Circuit Description; Receiver Circuit; Vhf Antenna Switching Circuit; Vhf Rf Circuit (Rf-A Unit) - Icom IC-821H Service Manual

SECTION 4 CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUIT
Note: [Main]=Main band, [Sub]=Sub band
4-1-1 VHF ANTENNA SWITCHING CIRCUIT
(PA-A UNIT)
Received signals from the antenna connector (J2) are
passed through the low-pass filter (L3-L5, C5-C8) then
applied to the antenna switching circuit {01-03, L6. L7}.
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very
high while transmitting by turning ON diodes (02, 03).
Thus transmit signals are blocked from entering the receiver
circuits.
The passed signals are then applied to the RF-A unit.
4-1-2 VHF RF CIRCUIT (RF-A UNIT)
Received signals from the PA-A unit via J6 are applied to
the RF amplifier circuit (Q7) via the RF attenuator (015) and
tunable bandpass filter (013. L 16) circuits.
The amplified signals are then passed through the another
tunable bandpass filter circuit (010-012, L 13-L 15) to sup-
press unwanted signals. The filtered signals are then
applied to the 1 st mixer circuit (Qll , Q12).
Varactor diodes (010-013) are used for the bandpass
filters to tune the center frequency for wide bandwidth
receiving and good image response rejection. PLL lock
voltages are applied to these diodes for tuning.
4-1-3 VHF 1ST MIXER CIRCUIT (RF-A UNIT)
The 1 st mixer circuit converts the received signals to a fixed
frequency of the 10 MHz IF signal with a PLL output fre-
quency. By changing the PLL frequency, only the desired
frequency will be passed through a pair of crystal filters at
the next stage of the VHF 1 st mixer .
• RECEIVER CONSTRUCTION
440 MHz
PA
unit:
RF·B
unit
014.015
4 -1
The filtered signals from the bandpass filter are mixed with
1 st LO signals at the mixer circuit (Ql1, Q12) to produce a
1st IF signal (10.85 MHz [Main] or 10.95 MHz [Sub]). The
1 st LO signals are the PLL output frequency which comes
from the VHF
veo
circuit (PLL unit. Q1. 01-03, 08).
The 1 st IF signal is then applied to either the Main or Sub
band 10 MHz IF circuit in the MAIN unit via P1 [Main] or P2
{Sub].
4-1-4 UHF RF CIRCUIT (RF-B UNIT)
The received signals from the UHF antenna connector are
passed through the low-pass tilter (PA-B unit, L3-L5,
C5-C10) and then the antenna switching circuit (PA-B unit,
01-03). The signals from the antenna switching circuit are
then applied to the RF amplifier circuit (Q16) via the tunable
bandpass filter (014, L35) and RF attenuator circuit {016}.
The amplified signals are passed through the tunable band-
pass filter (011-013), and then applied ~o the 1 st mixer
circuit (Q14, Q15).
4-1-5 UHF 1ST AND 2ND MIXER CIRCUIT
(RF-B UNIT)
The filtered RF signals from the bandpass filter are mixed
with a 1 st LO signal at the 1 st mixer circuit (Q14, Q15) to
produce a 1 st IF signal (71.25 MHz [Main] or 71.35 MHz
[Sub]). The 1 st IF signal is passed through a crystal filter
(F12 [Main), Fll [Sub]) to suppress out-ot-band signals. The
filtered IF signal is then applied to the 2nd mixer circuit
(Qll) to produce a 10 MHz IF signal with a 2nd LO signal.
The IF signal is then applied to the MAIN unit via P1 [Main]
or P2 [Sub].
The 1 st LO signal is generated at the UHF VCO circuit (PLL
unit, Q5, 04, 010), and the multiplied reference signal is
used for the 2nd LO signal.
I
MAIN
unit
I
I
I
I
BFO
for MAIN band
AF signals
to
SOL
control circuit
tor SUB band
AF signals to SOL
control circuit
10.9483-10.9515 MHz
to FM IF IC (IC20)