Circuit Description; Squelch Control - Standard Horizon HX280S Service Manual

1. Receive Signal Path
Incoming RF from the antenna jack passes through a low-
pass filter consisting of coils L1001, L1002, & L1004, ca-
pacitors C1005, C1014, C1015, & C1019, and antenna
switching diode D1004 (RLS135).
Signals within the frequency range of the transceiver en-
ter a band-pass filter consisting of coils L1013 and L1016,
capacitors C1060, C1064, and C1067, then amplified by
Q1015 and enter a Varactor-tuned band-pass filter con-
sisting of coils L1021 & L1022, capacitors C1106, C1107,
C1108, C1120, & C1221, and diodes D1025 & D1026 (both
HVC355B), before first mixing by Q1022 (3SK318).
Buffered output from the VCO is amplified by Q1007
(2SC5006) to provide a pure first local signal between
134.35 and 141.575 MHz for injection to the first mixer
Q1022 (3SK318).
The 21.7 MHz first mixer product then passes through
monolithic crystal filter XF1001/XF1002 to strip away all
but the desired signal, which is then amplified by Q1024
(2SC4915). The amplified first IF signal is applied to FM
IF subsystem IC Q1026 (BA4116FV), which contains the
second mixer, second local oscillator, limited amplifier,
noise amplifier, and RSSI amplifier.
A second local signal is produced from the PLL reference/
second local oscillator of X1001 (21.25 MHz). The 21.25
MHz reference signal is delivered to mixer section of FM
IF subsystem IC Q1026 (BA4116FV) which produce the
450 kHz second IF mixed with the first IF signal.
The second IF then passes through the ceramic filter
CF1001 to strip away unwanted mixer products, and is
then applied to the limited amplifier in the FM IF sub-
system IC Q1026 (BA4116FV), which removes amplitude
variations in the 450kHz IF, before detection of the speech
by the ceramic discriminator CD1001.
2. Audio Amplifier
The demodulated audio signal from the Q1026
(BA4116FV) passes through a band-pass filter and High-
pass filter, then applied to the de-emphasis of Q1005
(LM2902PWR). Then passes through the audio mute
switch Q1034 (SN74LVC1G66DCKR), the audio volume
VR1004 and the audio power amplifier Q1032 (TDA2822)
pin 7, providing up to 700 mW of audio power to the 16-
ohm loudspeaker.

Circuit Description

3. Squelch Control

The squelch circuitry consists of a noise amplifier and
band-pass filter and noise detector within Q1026
(BA4116FV). When no carrier received, noise at the out-
put of the detector stage in Q1026 (BA4116FV) is ampli-
fied and band-pass filtered by the noise amplifier section
of Q1026 (BA4116FV) and the network between pins 7
and 8, and then rectified by detection circuit in Q1026
(BA4116FV).
The resulting DC squelch control voltage is passed to pin
64 of the microprocessor Q1025 (UPD78F0484GK). If no
carrier is received, this signal causes pin 38 of Q1025
(UPD78F0484GK) to go low and pin 67 to go high. Pin 67
signals of Q1025 (UPD78F0484GK) to disable the supply
voltage to the audio amplifier Q1032 (TDA2822).
Thus, the microprocessor blocks output from the audio
amplifier, and silences the receiver, while no signal is be-
ing received (and during transmission, as well).
4. Transmit Signal Path
The speech input from the microphone MC1001 passes
through the audio amplifier Q1003 (NJM12902V), which
is adjusted the microphone gain. The speech signal pass-
es through pre-emphasis circuit to Q1003 (NJM12902V),
which contains the IDC, and low-pass filter. Then passes
through VR1001 which allows manual adjustment of the
transmitter deviation level.
The filtered audio signal is applied to varactor diode
D1014 (HVC306B), which frequency modulates the VCO
Q1008 (2SC5006).
The modulated signal from the VCO Q1008 (2SC5006) is
buffered by Q1007 (2SC5006). The low-level transmit sig-
nal is then passes through the TX switching diode D1017
(DAN235E) to the buffer amplifier Q1016 (2SC5226),
driver amplifier Q1013 (RQA0004PXDQS), then ampli-
fied transmit signal is applied to the final amplifier Q1009
(RQA0011DNS) up to 5.0 watts output power.
The transmit signal then passes through the antenna
switch D1003 (RLS135) and is low-pass filtered to sup-
press harmonic spurious radiation before delivery to the
antenna.
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