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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a receiver apparatus such as a tuner or
the like for receiving television signals or CATV signals, and more
particularly, to a tuner circuit having a favorable anti-disturbance
characteristic.
Conventional tuner circuits for receiving television signals are generally
provided with a trap circuit arranged in an input tuner circuit thereof
for removing a beat disturbance caused by an FM broadcasting wave or the
like, as described in Official Gazette of Japanese Laid-open Patent No.
80325/1985.
SUMMARY OF THE INVENTION
The above-mentioned prior art is effective when the frequency of a received
signal is far away from that of a spurious signal which may disturb images
or the like. However, if the frequency of a received signal is close to
that of a spurious signal as an FM signal disturbance affecting a
reception of the sixth channel of the U.S. channel system, a large amount
of a trap attenuation in an input tuner circuit causes an increase in loss
of a received signal.
It is an object of the present invention to provide a receiver apparatus
such as a tuner which is capable of solving the above-mentioned problem
and exhibiting a favorable anti-disturbance performance even when the
frequency of a received signal is close to that of a spurious signal.
To achieve the above object, the present invention provides a trap circuit
between an inter-stage circuit and a mixer circuit of a tuner for
attenuating a mutual modulation signal which causes a disturbance and also
employs a double balanced mixer which exhibits a favorable
anti-disturbance performance for the mixer circuit.
Upon receiving the sixth channel of the U.S. channel system, since the FM
signal frequency is extremely close, this FM signal cannot be removed by a
trap circuit arranged in an input tuner circuit of a receiver apparatus as
before. For this reason, the receiver apparatus inevitably simultaneously
receives the FM signal when a signal at the sixth channel is received. If
two or more signals are thus received at the same time, a mutual
modulation wave is produced in a nonlinear element arranged at a location
from an input circuit to an inter-stage circuit of the receiver apparatus.
The present invention is adapted to remove a spurious signal produced in an
IF band caused by the received two signals (RF signals) and a local
oscillation signal of a receiver apparatus. The spurious signal is
produced when a mutual modulation wave produced in a nonlinear element
from an input circuit to an inter-stage circuit on the basis of the
received two signals is converted to an IF band by a local oscillation
signal in a mixer circuit, and when the respective signals are converted
to the IF band in the mixer circuit. In the former case, a trap circuit
arranged between the interstage tuner circuit and the mixer circuit
attenuates mutual modulation wave components to improve the
anti-disturbance performance, while in the latter case, a double balanced
structure is employed for the mixer circuit to improve the
anti-disturbance performance.
Further, the above structure also produces an effect of offsetting the
mutual modulation wave components produced up to the inter-stage tuner
circuit and the mutual modulation wave components produced by the mixer
circuit in a double balanced structure, which results in further improving
the anti-disturbance performance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a first embodiment of the present
invention;
FIG. 2 is a diagram used for explaining a procedure of occurrence of a
disturbance;
FIGS. 3A and 3B are characteristic diagrams used for explaining a trap
circuit of the present invention;
FIG. 4 is a block diagram showing a second embodiment of the present
invention;
FIG. 5 is an ordinary block diagram showing a receiver apparatus of the
present invention; and
FIGS. 6, 7A and 7B are circuit diagrams respectively showing other
embodiments of a trap circuit of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of the present invention will hereinafter be explained
with reference to FIG. 1.
FIG. 1 shows a first embodiment of the present invention and specifically
shows a circuit diagram of a tuner for receiving television signals.
Reference numeral 1 designates a UHF signal input terminal, 2 a VHF signal
input terminal, 3 and 4 input variable tuner circuits respectively for UHF
and VHF, 5 and 6 RF amplifiers respectively for UHF and VHF, 7 and 8
inter-stage variable double tuner circuits for UHF and VHF, and 9 a trap
circuit according to the present invention.
Further, reference numeral 10 designates a mixer circuit, 11 an IF filter,
12 an IF amplifier circuit, 13 an IF output filter, 14 an IF output
terminal. Also, reference numeral 15 designates a UHF oscillator circuit,
16 a UHF oscillator variable resonance circuit, 17 a VHF oscillator
circuit, 18 a VHF oscillator variable resonance circuit, 19 an oscillation
signal coupling circuit, 20 an oscillation amplifier circuit, 21 a buffer
amplifier, 22 a bias supply circuit, and 23 a channel select control
circuit.
A UHF signal is inputted from the UHF signal input terminal 1 and supplied
through the UHF tuner circuit and the buffer amplifier 21 to the mixer
circuit 10 wherein the UHF signal is converted to an IF signal by a UHF
oscillating signal. The IF signal is outputted from the IF output terminal
14 through the IF filter 11, the IF amplifier circuit 12 and the IF output
filter 13. A VHF signal or a CATV signal, on the other hand, is inputted
from the VHF signal input terminal 2 and supplied through a VHF tuner
circuit to the mixer circuit 10 wherein it is converted by a VHF
oscillating signal to an IF signal which in turn is outputted from the IF
output terminal 14 through the IF filter 11, the IF amplifier circuit 12
and the IF output filter 13.
The mixer circuit 10 comprises a double balanced mixer 24 and an RF
differential amplifier circuit 25, while the VHF oscillator variable
resonance circuit 18 comprises a variable capacitance diode 30, a VHF high
band coil 31, a VHF low band coil 32 and a switching diode 33, wherein the
coils are changed over by a band voltage from the channel select control
signal 23 to change an oscillation frequency band, and also the
capacitance of the variable capacitance diode 30 is changed by a channel
selection voltage to vary an oscillation frequency. The UHF oscillator
variable resonance circuit 16 comprises a variable capacitance diode 34
and a resonance inductor 35 wherein the capacitance of the variable
capacitance diode 34 is changed by a channel selection signal to vary an
oscillation frequency. The inter-stage variable double tuner circuit 8
comprises variable capacitance diodes 40, 41, switching diodes 42, 43, VHF
high band coils 44, 45, VHF low band coils 46, 47 and capacitors 48-52,
wherein the coils are changed over by turning on and off the switching
diodes to change a received frequency band, and the capacitance of the
variable capacitance diodes is changed to vary a tuned frequency.
Next, a generating mechanism of an example of a disturbance which is
removed by the present invention and the action of the present invention
against such disturbance will be explained in detail
FIG. 2 shows a case where a U.S. sixth channel signal (a video carrier
frequency Fp: 83.25 MHz, an audio subcarrier frequency Fs: 8.5 MHz, an
oscillating frequency Fosc: 129 MHz) is received, and an FM educational
broadcast signal (a frequency Fm: 88.1-91.9 MHz) is disturbing the U.S.
sixth channel signal, wherein reference letter A designates a video
carrier signal of the sixth channel, B an FM signal, C an oscillation
signal of the sixth channel, D a sum signal of the signals A and B, E an
IF signal of the video carrier of the sixth channel, and F a difference
signal between the signals D and C. The signal F, that is, a beat signal
component Fs+Fm-Fosc is generated in the vicinity of the received IF
signal and causes a disturbance. For example, if an FM signal is at 91
MHz, the signal F is at 45.25 MHz, whereby a beat disturbing component of
0.5 MHz is generated against the IF signal of the sixth channel video
carrier at 45.75 MHz, and therefore the image quality is deteriorated.
Such a disturbing component is generated mainly by two processes which are
mentioned below.
One is that a nonlinear element such as a variable capacitance diode or the
like generates the above-mentioned sum signal of a video carrier signal of
the sixth channel and an FM signal and then the sum signal is converted to
the IF band by the oscillating signal in the mixer circuit. The other is
that the disturbing component is generated in the mixer circuit by the
above-mentioned three signals of a video carrier signal of the sixth
channel, an FM signal and the oscillating signal. In the latter case, it
is necessary to improve the performance of the mixer circuit, while in the
former case, the performance can be improved by attenuating the sum signal
component in stages before the mixer circuit.
The trap circuit 9 shown in FIG. 1 is provided for attenuating the sum
signal component, the operation of which will be next explained. In the
trap circuit 9, reference numerals 60, 61 designate coils, 62-66
capacitors, 67 a resistor and 68 a switching diode. Generally, the
capacitor 63 is chosen to be a small capacitance, and the coils 60, 61 are
changed over by the switching diode 68 to match the tuner circuit and the
mixer circuit in a low frequency region of the respective low and high
bands to reduce a loss. However, in the present invention, the sum signal
trap circuit 9 is formed merely by connecting the capacitor 62 in parallel
with the coil 61. FIG. 3A shows a reactance characteristic of the trap
circuit 9 and FIG. 3B an attenuation characteristic of the same, from
which it is understood that the trap circuit 9 attenuates a sum signal
component in the vicinity of 10 MHz by more than 10 dB and improves the
anti-disturbance performance.
The double balanced mixer 24 is composed of FETs 81-84, while the RF
differential amplifier circuit 25 has differential amplifier FETs 91 and
92. A resistor for improving the linearity is connected between the
sources of the FETs 91, 92, the gates of which are grounded. FETs 94, 95
for a constant current source are respectively connected to resistors 96,
97 for determining a current value thereof, whereby a supplied current is
determined. A gate bias for the FETs 91, 92 is determined by the bias
supply circuit 22 through resistors 72, 73. The buffer amplifier is also
utilized as a switch circuit, wherein reference numeral 75 designates a
bias resistor for a change-over operation and 76 a capacitor for the
ground. These elements have functions of amplifying a received UHF signal
and transmitting the same to the RF differential amplifier circuit 25, and
grounding the gate of the FET 92 in a high frequency manner when a VHI.
signal is received. Further, reference numeral 77 designates a high
frequency ground capacitor for converting an inbalanced oscillation signal
to a balanced signal.
Incidentally, in the embodiment shown in FIG. 1, a portion of the circuit
surrounded by a one-dot chain line 100 is formed by an integrated circuit.
Next, the operation of the mixer circuit 10 will be explained. When a VHF
signal is inputted to the gate of the FET 91 constituting the RF
differential amplifier circuit 25, the gate of the FET 92, coupled
therewith to form a pair is grounded in a high frequency manner by the
buffer amplifier 21 which is utilized also as a switching circuit, whereby
amplified and balancedly converted RF signals respectively having an
identical amplitude and a substantially opposite phase to each other are
outputted from the drains of the FETs 91, 92. The double balanced mixer 24
is fed with VHF oscillation signals respectively having a substantially
opposite phase to each other and an identical amplitude. Two pairs of the
FETs 81 and 83, 82 and 84, which are applied with the RF signals and the
oscillation signals both in opposite phases to each other, have their
respective drains connected to form an output terminal. The pair of FETs
81 and 83 are applied with the RF signals in the opposite phase to each
other at the respective sources thereof and the oscillation signals in the
opposite phases to each other at the respective gates thereof, whereby
converted IF signals in the same phase (a sum or difference component of
the oscillation signal frequency and the RF signal frequency) are
outputted from the respective drains of the FETs 81 and 83. In the other
pair of FETs 82 and 84 of the double balanced mixer 24, the RF signals and
the oscillation signals are frequency-converted to output IF signals in
phases opposite to those outputted from the FETs 81 and 83.
Next, explanation will be given of a characteristic relative to removing a
third distortion disturbance which is caused by the two RF signals and the
oscillation signal in the event of the frequency conversion operation.
In the FETs 81 and 83 of the double balanced mixer 24, a third distortion
having components Fs+Fm Fosc and so on is generated by a third distortion
coefficient of the respective FETs from two spurious signals in opposite
phases and the two oscillation signals in opposite phases. At this time,
in the FETs 81 and 83, since a third distortion component is also
generated due to a multiplication of the oscillation signals and the RF
signals in opposite phases, the third distortion components in opposite
phases having an identical amplitude are generated at the drains of the
FETs 81 and 83, respectively. Since the FETs 81 and 83 have the respective
drains connected with each other, the third distortion components are not
generated in the double balanced mixer 24 as a principle.
Thus, a mixer circuit in a double balanced mixer structure is considerably
effective in improving an anti-distortion performance. Therefore, if such
double balanced mixer circuit is combined with a filter circuit (a trap
circuit) for attenuating mutual modulation components generated in the
tuner circuit, a tuner exhibiting a favorable anti-disturbance
characteristic can be provided. Further, the double balanced mixer
structure can produce an additional effect of offsetting distortion
components in the IF band of the mutual modulation components generated in
the tuner circuit and distortion components generated in the mixer
circuit, which results in further improving the anti-distortion
characteristic.
FIG. 4 shows a second embodiment of the present invention. In FIG. 4, parts
for operating the same functions as those in FIG. 1 are designated by the
same reference numerals. A difference between FIG. 4 and FIG. 1 is that an
input terminal of a mixer circuit is provided with a variable resistor 99.
In the mixer circuit, a third distortion disturbance removing
characteristic varies due to variation or the line in pairs of FETs. In
the embodiment shown in FIG. 4, the resistor 99 offsets a bias of an RF
differential amplifier circuit 25 negatively or positively to reduce the
third distortion components generated in a double balanced mixer 25 to a
minimum, which results in presenting the most favorable anti-distortion
performance of the mixer circuit and further improving the anti-distortion
performance of the tuner.
FIG. 5 is a circuit block diagram showing an ordinary circuit arrangement
of the receiver apparatus according to the present invention. Also in FIG.
5, parts for operating the same functions as those in FIG. 1 are
designated with the same reference numerals. Reference numeral 100
designates an IC (Integrated Circuit) having the same functions as that in
FIG. 1. Reference numeral 110 designates a filter circuit or a trap
circuit provided for attenuating a mutual modulation wave in a received
signal or a spurious signal. Reference numerals 111, 112 designate input
and output terminals of the filter circuit 110, respectively.
FIG. 6 shows a second embodiment of the trap circuit, wherein reference
numerals 120 121 designate coils, 122-124 capacitors, 125 a switching
diode and 126, 127 resistors. The series resonance circuit formed of coils
120, 121 and the capacitor 122 constitutes a trap circuit.
FIGS. 7A, 7B respectively show third an fourth embodiments of the trap
circuit, wherein reference numerals 130-132 designate coils, 133-136
capacitors, 137, 138 switching diodes and 139-142 resistors. A parallel
resonance circuit composed of the coils 130, 131 and the capacitor 133 in
FIG. 7A constitutes a trap circuit while a parallel resonance circuit
composed of the coil 132 and the capacitor 135 in FIG. 7B constitutes a
trap circuit.
As explained above, according to the present invention, it is possible to
remove a disturbance caused by an FM educational channel affecting a
received U.S. sixth channel signal as well as a beat disturbance affecting
the sixth channel color signal caused by the video carrier, the audio
sub-carrier and the oscillation signal of the received U.S. sixth channel
signal. Also, the present invention is effective in removing disturbances
generally caused by mutual modulation waves and higher harmonic waves.
According to the present invention, a mixer circuit is provided with a
mixer in a double balanced mixer structure to improve an anti-distortion
and disturbance characteristic and a trap circuit in a simple structure
for attenuating distortion components in mutual modulation waves or higher
harmonic waves generated in a tuner circuit prior to the mixer circuit,
thereby producing effects of improving the anti-distortion and disturbance
characteristic and offsetting distortion components converted to an IF
band by the mixer circuit with distortion components converted from mutual
modulation waves generated by a tuner circuit to the IF band. It is
therefore appreciated that the present invention provides a receiver
apparatus such as a tuner which exhibits a satisfactory anti-distortion
and disturbance characteristic.
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