Circuit Description - Vertex Standard VX-351PMR446 Service Manual

1. Circuit Configuration by Frequency
The receiver is a Double-conversion Super-heterodyne with a
first intermediate frequency (IF) of 50.85 MHz and a second IF
of 450 kHz. 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 lo-
cal oscillator 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 RF signal from the antenna is delivered to the MAIN
Unit (MAIN-2 Unit) and passes through Low-pass filer, anten-
na switching diode D1003 (D3003) (RLS135), high pass filter
and removed undesired frequencies by the high-pass filter and
the band-pass filter. The passed signal is amplified in Q1021
(Q3021) (MSG33001) and moreover cuts an image frequen-
cy with the tuned band pass filter and comes into the 1st mixer.
2-2. First Mixer
The 1st mixer consists of the Q1036 (Q3036) (3SK293). Buff-
ered output from the VCO is amplified by Q1031 (Q3031)
(2SC5005) to provide a pure first local signal between
395.15625 and 395.24375 MHz for injection to the first mixer.
The IF signal then passes through monolithic crystal filters
XF1001 (XF3001) (±5.5 kHz BW) to strip away all but the
desired signal.
2-3. IF Amplifier
The first IF signal is amplified by Q1048 (Q3048) (2SC5226).
The amplified first IF signal is applied to FM IF subsystem IC
Q1053 (Q3053) (NJM2591) which contains the second mix-
er, second local oscillator, limiter amplifier, noise amplifier,
and S-meter amplifier. The signal from reference oscillator
X1002 (X3002) becomes 3 times of frequencies in Q1053
(Q3053), it is mixed with the IF signal and becomes 450 kHz.
The second IF then passes through the ceramic filter CF1001
(CF3001) (LTM450EW) to strip away unwanted mixer prod-
ucts, and is applied to the limiter amplifier in Q1053 (Q3053),
which removes amplitude variations in the 450kHz IF, before
detection of the speech by the ceramic discriminator CD1001
(CD3001) (ECDA450C24).
2-4. Audio amplifier
Detected signal from Q1053 (Q3053) is inputted to mute switch
Q1001-3 (Q3001-3) (SN74LV4066APWR) and option switch
Q1001-2 (Q3001-2). The signal which appeared from Q1001-2
(Q3001-2) is in band pass filter Q1066 (Q3066) (NJM12902V).
In the case an optional unit is installed, the Q1001-2 (Q3001-2)
is made "OFF" and the AF signal from Q1001-3 (Q3001-3) goes
the optional unit. In the case an optional unit is not installed,
Q1001-2 (Q3001-2) is made "ON" and the signal goes through
Q1001-2 (Q3001-2). The signal which passed Q1066 (Q3066)
goes to VR1001 (VR3001) AF volume . And then the signal goes
VX-351PMR446 Service Manual

Circuit Description

to audio amplifier Q1007 (Q3007) (NJM2070M). The output
signal from Q1007 (Q3007) is in audio speaker.
2-5. 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
"2" to "13" is established in the middle of threshold and tight.
The bigger figure is nearer the tight setting. The level "15"
becomes setting of carrier squelch.
2-5-1. Noise Squelch
Noise squelch circuit is composed of the band path filter of
Q1053 (Q3053), noise amplifier Q1058 (Q3058) (2SC4617),
and noise detector D1042 (D3042) and D1043 (D3043) (both
DA221). When a carrier isn't received, the noise ingredient
which goes out of the demodulator Q1053 (Q3053) is ampli-
fied in Q1058 (Q3058) through the band path filter Q1053
(Q3053), is detected to DC voltage with D1042 (D3042) and
D1043 (D3043) and is inputted to 48 pin (the A/D port) of the
Q1044 (Q3044) (CPU: LC87F5864B).
When a carrier is received, the DC voltage becomes low be-
cause the noise is compressed. When the detected voltage to
CPU is high, the CPU stops AF output with Q1001-1 (Q3001-
1) "OFF" by making the 39 pin (CPU) "L" level. When the
detection voltage is low, the CPU makes Q1001-1 (Q3001-1)
"ON" with making 39 pin "H" and the AF signal is output.
2-5-2. Carrier Squelch
The CPU (47 pin: A/D port) detect RSSI voltage output from
Q1053 (Q3053) 12 pin, and controls AF output. The RSSI out-
put voltage changes according to the signal strength of carrier.
The stronger signal makes the RSSI voltage to be higher volt-
age. 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 internal microphone MC1001 (MC3001)
or external microphone J1002 (J3002) is amplified with micro-
phone amplifier Q1069-2 (Q3069-2) (NJM12904), after passes
microphone gain volume Q1017-CH1 (Q3017-CH1)
(M62364FP). AF signal is passes a pre-emphasis circuit.
Q1001-1 (Q3001-1) (SN74LV4066APW) becomes "OFF"
when an option unit is attached and the AF signal from Q1068
(Q3068) goes via the option unit. When an option unit isn t
attached, Q1001-1 (Q3001-1) becomes "ON", the signal pass-
es Q1001-1 (Q3001-1) and is input to the limiter amplifier
Q1068-2 (Q3068-2) (BA2902). The signal passed splatter fil-
ter of Q1068 (Q3068) and adder amplifier Q1065 (Q3065) is
adjusted by maximum deviation adjustment volume Q1017-
CH4 (Q3017-CH4) (M62364FP). The AF signal ingredient
is amplified Q1065 (Q3065) (BA2902). After that, it is made
FM modulation to transmit carrier by the modulator D1023
(D3023) (HVC383) of VCO.
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