Icom ID-800H Service Manual page 12

• 400 MHz BAND RF CIRCUIT
(230–550 MHz EXCEPT UHF BAND)
The RF signals from the attenuator circuit are applied to the
band switch (D43). The switched signals are passed through
the tunable bandpass filter (D36, L37, L38, C242, C253,
C261) and are then amplified at the RF amplifier (Q40). The
amplified signals are applied to the another 2-stage tunable
bandpass filter (D25, D30, C196, C205, C212) to suppress
unwanted signals and improve the selectivity. The filtered
signals are applied to the RF amplifier (Q36). The ampli-
fied signals pass through the 400 MHz band switch (D17),
and then applied to the 400/800 MHz band 1st mixer circuit
(IC12).
• 800 MHz BAND RF CIRCUIT (810–1000 MHz)
The RF signals from the attenuator circuit are applied to
the antenna switch (D44). The switched signals are passed
through the tunable bandpass filter (D37, L39, C243, C254,
C260, C269) and are then amplified at the RF amplifier (Q41).
The amplified signals are applied to the another tunable
bandpass filter (D26, D31, L27, L31, C188, C197, C206,
C213, C223) to suppress unwanted signals and improve the
selectivity. The filtered signals are applied to the RF ampli-
fier (Q37). The amplified signals pass through the 800 MHz
band switch (D18), and then applied to the 400/800 MHz
band 1st mixer circuit (IC12) same as 400 MHz band sig-
nals.
The tunable bandpass filters (D22, D23, D25, D26, D29–
D31, D36, D37) employ varactor diodes to tune the center
frequency of the RF passband for wide bandwidth receiving
and good image response rejection.
4-1-5 1ST MIXER CIRCUIT (MAIN UNIT)
The 1st mixer circuit converts the received signal to a fixed
frequency of the 1st IF signal with the 1st LO frequency.
• VHF BAND
The filtered signals from the tunable bandpass filter (D22)
are mixed with the 1st LO signal at the 1st mixer circuit (Q32)
to produce the 46.05 MHz 1st IF signal. The 1st IF signal
passes through the IF switch (D8), and are then applied to
the 1st IF circuit.
• 2ND IF AND DEMODULATOR CIRCUIT
AF signals
("DET" signal)
AM/FM
switch
IC5
Q13,
Q16
AM
DET
8 7
Active
filter
Noise
detector
Limiter
amp.
Quadrature
detector
9 10 11
R5
X2
• UHF BAND
(EXP: 430–440 MHz, USA: 440–450 MHz)
The filtered signals from the bandpass filter (FI4) are mixed
with the 1st LO signal at the 1st mixer circuit (Q35) to pro-
duce the 46.05 MHz 1st IF signal. The 1st IF signal passes
through the IF switch (D9), and are then applied to the 1st
IF circuit.
• 400 MHz AND 800 MHz BAND
(230–550 MHz EXCEPT UHF BAND, 810–1000 MHz)
The switched signals from the band switch (D17, D18) are
mixed with the 1st LO signal at the 1st mixer circuit (IC12,
pins 1, 6) to produce the 46.05 MHz 1st IF signal. The 1st
IF signal passes through the IF switch (D10), and are then
applied to the 1st IF circuit.
The 46.05 MHz 1st IF signals from the each IF switches are
filtered at the monolithic filter (FI3) to suppress out-of-band
signals, and then pass through to the limiter circuit (D6).
The level adjusted signal from the limiter circuit is amplified
at the IF amplifier (Q18), and then applied to the 2nd mixer
circuit (IC8).
4-1-6 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT
)
The 2nd mixer circuit converts the 1st IF signal to the 2nd
IF signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC8) contains the 2nd mixer, limiter amplifier,
quadrature detector, and noise detector circuits, etc.
The 1st IF signal from the IF amplifier (Q18) is applied to
the 2nd mixer section of IC8 (pin 16), and is mixed with the
45.6 MHz 2nd LO signal to be converted into the 450 kHz
2nd IF signal.
The 2nd LO signal (45.6 MHz) is produced at PLL circuit by
tripling it's reference frequency (X1: 15.2 MHz) at the tripler
(Q7).
The 2nd IF signal from the 2nd mixer (IC8, pin 3) passes
through one of the ceramic filters (FI1: Narrow, FI2: Wide),
where unwanted signals are suppressed.
Narrow
IC10
IC9
FI1
FI2
Wide
5 3 2
Mixer
RSSI
12 14 16
"SQL" signal to the main CPU (IC505)
"RSSI" signal to the main CPU (IC505)
4 - 2
45.6 MHz
BPF
3
Q7
X1
15.2 MHz
IC8
TA31136FN
1st IF from the
IF amplifier (Q18)