Vertex Standard VX-410 - Manual (Page 13)

13
Circuit Description
Receive Signal Path
Incoming RF from the antenna jack is delivered to the
RF Unit and passes through a low-pass filter consisting of
coils L1001, L1002, and L1003, capacitors C1001, C1002,
C1021, C1022, C1023, C1024, C1025, C1026, and C1027,
and antenna switching diode D1004 (RLS135).
Signals within the frequency range of the transceiver
enter a varactor-tuned band-pass filter consisting of coils
L1012 and L1015, capacitors C1058, C1059, C1085,
C1118, and C1119, and diodes D1007, D1008, D1009,
and D1010 (all HVD350B), then are amplified by Q1019
(2SC5006) and enter a varactor-tuned band-pass filter
consisting of coils L1019, L1020, and L1025, capacitors
C1122, C1150, C1151, C1152, C1153, C1154, C1155,
C1156, C1177, C1178, C1179, C1180, and C1181, and
diodes D1012, D1013, and D1016 (all HVD350B), be-
fore application to the first mixer, Q1029 (3SK320).
Buffered output from the VCO is amplified by Q1026
(2SC5005) to provide a pure first local signal between
399.15 and 439.15 MHz for injection to the first mixer
Q1029. The 50.85 MHz first mixer product then passes
through monolithic crystal filter XF1001 (MFT50.85PT,
5.5 kHz BW) to strip away unwanted mixer products, and
is then amplified by Q1041 (2SC4215Y).
The amplified first IF signal is applied to FM IF sub-
system IC Q1048 (TA31136FN), which contains the sec-
ond mixer, second local oscillator, limited amplifier, noise
amplifier, and RSSI amplifier.
The second local signal is produced from the PLL ref-
erence/second local oscillator of X1002 (TCXO
TTS05VS-M1 16.80 MHz). The 16.80 MHz reference
signal is tripled by Q1048, capacitor C1251, and coil
L1042, and the resulting the 50.4 MHz second local sig-
nal is then delivered to the mixer section of Q1048 which
produces the 450 kHz second IF when mixed with the
first IF signal.
The second IF then passes through the ceramic filter
CF1001 (ELFC450G on “Narrow” channels) or CF1002
(ELFY450E on “Wide” and “Narrow” channels) to strip
away all but the desired signal, and is then applied to the
limiter amplifier in Q1048, which removes amplitude
variations in the 450 kHz IF, before detection of the speech
by the ceramic discriminator CD1001 (ECDA450C24).
Squelch Control
The squelch circuitry consists of a noise amplifier,
band-pass filter, and noise detector within Q1048
(TA31136FN).
When no carrier is received, noise at the output of the
detector stage in Q1048 is amplified and band-pass fil-
tered by the noise amplifier section of Q1048 and the net-
work between pins 7 and 8, and then is rectified by detec-
tion circuit in Q1048.
The resulting DC squelch control voltage is passed to
pin 19 of the microprocessor Q1047 (LC87F74C8A). If
no carrier is received, this signal causes pin 19 of Q1047
to go high and pin 30 to go high. Pin 35 signals Q1006
(2SC4154E) to disable the supply voltage to the audio
amplifier Q1005, while pin 30 holds the green (Busy) half
of the LED off, when pin 35 is high and pin 30 is high.
Thus, the microprocessor blocks output from the au-
dio amplifier, and silences the receiver, while no signal is
being received (and during transmission, as well).
When a carrier appears at the discriminator, noise is
removed from the output, causing pin 19 of Q1047 to go
low and the microprocessor to activate the “Busy” LED
via Q1047.
The microprocessor then checks for CTCSS or CDCSS
code squelch information, if enabled. If not transmitting
and CTCSS or CDCSS is not activated, or if the received
tone or code matches that programmed, audio is allowed
to pass through the audio amplifier Q1005 (NJM2070M)
to the loudspeaker by the enabling of the supply voltage
to it via Q1048.
Transmit Signal Path
Speech input from the microphone MC1001 passes
through the audio amplifier Q1021 (AK2342B) to Q1015
(M62364FP) which adjusts the microphone gain. The ad-
justed audio is applied to the compander Q1010
(LA8630M) which compresses the speech signal accord-
ing to a control command from the microprocessor Q1047
(LC87F74C8A).
The compressed speech signal passes through the
dummy unit and pre-emphasis circuit to Q1021, which
contains the low-pass filter, Voice Scrambler selector, and
high-pass filter.
The output from Q1021 is applied to the AF mute gate
Q1055 (2SK3541), then returns to Q1021, which con-
tains the limiter amplifier, splatter filter and audio amplifier.
The filtered audio signal is applied to Q1015
(M62364FP) which is adjusts the audio level, then is ap-
plied to varactor diode D1017 (HVD350B), which fre-
quency modulates the VCO Q1033 (2SK508). A portion
of the audio signal from Q1015 is applied to TCXO X1002
(TTS05VS).
The processed audio may then be mixed with a CTCSS
tone generated by Q1047 (LC87F74C8A) for frequency
modulation of the PLL carrier (up to ±5 kHz from the
unmodulated carrier) at the transmitting frequency.
If a CDCSS code is enabled for transmission, the code
is generated by microprocessor Q1047 and delivered to
X1002 (TCXO TTS05VS) for CDCSS modulating.
The modulated signal from the VCO Q1033 (2SK508-
K52) is buffered by Q1026 (2SC5005). The low-level
transmit signal then passes through the T/R switching di-