Icom IC-725 Service Manual page 9

4·1·5 IF CIRCUITS (MAIN UNIT)
The 1st IF signal from 015 is converted to a 9 MHz 2nd
IF signal at the 2nd mixer (IC1). IC1 is a DBM (Double
Balanced Mixer).
EXACTNESS 2ND IF FREQUENCY
MODE FREQUENCY (MHz)
SSB 9.0115
cw
9.0106
AM, FM
9.0100
The 2nd LO signal (61.44 MHz) from the PLL UNIT via J4
is applied to the 2nd mixer. The converted 2nd IF signal
passes through D4 (D35 for transmitting) and is applied
to the MCF (Fl2) to suppress unwanted signals.
The signal output from Fl2 passes through the noise
blanker gate (D5-D8) and is amplified at the 2nd IF am-
plifier (021). The signal enters one of the three 9 MHz
filters (Fl3, Fl4, optional CW narrow filter) or optional AM
· FM UNIT via D52. The filters are selected with mode
selecting signals (SSB ·CW, AM, CW-N) and the "TB"
voltage line.
The signal from a 9 MHz filter is amplified at the 2nd IF
amplifiers (027-029) and applied to the demodulator
circuit.
Dual-gate FETs are used on the 1st and 2nd IF amplifiers
(015, 021, 027). The 2nd gates of 015, 021 and 027 are
controlled by AGC bias voltage. A rapid time constant
is used for 027 to prevent raising the edge distortion of
receive signals.
R140, connected to the gate of 028, improves the
temperature characteristics of the receiver gain. R138
adjusts the receiver gain.
IF CIRCUIT
AF signal
SQL
IC8
BFO
SSB DET
AM OET
062
4·1·6 NOISE BLANKER CIRCUITS
(MAIN UNIT)
The IC-725 uses a noise trigger noise blanker circuit that
cuts out pulse-type noise signals at the noise blanker gate
(D5-D8).
A portion of the signals from Fl2 is amplified at the noise
amplifiers (016, IC2) and detected at the noise detector
(D12, D13). The detected voltage from the noise detector
is applied to the noise blanker switch (019).
The threshold level of the noise blanker switch is set at
0.9 V. When the detected voltage exceeds the threshold
level, 020 outputs a blanking signal to activate the noise
blanker gate (05-08).
A portion of the detected voltage is applied to the noise
AGC circuit (018) and fed back to the noise amplifier (IC2)
as noise AGC voltages. The time constant of the noise
AGC circuit is determined by R43, R47 and C60. This
AGC circuit does not operate to detect pulse-type noise.
When the operating frequency or mode is changed, the
"ONB" signal line becomes "LOW," turning 020 ON. The
noise blanker gate prevents PLL click noise.
4·1·7
BFO
CIRCUIT (MAIN UNln
A 9 MHz signal oscillated at the BFO circuit (031, X1)
is buffer-amplified at 042 and used at the balanced
modulator (IC6) and a product detector (IC5). The BFO
frequency is shifted with a mode signal using 067-069.
In USB mode, the "USB" signal line becomes "HIGH,"
turning ON 069. The frequency is then adjusted with
C294 to set the USB carrier point.
At CW mode transmitting, the "CW" signal line becomes
"HIGH" and 033 becomes OFF, turning ON 068. The
frequency is then adjusted with L83 to set the CW
transmit carrier point.
In LSB mode, the "LSB" signal line becomes "HIGH,"
turning ON 067. The frequency is then adjusted with
L82 to set the LSB carrier point.
NOISE
BLANKER
Fig. 2
059, 060
4-2