Sony ICF-6800W - Page 13

However, in the case of a balanced-type mixer,
the input signal that comes in from the balanced
end does not itself emerge at the output end; only
the converted frequencies emerge.
The ICF-6800W uses a balanced-type mixer like
this, to balance the input signal end and eliminate
the aforementioned interference.
Let us say that a signal that is the same fre-
quency as the 19.055 MHz Ist intermediate frequency
comes into the input of the mixer in the schematic
shown in Fig. 26. This signal will enter the Q2 and
Q3 gates in the same phase, and will appear at the
drains also in phase. They will then be fed to each
end of the IFT A1, and so will cancel out each other.
Meanwhile, the local-oscillator signal is fed to
the source sides of Q2 and Q3 in opposite phase,
and so the signal that goes through a frequency-
conversion process by means of the local oscillator
signal fed to the source side appears at the drain
in opposite phase in the additive form.
Therefore, IFT Al is fed only with the output
of the converter circuit, and so it is possible to obtain
an IF signal that is stronger than that which can be
obtained with a single-conversion type mixer.
VT2 is provided for adjustment purposes to
match the gain figures of Q2 and Q3, and to enable
even more complete cancellation.
Note: The local oscillation components are also generated
in IFT Al, but they can be eliminated readily by
means of the filter effect.
INPUT FROM ae
ANTENNA
Lf
FROM LOCAL
OSCILLATOR
Fig. 26
Functions of Q53—Q57 (phase detector)
(Refer to Fig. 27)
This circuit is provided in order to maintain
the oscillating frequency of VCO2 at a constant
figure. In other words, locks it at each point, wheth-
er it is 28 MHz, 29 MHz and so on up to 37 MHz,
so that frequency will not drift. It functions as
follows.
Basically, it mixes the signal from VCO2 that
is being fed to the gate with the pulse signal that
is being fed to the source, and extracts the differ-
ential (AC component) to regulate VCO2.
Q53 comprises a balanced-type mixer. Signals
that are fed to each of the gates of Q53 in the same
vco2
INPUT
—-13-—
ICF-6800W
phase also are produced at the drain in the same
phase, and so only the DC components are produced
at the output of the differential amplifier Q55
through Q57.
OUTPUT TO VARICAP
OF vCO2
PULSE
INPUT 055
Fig. 27
The pulse signals that are being fed to the sources
are each being fed in opposite phase, and so the
signals
that
have
gone
through
the
frequency
con-
version process are fed to the differencial amplifier
each in opposite phase, so that amplified AC com-
ponents are produced at the output of the dif-
ferencial amplifier.
Let us say, for example, that the frequency of
VCO2is 30.00 MHz.
As shown in Fig. 28, the pulse signals that are
added at the source include pulse signals that are
intergral 1 MHz. These pulse signals are generated by
means of a pulse generator.
frequency spectrum of
\ the pulse
level
| ee Se 40 41 42 43
—a MHz
A beat signal is generated between the frequency
of VCO2(30.001 MHz) and the pulse signal (30 M Hz).
(In other words, a 0.001 MHz beat signal.)
This beat signal is fed to the differential am-
plifier, each in opposite phase, and then derived as
the output. At this time, the harmonic components
are eliminated by the filter circuit.
This AC-component output is fed to the vagable
capacitance diode of VCO2 to vary the oscilating
frequency of VCO2,
If the frequency of VCO2 is 30MHz, then a
zero beat is generated, and so there will be no AC
components at the output; only the DC compongsts.