Midland 70-1341, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
70-1341/1344/1441/1444
5 - 3
CIRCUIT DESCRIPTION
The Titan radio is made up of four major sections: the RF Section, the PA Section, the Logic Section and the Display
section.
RF SECTION
The RF Section consists of a frequency synthesizer, a
transmit modulator, a receiver, and receive audio
amplifier circuits.
SYNTHESIZER
Radio frequency signals for transmission and receiver
injection are produced by voltage-controlled oscillators
(VCO’s) in a Phase-Lock Loop (PLL) configuration.

Voltage Controlled Oscillator
In this radio, a VCO is used. Q706 operates in transmit
mode to generate transmit frequencies, and in receive
mode to receive injection frequencies. In each mode,
the output of the buffer is amplified by IC703. RF signal
at receiver injection frequency (F
C
= 45.0 MHz) is
applied from the LO amplifier Q717 in the receiver
circuit. RF signal from Q708 is amplified further by the
PA portion.
When the frequency of the VCO output drifts away
from the desired value, the loop adjusts the steering
voltage to compensate. A single VCO tank can tune
across the entire channel spread 24 MHz.
Resonance of each VCO tank is voltage-tuned by
varactor diodes D701 and D705 respectively. Loop
steering voltage applies reverse bias to all these
varactor diodes simultaneously. As steering voltage
increases, varactor diode capacitance decreases.
Thus, net capacitance in each tank decreases, which
increases resonant frequency of the tanks.

Loop Dividers
The amplitude of the VCO signal from IC705 for TX
and RX are sufficient to feed prescaling frequency
divider, IC701, which applies an output pulse to once
every 64 or 65 input cycles. Additional frequency
division is also performed within IC701 to produce
5/6.25
kHz. X701 is a temperature-compensated
crystal oscillator that produces a reference frequency
of exactly 12.8
MHz. The reference frequency is
divided by IC701 to produce 5/6.25 kHz that is
compared to the down-counted 5/6.25 kHz sample of
VCO output.
Normally the loop response is slowed enough by the active
filter to block 5.0 kHz reference noise and prevent loop
correction of voice modulation during transmit. Higher
active filter rolloff frequency is selected by the
microcomputer system on the Logic Board when the radio
changes channels or it is keyed and unkeyed, by a logic
low applied to the base of Q705. This increase in loop
response speeds locking time.
A connection from an intermediate point in the
phase/frequency comparator in IC701 is made at pin 7.
When the loop is out of lock, the down-counted VCO
sample is not in phase with the 5.0 kHz reference and low
going pulses appear here, which produce a logic low at pin
7. This logic low is applied to Q710 through Q709 to switch
to Q509. Q509 then clamps off bias to transmit PA
preamplifier Q501 to prevent emission of erratic signals
generated by the uncontrolled VCO.
RECEIVER

Preselector
Through PIN-diode gates in the PA, RF signals are routed
to the receiver input. Signals at image frequencies and
frequencies far removed from the desired channel are
rejected by a preselector comprised of six top-coupled,
parallel tanks: L201, L202, L204, L206, and L207. No
tuning of these tanks is required for the entire 24 MHz
channel frequency spread. Q201 provides adequate gain
to overcome preselector signal losses and maximize
receiver sensitivity.

Injection
First Local Oscillator signal (channel frequency plus
45 MHz) is synthesized by the phase-lock loop and applied
to Q717. A low pass filter is provided at the output of Q717;
this rejects extraneous synthesized signals. No alignment
for the first local oscillator signal is required.

First Mixer
To maximize intermodulation immunity, a balanced
configuration is used for the first mixer stage. High injection
is applied to L210-primary and preselector output is
applied to its secondary center tap. A diode double
balanced mixer using quad-diode D202 is employed. High
injection is applied to the push-pull input of the mixer.
Some of this signal appears at mixer output, but most is
lost because L209 is designed to operate at the 45 MHz
first IF frequency.