Icom IC-Q7A Service Manual page 8

4-1-7 1ST IF AND 2ND MIXER CIRCUITS (RF UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal.
The filtered 266.7 MHz 1st IF signal from the bandpass filter
(ҒІ1) is mixed with the 2nd LO signal at the 2nd mixer (ІС10)
to produce a 19.65 MHz 2nd IF signal. The 2nd IF signal
passes through (except WFM mode) or bypasses (WFM
mode) the bandpass filter (FI3), and is then amplified at the
buffer amplifier (Q5). The amplified signal is applied to the
demodulator circuit. |
4-1-8 DEMODULATOR CIRCUITS (RF UNIT)
The demodulator circuit converts the 2nd IF signal into AF
signals.
The 19.65 MHz 2nd IF signal from the buffer amplifier (Q5) is
applied to the 3rd mixer section of the FM IF IC (IC2, pin 16),
and is then mixed with the 3rd LO signal for conversion into
a 450 kHz 3rd IF signal.
IC2 contains the 3rd mixer, limiter amplifier, quadrature
detector and S-meter detector, etc. A frequency from the PLL
reference oscillator is used for the 3rd LO signal (19.20
MHz).
(1) FM mode
The 3rd IF signal is output from FM IF IC (IC2, pin 3) and
passes through the ceramic bandpass filter (F12). The filtered
signal is fed back and amplified at the limiter amplifier section
(pin 5), then demodulated into AF signals at the quadrature
detector section (pins 10, 11) and detector coil (L21). The
demodulated AF signals are output from pin 9 and are
applied to the AF circuit (LOGIC unit).
(2) WFM mode
The 3rd IF signal from the 3rd mixer bypasses the ceramic fil-
ter (Fl2) and fed back to the limiter amplifier section (pin 5).
е 2nd IF AND DEMODULATOR CIRCUITS
R56
AF signal 'WFMS" 7095
The amplified signal is demodulated at the quadrature detec-
tor section (pins 10, 11) and detector coil (L21). The AF sig-
nals are output from pin 9 and are applied to the AF circuit
(LOGIC unit).
By connecting R55 to R54 in parallel, the output characteris-
tics of pin 12, "RSSI", change gradually. Therefore, the FM IF
IC can detect WFM components.
(3) AM mode
The filtered 3rd IF signal from the bandpass filter (Fl2) is
amplified at the 3rd IF amplifier (Q1). The amplified IF signal
is applied to the AM detector circuit (Q4) to convert into AF
signals, and the signals are applied to the AF circuit (LOGIC
unit).
4-1-9 AF AMPLIFIER CIRCUIT (LOGIC UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
While in FM mode, AF signals from the demodulator circuit
(RF unit) are passed through the de-emphasis (R118, C66,
C68) and bandpass filter (Q30) and are then applied to the
pre-amplifier (Q31).
While in AM mode, AF signals are pass through the band-
pass filter (Q30) and are then applied to the pre-amplifier
(Q31).
While in WFM mode, AF signals are applied to the pre-ampli-
fier (Q31) directly.
The pre-amplified AF signals pass through the AF mute
switch (Q37), and are then applied to the electronic volume
control circuit (IC14, pin 6). The level controlled AF signals
are output from pin 7 and are applied to the AF power ampli-
fier (IC15, pin 1) via the buffer amplifier (Q36). The power
amplified AF signals are applied to the internal speaker via
the [EXT SP] jack.
3rd IF filter
50 kH
Х1
19.2 MHz
IC2 TA31136F
2nd IF (19.65 MHz)
from Q5
"SDET" signal to the IC11 (Logic unit)
"RSSI" signal to the CPU
O WFM pin 7 (Logic unit)
Q41 16242