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1. 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 L1003, L1006 and L1007, capacitors C1002, C1007,
C1013, C1017, C1022, C1025, and C1169 and antenna
switching diode D1007 (RLS135).
Signals within the frequency range of the transceiver
enter a varactor-tuned band-pass filter consisting of coils
L1014 and L1015, capacitors C1057, C1058, C1064, C1071
and C1073, and diodes D1012 (HVC355B), D1013
(1SV280), D1016 (HVC355B) and D1017 (1SV280). The
signals are then amplified by Q1015 (2SC5006), and en-
ter a varactor-tuned band-pass filter consisting of coils
L1018, L1021 and L1024, capacitors C1084, C1088, C1089,
C1094, and C1096, and diodes D1022, D1024, D1026
(HVC355B) and D1023, D1025 and D1027 (1SV280), be-
fore mixing by Q1025 (SGM2016).
Buffered output from the VCO is amplified by Q1009
(2SC5005) to provide a pure first local signal between
406.05 and 441.05 MHz for injection to the first mixer,
Q1025. The 43.95 MHz first mixer product then passes
through monolithic crystal filters XF1001 and XF1002
(43Y12B6, ±6.0 kHz BW) to strip away unwanted mixer
products, and is then amplified by Q1032 (2SC4215Y).
The amplified first IF signal is applied to FM IF sub-
system IC Q1036 (BA4116FV), which contains the second
mixer, second local oscillator, limiter amplifier, noise am-
plifier, and S-meter amplifier.
A second local signal is generated by the PLL reference/
second local oscillator, 14.50 MHz crystal X1001, to pro-
duce the 450 kHz second IF when mixed with the first IF
signal within Q1036.
The second IF then passes through the ceramic filter
CF1001 (ALFYM450F) or CF1002 (SFPC450G: only on
"Narrow" channels) to strip away all but the desired sig-
nal, and is then applied to the limiter amplifier in Q1036,
which removes amplitude variations in the 450kHz IF,
before detection of the speech by the ceramic discrimina-
tor CD1001 (CDBC450CX24).
Detected audio from Q1036 is applied to the audio high-
pass filter, and then passes via the volume control to the
audio amplifier Q1039 (TDA7233D), which provides up
to 0.5 Watts to the optional headphone jack or a 4-ohm
loudspeaker.
2. Squelch Control
The squelch circuitry consists of a noise amplifier and
band-pass filter within Q1036, and noise detector D1030
(1SS355).
When no carrier is received, noise at the output of the
Circuit Description
detector stage in Q1036 is amplified and band-pass fil-
tered by the noise amplifier section of Q1036 and the net-
work between pins 7 and 8, and then is rectified by D1030.
The resulting DC squelch control voltage is passed to
pin 37 of the microprocessor Q1014 (M37515M4-117HP).
If no carrier is received, this signal causes pin 7 of Q1014
to go high and pin 20 to go low. Pin 7 signals Q1037
(IMD10A) to disable the supply voltage to the audio am-
plifier Q1039, while pin 20 makes Q1023 (IMX1) hold the
green (Busy) half of the LED off, when pin 7 is high and
pin 20 is low.
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 37 of Q1014 to go
low and the microprocessor to activate the "Busy" LED
via Q1014.
The microprocessor then checks for CTCSS or CDCSS code
squelch information, if enabled, or for DTMF data on the
optional DTMF Unit. If not transmitting, and CTCSS or
CDCSS is not activated, or if the received tone or code
matches that programmed for that channel, audio is al-
lowed to pass through the audio amplifier Q1039
(TDA7233D) to the loudspeaker because of the enabling
of the supply voltage via Q1037.
3. Transmit Signal Path
Speech input from the microphone is amplified by
Q1017 (NJM2902V); after pre-emphasis by C1059 and
R1045, the audio passes through another section of Q1017.
The processed audio may then be mixed with a CTCSS
tone generated by Q1014 (M37515M4-117HP), and then
delivered to D1005 (HVU350) for frequency modulation
of the PLL carrier (up to ±5kHz from the unmodulated
carrier) at the transmitting frequency.
If a CDCSS code is enabled for transmission, the code
is generated by microprocessor Q1014 and delivered to
D1004 (1SV230) for CDCSS modulating.
If DTMF is enabled for transmission, the tone is gener-
ated by the microprocessor Q1014 and applied to the lim-
iter amplifier section in place of the speech audio. Also,
the tone is amplified for monitoring in the loudspeaker.
The modulated signal from the VCO Q1005 (2SK508-
K52) is buffered by Q1008 (2SC5005) and amplified by
Q1009 (2SC5005). The low-level transmit signal is then
passed through the T/R switching diode D1014
(DAN235U) to the driver amplifiers Q1012 (2SC3357) and
Q1016 (2SK2973). The amplified transmit signal is ap-
plied to the final amplifier Q1021, providing up to 5 Watts
9
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