Vertex Standard VX-900 - Manual (Page 18)

18
Circuit Description
The resulting DC squelch control voltage is passed
to pin 19 of the microprocessor Q1050. If no carrier
is received, this signal causes pin 6 of Q1052
(BU2090FS) to go low and pin 89 of Q1050 to go
high. Pin 6 of Q1052 signals Q1008 (CPH6102),
Q1017
(UMC5N) and Q1020 (UMC5N) to disable the
supply voltage to audio amplifiers Q1007 and
Q1011
, while pin 89 makes Q1014 (UMG2N) hold
the green (Busy) half of the LED off, when pin 6 of
Q1052
is low and pin 89 of Q1052 is high.
Thus, the microprocessor blocks output from the au-
dio amplifier, and silences the receiver while no sig-
nal is being received, and also during transmission.
When a carrier appears at the discriminator, noise
is removed from the output, causing pin 19 of Q1048
to go low and the microprocessor to blink the busy
LED via Q1052.
The microprocessor then checks the DTMF decoder
chip on the Optional Unit, the CTCSS and the CDCSS
code for DTMF or CTCSS or CDCSS code squelch in-
formation, if enabled, respectively. If not transmitting
and CTCSS or CDCSS is not activated, or if the re-
ceived tone or code matches that programmed, the
microprocessor stops scanning, if active, and allows
audio to pass through the audio amplifier Q1007 and
Q1011
(TDA2822D) to the loudspeaker by enabling
the supply voltage to it via Q1008, Q1017 and Q1020.
Transmit Signal Path
Microphone Amplifier
Speech input from the microphone is amplified in
Q1022
(NJM2902V), is filtered, and is sent to Q1021
(M62364FP) and sent to the Dummy Unit (or Op-
tional Unit). The audio which returned from the
Dummy Unit passes through Q1027 (AK2345) to be
pre-emphasized.
The processed audio is then mixed with a CTCSS tone
generated by microprocessor Q1050 and delivered to
D1043
(HVC350) for frequency modulating the PLL
carrier up to ±5 kHz from the unmodulated carrier at
the transmitting frequency.
If an external microphone is used, PTT switching is
controlled by Q1003 (UMZ2N), which signals the mi-
croprocessor when the impedance at the microphone
jack drops.
If a CDCSS code is enabled for transmission, the code
is generated by Q1027 and delivered to D1061
(HVC350) for CDCSS modulation.
If DTMF is enabled for transmission, the tone is gen-
erated by the microprocessor Q1050 and applied to
the splatter filter (Q1028) in place of speech audio.
Also, the tone is amplified for monitoring in the
loudspeaker.
Noise Canceling Microphone Circuit
Two signals from separate internal microphones
(main and sub) are fed to the positive input (sub)
and to the negative input (main) and of Q1022
(NJM2902V). If the same signal level is present at
both main and sub, the main signal will be canceled
at the output of Pin 1 of Q1022. In other words, noise
from nearby sources not directly connected to the
transceiver enters the main and sub input at the same
signal and, in the absence of (stronger) voice input,
is therefore canceled out.
When a signal is only input to the main microphone,
and there is no signal at the sub microphone, the
main signal is passed as-is from Q1022.
Drivers and Final Amplifiers
The modulated signal from the VCO, Q1041
(2SC4227-R34), is buffered by Q1032 (2SC5226-4/
5
) and amplified by Q1029 (2SC5226-4/5) and
Q1026
(2SC3356-R25). The low-level transmit sig-
nal is then applied to the Power Module Q1016
(M68732HA-22) for final amplification up to a maxi-
mum of 5 Watts of output power.
The transmit signal then passes through the antenna
switch D1016 (HVU131), and is low-pass filtered to
suppress harmonic spurious radiation before deliv-
ery to the antenna.
Automatic Transmit Power Control
RF power output from the final amplifier is sampled
by C1020 and C1025, and is rectified by D1021
(RB715F). The resulting DC is fed back through
Q1018
(NJM2902V) to the Power Module, provid-
ing control of the power output.
The microprocessor selects either "High" or one of
three "Low" power levels.