VX-261 VHF FM Transceiver Service Manual
6
Circuit Description
1. Receiver System
1-1. Front-end RF amplifier
Incoming RF signal from the antenna is delivered to the
Main Unit and passes through low-pass filter, antenna
switching diode
D1004
and
D1005
(both
1SS390
), high-
pass filter and removed undesired frequencies by varactor
tuned band-pass filter
D1007
and
D1009
(both
1SV325
).
The filtered RF signal is amplified by
Q1015
(
BFS505
)
and then passes through another varactor tuned band-pass
filter
D1013
and
D1014
(both
1SV325
) to remove the
undesired frequencies, and then applied to the 1st mixer
Q1020
(
3SK293
).
1-2. First Mixer
The RF signal is mixed with the 1st local signal be-
tween 186.85 and 224.85 MHz in the 1st mixer
Q1020
(
3SK293
), to produce 50.85 MHz 1st IF signal.
The 1st local signal is generated by the VCO, which
consists of
Q1029
(
CPH3910
), varactor diodes
D1021
,
D1022
,
D1023
, and
D1025
(all
1SV325
). The 1st lo-
cal signal is supplied to the 1st mixer
Q1020
(
3SK293
)
through the buffer amplifier
Q1021
and
Q1026
(both
2SC5010
).
1-3. IF Amplifier & Demodulator
The 1st IF signal passes through monolithic crystal filters
XF1001
(±7.5 kHz BW) to strip away all but the desired
signal, and then supplied the buffer amplifier
Q1035
(
2SC5226
).
The amplified 1st IF signal is applied to the FM IF subsys-
tem IC
Q1040
(
AA32416
) which contains the 2nd mixer,
2nd local oscillator, limiter amplifier, noise amplifier, and
RSSI amplifier.
The signal from reference oscillator
X1002
(16.8 MHz)
becomes three times of frequencies in
Q1040
(
AA32416
),
it is mixed with the 1st IF signal and becomes 450 kHz.
The 2nd IF signal passes through the ceramic filter
CF1001
(
LTM450GW
) or
CF1002
(
LTM450EW
) to strip
away unwanted mixer products, and is supplied to the
limiter amplifier in
Q1040
(
AA32416
), which removes
amplitude variations in the 450 kHz IF, before detection
of the speech by the ceramic discriminator
CD1001
(
JT-
BM450CX24
).
1-4. Audio amplifier
The detected signal from
Q1040
(
AA32416
) is supplied
to the AF adder
Q1045-1
(
NJM12902V
) through the TX/
RX switch
Q1057-2
(
SN74LV4066A
), high-pass filter
Q1045-2
(
NJM12902V
), and mute switch
Q1057-4
(
SN74LV4066A
).
The audio signal from
Q1045-1
(
NJM12902V
) is sup-
plied to the audio amplifier
Q1008
(
TDA2822L
) through
the AF volume potentiometer (VR1001). As a result, the
audio signal provides up to 700 mW (@16-ohm BTL) for
internal speaker or up to 500 mW (@4-ohm OTL) for ex-
ternal speaker.
1-5. Squelch Circuit
There are 16 levels of squelch setting from “0” to “15”.
The level “0” means open the squelch. The level “1”
means the threshold setting level and level “14” means
tight squelch. From level “2” to level “13” is established
in the middle of threshold and tight. The level “15” be-
comes setting of carrier squelch.
1-5-1. Noise Squelch
The noise squelch circuit is consisted of the band-path fil-
ter, noise amplifier
Q1047
(
2SC4617
), and noise detector
D1035
(
DA221
).
When a carrier isn’t received, the noise ingredient which
goes out of the demodulator section of
Q1040
(
AA32416
)
is amplified by noise amplifier
Q1047
(
2SC4617
)
through the band-path filter, and then is detected to DC
voltage by
D1035
(
DA221
). The DC voltage is inputted
to pin 54 (A/D port) of the CPU
Q1028
(
R5F100LHDFB
).
When a carrier is received, the DC voltage becomes low
because the noise is compressed.
When the detected voltage to CPU is “High”, the CPU
stops the AF output of
Q1013
(
DTC144EE
) by making to
“low” of the pin 39 of CPU.
When the detection voltage to CPU is “low”, the CPU al-
lows the AF output of
Q1013
(
DTC144EE
) by making to
“High” of the pin 39 of CPU.
1-5-2. Carrier Squelch
The detected RSSI voltage from pin 12 of
Q1040
(
AA32416
) supplied to pin 53 (A/D port) of
Q1028
(
R5F100LHDFB
). It is controls the AF output.
The RSSI output voltage changes according to the signal
strength of carrier. The stronger signal makes the RSSI
voltage to be higher voltage. The process of the AF signal
control is same as Noise Squelch. The shipping data is ad-
justed 3 dBµ (EMF) higher than squelch tight sensitivity.
