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TM 11-5820-890-30-5
5-4.
RF SIGNAL PATHS.
a.
High Power Transmit Path. When the HIGH POWER XMT control line is at logic 1, the control circuits
set the RF switches in the output filter switch (6A1A1) to the XMT position. See figure FO-16. The RF signal is
input at connector J2. It passes through E1 to the first RF switch. With the switch as shown in figure FO-16, the
signal is output at E3. It travels through cables W1 and W3 to the power amplifier heat sink (6A3) at E1. The
power amplifier heat sink provides 12 dB of RF gain. (1 dB is lost in the filters and switches.) It is a push-pull
amplifier with input and output impedance matching. The output at E5 goes through cables W4 and W2 into the
input filter switch at E1. It switches the signal through one of three low-pass filters in filter FL14. The low-pass filter
attenuates any harmonics or spurious signals outside the filter’s frequency range. The output of the low-pass
filter is routed to the second RF switch and out to connector J1 through the power detector. The power detector
measures the output power level to provide an ALC feedback signal. It also measures the reflected power to
provide a VSWR feedback signal.
b.
Low Power Transmit Path and Receive Path. When the RT RF switch is set to LO, M, or Hl, the HIGH
POWER XMT input line is held at logic 0. The HIGH POWER XMT line will also be at logic 0 any time the radio is not
transmitting. This causes the two electronic RF switches in the output filter switch (6A1A1) to be set to the RCV
position. This basically connects connector J1 to connector J2 so that signals can pass between the RT and the
antenna.
5-5.
FEEDBACK AND OUTPUT SIGNAL PATHS.
The two main feedback signals originate at the power detector in the output filter switch (6A1A1). See figure
FO-16. The forward power is checked and is used for the automatic level control (ALC) signal. The reverse or
reflected power is checked and is used for the VSWR signal. Both of these are 0 to 4 V dc signals and are fed
back to the decoder control (6A2). The ALC signal is used to provide the POWER LEVEL signal that goes back to
the RT to drive the SIG display. The VSWR and the ALC signals are compared. If the VSWR is greater than 5:1, the
SIDETONE DISABLE line is set to 6.5 V dc. Adjustments are provided on the decoder control to adjust the
sidetone disable limits.
The power amplifier heat sink (6A3) uses the 27 V dc power from the constant current source circuit of 6A1Q1
and 6A1Q2. Most of the current is supplied from Q1 through filter FL1. It is used to regulate the RF gain of the
power amplifier heat sink. The gain is reduced if any of the following occurs:
The RF power output exceeds its maximum limit.
The VSWR exceeds 5:1.
The temperature of the power amplifier exceeds 70° C (160° F).
The temperature of the power amplifier is sensed by two thermistors in the power amplifier heat sink. They are
connected in series between E4 and ground. The decoder control monitors the resistance at pin 24 of J1.
5-3
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