Circuit Description; Power Switching And Regulation; Power Switching; Five-Volt Regulators (U205, U206) - E.F. Johnson LTR-Net 7243 Service Manual

NOTE: A block diagram of the RF and logic boards is
located on page 8-8.

4.1 POWER SWITCHING AND REGULATION

4.1.1 POWER SWITCHING

Unswitched battery voltage is applied to the RF
power amplifier and power control stages on the RF
board. Otherwise, all power is switched by P-channel
MOSFETs Q205 and Q206 on the audio/logic board.
When the on-off switch is turned on, the SW BAT
supply is applied to switching transistor Q206. That
transistor then turns on and pulls the gates of Q205A
and Q205B low. This turns Q205A and Q205B on and
applies switched 7.5 volts to 5-volt regulators U205
and U206 and other parts of the transceiver.
When the power switch is turned off, the base of
Q206 is pulled low by R226 which turns Q206 off.
However, because of diode D201, Q205A and Q205B
do not turn off until C233 charges through R221. This
provides a slight turn-off delay to give the micropro-
cessor time to take over power off control. The micro-
processor immediately detects power turn off from the
signal on pin 32. It then changes the P_HOLD output
on pin 71 to a high level which turns Q206 again on
until current settings have been saved to memory and
other required power down functions have been
performed.

4.1.2 FIVE-VOLT REGULATORS (U205, U206)

Regulator U206 in the logic unit provides the
L5V logic supply, and regulator U205 provides the
+5V supply. The +5V supply is in turn switched by
Q202-Q204 to provide the R5V, T5V, and +5S
supplies. These transistors are controlled by signals
from the microprocessor.
The low battery detect signal is provided by a
voltage divider formed by R237 and R238. These
resistors divide down the 7.5V battery supply to
produce the signal that is applied to pin 95 of the
microprocessor.
SECTION 4 CIRCUIT DESCRIPTION
Regulator U601 on the RF board provides the
C5V supply, and regulator U602 provides the T5V and
R5V supply. The T5V supply is enabled when the
DPTT (delayed PTT) signal goes low, and the R5V
supply is enabled when it goes high. The DPTT signal
is from pin 85 of the microprocessor, and it also
controls R5V supply switch Q204 on the audio/logic
board.

4.2 SYNTHESIZER DESCRIPTION

4.2.1 INTRODUCTION

The synthesizer output signal is produced sepa-
rate receive and transmit VCOs on the RF board. The
frequency of these VCOs is controlled by a DC
voltage produced by the phase detector in synthesizer
chip U202. The phase detector senses the phase and
frequency of the two input signals. One signal is the
reference frequency from TCXO U201, and the other
is from the VCO. If these signals are not the same
frequency, the phase detector changes the VCO control
voltage which changes the VCO frequency until both
signals are synchronized. The VCO is then "Locked"
on frequency.
Channels are selected by programming the
counters in U202 to divide by different numbers. This
programming data comes from the microprocessor on
the logic board. The frequency stability of the synthe-
sizer in both the receive and transmit modes is deter-
mined by the stability of TCXO U201. The stability of
this device is 1.5 PPM from –22° to +140° F (–30° to
+60°). The receive VCO output signal is buffered and
then fed to the receiver as the first injection signal, and
the transmit VCO signal is also buffered and fed to the
transmitter as the transmit signal.
4.2.2 RECEIVE AND TRANSMIT VCOS, BUFFER
AMPLIFIERS (Q503, Q533-Q535)
There are separate receive and transmit VCOs on
the RF board. The VCO control voltage is applied to
both VCOs, but only one is active at a time because
they are powered by separate R5V (receive) and T5V
(transmit) power sources.
4-1

CIRCUIT DESCRIPTION

November 2001
Part No. 001-7240-001