HM135
Service Manual
Preliminary version
Page 7 of 13
2.5.d
Transmitter Audio Circuits
The speech audio coming from the MIC connector (line
AUX_MIC
) is applied to the FET
VT804
which acts as
sensitivity switch (function high/low MIC sensitivity) in 2 levels and is directly controlled by the microprocessor (pin
27
) via the transistor
VT807
. The signal is then fed to the amplifier made by
DA801:A
(
DA801:B
is used to supply
the reference voltage at its positive input) providing a stronger speech signal (line
MIC_INPUT
) which is fed to the
6dB per octave pre-emphasis circuit provided by the capacitor
C277
and the resistor
R213
and applied to pin
6
of
DA11:B
.
NOTE: Between the
C277
and the
MIC_INPUT
line, the transistor
VT48
acts as a mute switch which disable the speech
audio when a Selcall is sent.
Selcall audio PWM signaling coming from the pin
37
of the microprocessor (line
MCU_SELCALL_PWM
) is fed to
the 3 KHz low pass filter which consists of
DA9:B
and related circuitry. Its output is then fed to the input of the
DA11:B
, so routed the same way of the speech audio.
The speech/Selcall signal is applied to the input of
DA11:B
which limits the peak-to-peak output, then is fed to the
pin
3
of IC
DA10
which is a double digital audio regulator. After the level regulation, the speech/Selcall is output at
pin
11
and sent to the first input (pin
9
) of
DA9:C
in order to be summed with the CTCSS/DCS signal.
NOTE: The level regulation of
DA10
comes in form of digital data from the pins
6
,
7
and
8
(lines
POT_DATA
,
POT_CLK
and
DAC_CS
respectively) which are directly controlled by the microprocessor
DD5
(pins
43
,
46
and
36
respectively) via
related logical inverters.
CTCSS/DCS sub-audio tone PWM signaling coming from the pin
39
of the microprocessor (line
MCU_CTCSS_DCS_PWM
) is fed to the CTCSS/DCS 300 Hz low pass filter which consists of
DA9:A
and related
circuitry, then fed in the pin
2
of the IC
DA10
which regulates the level and outputs the regulated CTCSS/DCS at
its pin 12. This signal is then applied to the second input of
DA9:C
in order to be summed with the speech/Selcall
signal.
DA9:C
sums the two signals (speech/Selcall coming from pin
11
of
DA10
and CTCSS/DCS coming from pin
12
)
feeding its combined output to
DA9:D
which is a 3 KHz low pass filter. The output of
DA9:D
is then applied to pins
4
and
5
of the
DA10
.
2.5.e Double-point
modulation
The outputs (lines
REF_MOD
and
VCO_MOD
) coming, respectively, from pins
10
and
9
of
DA10
are fed to the
PLL area. The line
REF_MOD
is directly applied to the trimmer
RP3
which provides the Ref. Modulation control
(low audio frequencies) directly applied to the TCXO. The line
VCO_MOD
is applied via the resistors
R116
to the
trimmer
RP4
which provides the VCO modulation control (high audio frequencies) directly sent to the TX VCO.
2.6 Receiver
2.6.a Receiver’s
Front-End
The RX signal coming from the antenna connector is fed in sequence through the coupling line
TA1
, the low pass
filter (consisting of
L46
to
L49
and connected capacitors) and the antenna switching (
VD21
and
VD22)
. The
output of the antenna switching (line
RF_RX
) is sent to input of the receiver and coupled to the input band-pass
filter. The transistor
VT2
is the front-end amplifier and its output is applied to the second band-pass filter (
L2
,
L3
,
L4
,
L5
and related capacitors). The output from the band-pass filter is applied to the pin
4
of the RF mixer
A1
.
The diode
VD1
is used for the function local/distance. Normally the line
LOCAL_DIST
issued by the pin
25
of the
microprocessor (properly adapted by
R317
and
C352
) goes to high so
VT1
is switched on forward biasing the
diode
VD1
. This provides a bypass of the resistor
R5
for the RF, so the RF signal fed to the mixer is higher
(distance mode). On the other side, if the line
LOCAL_DIST
drops to low, the diode
VD1
is not forward biased and
the RF is attenuated of 10dB approx. due to the
R5
(local mode).
2.6.b
Local Oscillator (LO)
As already explained in the PLL section, the output coming from the RX VCO (working at 45,1 MHz above the RX
frequency) is sent to the synthesizer buffer
VT19
, then sent to
DA5
for an additional buffering. The output of
DA5
is connected to the low-pass filter (
L27
,
L28
,
L29
and related capacitors), then sent to the RX stages (line
HET_RX
) which is sent to
the RX mixer
A1
via an attenuator which consists of
R13
,
R14
and
R15
.
Содержание HM135
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