Vhf Af Amplifier Circuit; Vhf Squelch Circuit; Tone Squelch; Af Amplifier Circuit - Icom IC-2800H Service Manual

The 1st IF signal from the 2nd IF amplifier is applied to the
2nd mixer section of IC28 (pin 16), and is mixed with a 15.2
MHz 2nd LO signal generated by the reference oscillator cir-
cuit (X1, IC2) to produce a 450 kHz VHF 2nd IF signal.
The 2nd IF signal from the 2nd mixer passes through the 2nd
IF filter (FI4) (during wide channel spacing selection, or pass-
es through FI5 during narrow channel spacing selection;
[EUR], [ITA] versions only), where unwanted signals are
suppressed. It is then amplified at the limiter amplifier section
(IC28, pin 5) and applied to the FM detector section (X2,
IC28, pins 10, 11) for demodulation the 2nd IF signal into AF
signals.
The FM detector circuit employs a quadrature detection
method (liner phase detection), which uses a ceramic dis-
criminator (X2) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC28 (pin 9) is applied to the
AF circuit.

4-1-8 VHF AF AMPLIFIER CIRCUIT

The AF amplifier circuit amplifies the detected signals to drive
a speaker. The AF circuit includes an AF mute circuit for the
squelch.
AF signals from FM IF IC (IC28, pin 9) pass through the AF
selector (IC21, pins 9, 8), and are then applied to the low-
pass (Q83, R370–R373, C406–C409) and high-pass (Q84,
R375–R379, C415–C418) filters. The filtered signals are
level adjusted at the volume control IC (IC10), and are ampli-
fied at the AF power amplifier (IC12, pin 2) passing through
the V-AF mute switch (Q70).
The output signal from IC12 (pin 11) drives the external or
internal speaker.

• AF amplifier circuit

IC21
IC28
VDET
9
VHF FM
Detector
AF selector
IC8
4
UDET
UHF FM
Detector
Q83, Q84
VAF
8 1
BPF
6
VDMUTE
Q85, Q86
8
3 UAF
BPF
5
UDMUTE

4-1-9 VHF SQUELCH CIRCUIT

• NOISE SQUELCH
A noise squelch circuit cuts out AF signals when no RF sig-
nals are received. By detecting noise components in the AF
signals, the squelch circuit switches the AF mute switch.
Some of the noise components in the AF signals from the FM
IF IC (IC28, pin 9) are passed through the active filter section
(IC28, pin 8, 7), and then applied to the noise detector sec-
tion (IC28). The variable resister (R196) adjusts the input
level of the active filter, and the level is used for squelch
threshold reference. The detected noise signals are applied
to the CPU (IC19 pin 97) via the "VSQL" line.
The [SQL] (CONTROL unit; R154) controls the input level of
the sub-CPU (CONTROL unit; IC10, pin 59) in DC voltage.
The sub-CPU reads the angle of the [SQL] rotation, then
send the squelch data to the CPU incorporated in the RDATA
line. Then the CPU controls V-AF mute switch (Q70) via the
"VAMUTE" line.
Even when the squelch is closed, the V-AF mute switch
(Q70) opens at the moment of emitting beep tone.

• TONE SQUELCH

The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS). When tone squelch is in use, and
a signal with a mismatched or no subaudible tone is
received, the tone squelch circuit mutes the AF signals even
when noise squelch is open.
A portion of the AF signals from the FM IF IC (IC28, pin 9)
passes through the active filter (IC20) to remove AF (voice)
signals via the AF selector (IC29, pins 5, 4), and is then
applied to the CTCSS decoder inside the CPU (IC19, pin 1)
via the "TONEIN" line to control the AF mute switch.
Q70
VAMUTE
VMUTE
IC10
2
IC11, Q69
Volume
Common
controller
MUTE
7
Q71
UAMUTE
UMUTE
4 - 2
IC12
2 11
J1
AF power
Amplifier
5
7
J2
SP