Icom IC-751A Service Manual page 19

TWO SIGNAL RECEIVE CHARACTERISTICS
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The ist mixer circuit is an upconverter which converts
receive signals into 70.4515MHz 1st IF signals.
The ist LO output signal from the PLL UNIT is fed througha
high-pass filter, amplified by Q2, filtered by a low-pass filter,
and then is applied to the 1st mixer as a local oscillator signal
(70.5515~100,4515MHz). R18, L13, and C14 are designed as
feedback functions to improve the frequency characteristics
of Q2.
The 1st IF signal is filtered by a monolithic crystal filter (FI,
+7.5kHz/-3dB) and then is amplified by a dual-gate FET
(Q8), the 2nd gate of which is controlled by the AGC voltage.
Signals are fed through T/R Switching Diode D19 and a
high-pass filter to the 2nd IF mixer of Double-Balanced
Mixer IC3 where the signals are converted into 9.0115MHz
2nd IF signals. The signals are then filtered in order to have
local oscillation components removed by a low-pass filter
before being fed to the MAIN UNIT through P3.
2nd LO signals (61.44MHz) from the PLL UNIT are fed to IC3
as local oscillator signals for the 2nd mixer.
2. MAIN UNIT
9MHz 2nd IF signals from J4 pass through Q33, a noise-
blanker gate and amplifier. After being amplified at Q33,
signals are fed through a filter select switch circuit and intoa
9MHz IF filter.
Noise blanker gate D68~D71 is a diode balanced-type
switch circuit which passes signals through it. Signals are
cut by this gate when control voltage from the noise blanker
circuit is applied to D72.
Mode switches and the [FILTER] SWITCH on the front pane!
send signals into the circuit which select a 9MHz IF circuit
section consisting of Fl2 and FI8.
Filtered signals amplified at Q84 are fed into the 3rd mixer,
1C10. 9.4665MHz (+ SHIFT frequency) signals are supplied
as local oscillator signals from Q55 to 1C10 (pin 7) in order to
obtain 455kHz 3rd IF signals. 3rd IF signals are buffer
amplified at Q39 and fed into the 455kHz filter section of the
circuit. The 455kHz section consists of FIS~FI6 and an
optional narrow filter (in CW and RTTY modes). 3rd IF
signals are selected as in the 9MHz section.
Signals from the 455kHz filter are converted to 9MHz again
by C11, the 4th mixer. When the transceiver is in FM mode,
output from FI6 (the FM filter) is applied to the FM receive
circuit. 9MHz-converted signals pass through the notch
circuit and are amplified at Q43 and Q44 before being fed
into the detector and APC circuits.
4-1-3 FILTER SECTION (MAIN UNIT)
The IC-751A has two filter sections (Q9MHz and 455kHz) for
passband tuning and high selectivity.
The 9MHz filter section consists of a through circuit in AM
and FM modes, FI2 in SSB, CW, and RTTY modes, and FI8 in
CW Narrow and RTTY Narrow modes. 9MHz 3rd IF
frequencies are 9.0115MHz in SSB mode, 9.0100MHz in AM
and FM modes, and 9.00706MHz in CW and RTTY modes.
The 455kHz filter section consists of FI3, Fl4, FI5, and FI6.
These filters consist of several other filters which are listed
beiow.
FI3: Contains an SSB High Shape Factor Filter, an
SSB Filter, a CW Filter, and an RTTY Filter.
Fl4: Contains an SSB Wide and RTTY Narrow Filter.
FI5: Contains an AM Filter.
FI6: Contains an FM Filter and optional CW Narrow
and RTTY Narrow Filters.
The above filters are selected by control signals from the
filter switching circuit using the switch circuit. Refer to
SECTION 4-2-19 FILTER SWITCHING CIRCUIT for more
information regarding filters and a combination of filter
switches or modes.
4-1-4 NOISE BLANKER CIRCUIT
A portion of 2nd IF signals from the RF UNIT is fed into a
noise amplifier circuit consisting of Q8, Q9, and Q10. This
circuit has high gain and a wide AGC dynamic range. It
amplifies weak signals, giving them higher sensitivity with a
wider dynamic range.
Amplified signals from Q10 are detected by D17 and D18,
and are then fed into Q11 (the noise AGC) and Q13 and Q15
(noise switches). The bias voltage of Q8, Q9, and Q10 is
decreased by Q11 to control the gain of the noise amplifier.
This noise AGC has atime constant determined by R46, R47,
and C31, and functions as an average-type AGC circuit. The
AGC therefore responds to SSB signals such as those
without sharp leading edges or those that are constant-
amplituded. Rectified voltage of the noise AGC is constant.
However, rectified voltage will exceed the threshold level of
the determined voltage when noise with a sharp leading
edge is received.
Q15 controls the gate control circuit when the rectified
voltage exceeds the threshoid level. The threshold level is
controlled by the NOISE BLANKER [NB] CONTROL on the
4—2