Ericsson KRD 103 117/1, Руководство по обслуживанию, Страница: 5 / 54

First IF Pre-Amplifier (Q2)
The filtered IF signal is applied to the first IF pre-ampli-
fier Q2 to recover the losses of the filters and increase the
sensitivity of the received signal. Then it is sent to pin 24 of
the IF IC.
IF IC (IC1)
IC1 is a bipolar monolithic integrated circuit that pro-
vides all the functions of a comprehensive FM IF system.
This IC contains a second local oscillator, second IF mixer,
second IF amplifier, second IF limiting amplifier and a quad-
rature FM detector.
The second mixer combines the first IF signal with the
second local frequency (59.395 MHz) to produce the second
IF signal at 455 kHz.
The second IF signal is supplied to 455 kHz ceramic fil-
ters FT3 and FT4 to reduce noise near the second IF. The
signal is then applied to the second IF amplifier and limiting
amplifier. The limiting amplifier limits the signal to a spe-
cific level, removing any amplitude noise by clipping the
455 kHz signal. The limited 455 kHz signal is fed to the
quadrature detector and releases a demodulated audio signal
at pin 15 of IC1. This signal contains CTCSS tone (or
DCSS) and voice audio, or just a white noise at no rf signal
input.
De-Emphasis/Highpass Filter (R21, C33 And
IC2)
The output audio signal (with sub-audible data) at pin 15
of IF IC goes through de-emphasis cirucit (R21 and C33)
and highpass filter IC2. The de-emphasis circuit performs a
specific audio frequency response. The highpass filter at-
tenuates the low frequency signal of CTCSS (or DCSS) con-
tained in the demodulated signal.
RX Audio Muting (Q4 And Q7)
The audio output signal from the highpass filter is sent to
the audio frequency muting circuit Q4. The microprocessor
(pin 47) sends a mute control signal to Q4 through inverter
Q7.
The audio signal passes through the mute circuit when
the microprocessor sends a logical low level signal to the
mute circuit.
Volume Controller (IC3)
The audio signal is sent to the volume controller (pin 4 of
IC3). The microprocessor provides serial data to IC3 such as
clock, data and enable signals to control the audio output
level to the audio amplifier. Another volume controller (pin
14 of IC3) is connected to R218 of the transmit VCO and
controls the modulation flatness by the microprocessor.
The microprocessor (pin 10) generates a tone to indicate
radio conditions such as channel busy lockout, time out
timer and switch tone. This tone is applied to the volume
controller.
Audio Amplifier (IC4)
Pin 9 of IC4 receives and amplifies the audio signal from
the volume controller (pin 6 of IC3). Audio power output de-
livered to the internal speaker is normally more than 0.5
watts at 4 ohms. Pin 3 of IC4 releases the amplitude received
audio and sends it to the external speaker jack J3 then ap-
plies it to the internal speaker.
Squelch Circuit (IC1)
The IF IC also has a noise amplifier for squelch control-
ler. RT1 adjust the input level of noise from the output of the
discriminator to set the squelch threshold level. Pin 19 of
IC1 sends the squelch signal to pin 56 of the microprocessor.
If the radio received a carrier, pin 19 of IC1 sends a logi-
cal low signal to the microprocessor.
TRANSMITTER
The transmitter converts an audio message to an RF sig-
nal and amplifies the RF signal to 5 watts at the antenna ter-
minal. The transmitter frequency range is 146 to 174 MHz
and the bandwidth without alignment is 28 MHz (VHF) or
438 to 470 MHz and the bandwidth, without alignment, is
32 MHz (UHF). The transmitter contains the following cir-
cuits:
•
Microphone amplifier
•
Pre-emphasis and limiter
•
Low pass filter
•
Modulation buffer
•
Pre-amplifier
•
FM modulator
•
Pre-driver
•
RF power amplifier
•
RF power controller
•
Transmit inhibit
•
Antenna switch
When the PTT switch is pressed, the negative side of the
microphone is grounded and DC bias voltage is applied to the
internal microphone to activate it. The microprocessor (pin 6)
detects the pressed PTT switch and switches Tx/Rx control
(pin 52) to the Tx mode (a logical low).
Microphone Amplifier (Q701)
The speech audio from the microphone enters the base of
Q701 and is amplified. Then it is released to the high pass filter
(IC701).
High Pass Filter/Pre-Emphasis/Limiter Amplifier
The output signal of the microphone amplifier enters the
pre-emphasis/pre-amplifier (C703, Q701). The high pass filter
(IC701) removes any components in the sub-audible range (be-
low 300 Hz) that would interfere with the CTCSS or DCSS
tone.
The output signal of the high pass filter enters the pre-em-
phasis/limiter amplifier (IC702) to provide limiting modula-
tion.
MIC Mute (Q702 And Q703)
The output of the limiter is sent to the microphone mute
circuit Q702. The microprocessor (pin 48) sends a mute con-
trol signal to Q702 through an inverter Q703.
The audio signal passes through the mute circuit when the
microprocessor sends a logical low level to the mute circuit.
Low Pass Filter (IC703)
The limiter output is supplied to the low pass filter IC703
through a sum-amplifier IC702. The limiter output contains
high frequencies above 3 kHz. The low pass filter filters in
these high frequencies and releases them to pin 1 of IC703 and
then to the synthesizer.
TX Pre-Amplifier (Q508)
The output of the synthesizer is supplied to the pre-ampli-
fier Q508. The Tx pre-amplifier increases the transmit signal to
a level sufficient to drive the power module.
Power Module And Power Controller (IC501,
L401, D401, Q501, Q506 And Q507)
The power amplifier circuit amplifies the output of the syn-
thesizer to the proper output wattage (normally 5 watts at the
antenna connector). The transmit power detect circuit (L401
and D401) is connected to the output of the antenna switch
(D403) and monitors the transmit power.
The power control circuitry is a closed negative feedback
loop system (IC501, D401, Q501, Q506, Q507, IC501). It
monitors the transmit power and increases the control current
to the power module (pin 3 of IC501). If the RF output power
decreases, the transmit power is stabilized to the level adjusted
by RT501.
In the low power transmission, the microprocessor (pin 9 of
IC601) sends a logical low signal to the power control circuit
(base of Q501 through R513). The threshold level of the trans-
mit power controller changes to the low power level. The
power amplifier circuit amplifies the output of the synthesizer
and the transmit local oscillator to the proper output wattage
(normally 1 watt at the antenna connector).
Transmit Inhibit (Q502 And Q503)
While the PLL is out of lock, pin 7 of PLL IC301 releases a
logical low to the transmit inhibit circuit Q503 through in-
verters Q303 and Q504. Since the output of Q503 turns Q502
off, no DC power is applied to the first stage of the power am-
plifier and the power controller to prevent transmission on an
improper frequency.
When the PLL is locked, pin 7 of IC301 becomes logical
high. This signal is supplied to Q503. The base of Q503 is
pulled to the transmit 5V line. The output of Q503 forces the
base of Q502 to ground; then the DC power is supplied to the
first stage of the power amplifier.
Antenna Switch (D402 And D403)
In the transmit mode, D402 and D403 receive the voltage,
transmit battery (7.5V). The circuit switches off the receiver
and allows RF signals to flow from the transmitter to the an-
tenna, passing through the transmit harmonic filter that reduces
second and third harmonic frequencies generated within the RF
amplifier.
AE/LZB 119 1658 R1A
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