Circuit description, Circuit configuration by frequency, Receiver system – Vertex Standard VX-4200 User Manual
Page 9: 1. front-end rf amplifier, 2. first mixer, 3. if amplifier, 4. audio amplifier, 5. squelch circuit, 5-1. noise squelch, 5-2. carrier squelch
1. Circuit Configuration by Frequency
The receiver is a double-conversion superheterodyne with a first
intermediate frequency (IF) of 50.85 MHz and a second IF of
450kHz. Incoming signal from the antenna is mixed with the
local signal from the VCO/PLL to produce the first IF of 50.85
MHz. This is then mixed with the 50.4 MHz second local oscil-
lator output to produce the 450 kHz second IF. This is detected
to give the demodulated signal. The transmit signal frequency is
generated by the PLL VCO, and modulated by the signal from
the microphone. It is then amplified and sent to the antenna.
2. Receiver System
2-1. Front-end RF amplifier
Incoming FR signal from the antenna is delivered to the RF Unit
and passes through Low-pass filer, antenna switching diode, and
removed undesired frequencies by varactor diode (tuned band-
pass filer: T1001, T1002 and T1003). The passed signal is am-
plified in Q1022 (2SC3357) and moreover cuts an image fre-
quency with the band path filter and comes into the 1st mixer.
The Q1020 (2SA1774) controls bias of Q1022 (2SC3357).
2-2. First Mixer
The 1st mixer consists of the Q1035 (SPM5001), T1004, T1005
and T1006. Buffered output from the VCO is amplified by Q1038
(2SC5226) to provide a pure first local signal between 184.85
and 224.85 MHz for injection to the first mixer. The output IF
signal is amplified by Q1046 (2SC3357) and enters from the
mixer to the crystal filter. The IF signal then passes through
monolithic crystal filters XF1001 (±5.5 kHz BW) to strip away
all but the desired signal.
2-3. IF Amplifier
The first IF signal is amplified by Q1059 (2SC5226). The am-
plified first IF signal is applied to FM IF subsystem IC Q1079
(TA31136FN) which contains the second mixer, second local
oscillator, limiter amplifier, noise amplifier, and S-meter ampli-
fier. The signal from reference oscillator X1003 becomes 3 times
of frequencies in Q1079 (TA31136FN), it is mixed with the IF
signal and becomes 450 kHz. The second IF then passes through
the ceramic filter CF1001 (for wide channels), CF1002 (for
narrow channels) to strip away unwanted mixer products, and is
applied to the limiter amplifier in Q1038 (2SC5226), which
removes amplitude variations in the 450 kHz IF, before detec-
tion of the speech by the ceramic discriminator CD1001.
2-4. Audio amplifier
Detected signal from Q1079 (TA31136FN) is inputted to Q1037
(AK2345) and is output through the band path filter inside
Q1037 (AK2345). When the optional unit is installed, the Q1063
(BU4066BCFV) is made “OFF” and the AF signal from Q1037
(AK2345) goes the optional unit. When the optional unit is not
installed, Q1037 (AK2345) is made “ON” and the signal goes
through Q1063 (BU4066BCFV). The signal then goes through
Circuit Description
AF mute switch Q1068 (BU4066BCFV), de-emphasis part and
expander Q1021 (LA8630M). When the function of expander
is off, the 1021 will be bypassed by Q1069 (TAR5S50). The
output signal of expander (or a signal from de-emphasis) is am-
plified by Q1088 (NJM12902V) and it is amplified with AF
power amplifier Q1005 (TDA1519CTH) after passing AF vol-
u m e Q 1 0 2 9 ( M 6 2 3 6 4 F P ) . T h e o u t p u t o f Q 1 0 0 5
(TDA1519CTH) drives a speaker (it chooses the external SP
or internal SP in J6001).
2-5. Squelch Circuit
There are 13 levels of squelch setting from 0 to 12. The level 0
means open the squelch. The level 1 means the threshold setting
level and level 11 means tight squelch. From 2 to 10 is estab-
lished in the middle of threshold and tight. The bigger figure is
nearer the tight setting. The level 12 becomes setting of carrier
squelch.
2-5-1. Noise Squelch
Noise squelch circuit is composed of the band path filter of
Q1079 (TA31136FN), noise amplifier Q1091 (2SC4617), and
noise detector D1059, D1061 (both DA221). When a carrier
isn’t received, the noise ingredient which goes out of the de-
modulator Q1079 (TA31136FN) is amplified in Q1091
(2SC4617) through the band path filter Q1079 (TA31136FN),
is detected to DC voltage with D1059, D1061 (both DA221)
and is inputted to 19 pin (the A/D port) of the Q1083 (CPU:
LC87F74C8A). When a carrier is received, the DC voltage be-
comes “LOW” because the noise is compressed. When the de-
tected voltage to CPU is “HIGH,” the CPU stops AF output
with Q1068 (BU4066BCFV) “OFF” by making the 40pin “L”
level. When the detection voltage is low, the CPU makes Q1068
“ON” with making 40pin “H” and the AF signal is output.
2-5-2. Carrier Squelch
The pin 18 (A/D port) of Q1083 (CPU: LC87F74C8A) detect
RSSI voltage output from pin 12 of Q1079 (TA31136FN), and
controls 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 adjusted
3dB higher than squelch tight sensitivity.
3. Transmitter System
3-1. Mic Amplifier
The AF signal from microphone jack J4001 (4ch) or J5001
(501ch) is amplified with microphone amplifier Q1066, is am-
plified with 2nd microphone amplifier Q1088 after microphone
selection switch Q1016, and passes microphone gain volume
Q1029. The AF signal which was controlled in the correct gain
passes compandor Q1021. When not using a compandor, with
the control from the CPU, it passes in the compandor circuit
inside of Q1021 (LA8630M) and it is output and it passes a
E-1