gain of 50. Audio from from the
FM discriminator (U1) is fed through C7 through the volume control (R7)
to pin 2, the amplifier input. The amplified output at pin 8 is available
through C34 to both the speaker jack and pin 4 of the Packet 1/0 Jack.
C41 in series with R108 across this amplified output are good practice
recommended to prevent self-oscillation of the IC. Pin 1 is bypassed to
ground through C48 in normal operation.
If pin 1 is grounded directly, the internal bias of the LM380 is upset, and
the amplifier is silenced. 06 is a simple switch. When 8 volts is applied
through R107 and 022 to the base of 06, the transistor collector grounds
pin 1 of U2, thus silencing the receiver during transmit. The COR output
of U1 (pin 1
O)
also mutes the amplifier.
.
.
- FX-440
Ref.
19
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