Double-tuned circuit

Details Double-tuned circuit Double-tuned circuit

1998-12-18

2001-05-01

Le, Thanh Cong (Department: 2684)

Telecommunications

Receiver or analog modulated signal frequency converter

Signal selection based on frequency

C455S188100, C455S191200

Reexamination Certificate

active

06226503

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a double-tuned circuit used as an inter-stage tuning circuit of a television-set tuner, and more specifically, to a double-tuned circuit in which a high-band coupling coil and a low-band coupling coil are independently provided to couple a primary tuning circuit with a secondary tuning circuit.
2. Description of the Related Art
FIG. 5
shows a conventional double-tuned circuit. In
FIG. 5
, a double-tuned circuit
51
is connected to a high-frequency amplifier (not shown) at an input end
51
a
and to a mixer (not shown) at an output end
51
b.
The double-tuned circuit
51
is switched between a high band and a low band by a so-called band switching method.
A primary tuning circuit
51
c
is provided with a high-band tuning coil
52
and a low-band tuning coil
53
connected in series by one end of each coil. Between the connection point thereof and the ground, a switch diode
54
and a DC-blocking capacitor
55
connected in series are connected.
The cathode of the switch diode
54
is connected to the connection point of the high-band tuning coil
52
and the low-band tuning coil
53
, and the other end of the high-band tuning coil
52
is connected to the input end
51
a.
Between the input end
51
a
and the ground, a DC-blocking capacitor
56
and a varactor diode
57
connected in series are connected, and the anode of the varactor diode
57
is grounded.
A secondary tuning circuit
51
d
is provided with a high-band tuning coil
58
and a low-band tuning coil
59
connected in series by one end of each coil. Between the connection point thereof and the ground, a switch diode
60
and a DC-blocking capacitor
61
connected in series are connected.
The cathode of the switch diode
60
is connected to the connection point of the high-band tuning coil
58
and the low-band tuning coil
59
, and the other end of the high-band tuning coil
58
is connected to the output end
51
b.
Between the output end
51
b
and the ground, a DC-blocking capacitor
62
and a varactor diode
63
connected in series are connected, and the anode of the varactor diode
63
is grounded.
The other end of the low-band tuning coil
53
in the primary tuning circuit
51
c
is connected to the other end of the low-band tuning circuit
59
in the secondary tuning circuit
51
d.
Between the connection point thereof and the ground, a coupling coil
64
and a DC-blocking capacitor
65
connected in series are provided. The DC-blocking capacitor
65
is connected to the ground.
The connection point of the coupling coil
64
and the DC-blocking capacitor
65
is connected to one end of a feed resistor
66
, and the other end thereof is connected to a low-band switching terminal
67
.
The connection point of the anode of the switch diode
54
and the DC-blocking capacitor
55
in the primary tuning circuit
51
c
is connected to one end of a feed resistor
68
, and the other end thereof is connected to a high-band switching terminal
69
. In the same way, the connection point of the anode of the switch diode
60
and the DC-blocking capacitor
61
in the secondary tuning circuit
51
d
is connected to one end of a feed resistor
70
, and the other end thereof is connected to the high-band switching terminal
69
.
The connection point of the DC-blocking capacitor
56
and the varactor diode
57
in the primary tuning circuit
51
c
is connected to one end of a feed resistor
71
, and the other end thereof is connected to a tuning voltage terminal
72
. In the same way, the connection point of the DC-blocking capacitor
62
and the varactor
63
in the secondary tuning circuit
51
d
is connected to one end of a feed resistor
73
, and the other end thereof is connected to the tuning voltage terminal
72
.
In the above configuration, a tuning voltage is applied to the tuning voltage terminal
72
. The voltage is changed to alter the capacitance of the varactor diode
57
in the primary tuning circuit
51
c
and the capacitance of the varactor diode
63
in the secondary tuning circuit
51
d,
and thereby the tuning frequency of the double-tuned circuit
51
is to be changed.
Band switching will be described below, in which the double-tuned circuit
51
is switched between a state in which a high-band television signal is received and a state in which a low-band television signal is received.
To switch the double-tuned circuit
51
shown in
FIG. 5
to a state in which a low-band television signal is received, a band switching voltage of, for example, 5 V is applied to the low-band switching terminal
67
. A voltage is applied to the switch diode
54
and the switch diode
60
in the reverse directions and both switch diodes
54
and
60
become a non-continuity state. Therefore, the double-tuned circuit
51
shown in
FIG. 5
works as an equivalent circuit shown in FIG.
6
.
In the equivalent circuit shown in
FIG. 6
, a coil
74
in the primary tuning circuit
51
c
indicates the high-band tuning coil
52
and the low-band tuning coil
53
connected in series, and a coil
75
in the secondary tuning circuit
51
d
indicates the high-band tuning coil
58
and the low-band tuning coil
59
connected in series.
The coupling coil
64
connected between the ground and the connection point of the coil
74
in the primary tuning circuit
51
c
and the coil
75
in the secondary tuning circuit
51
d
determines the coupling state between the primary tuning circuit
51
c
and the secondary tuning circuit
51
d
at a low band. The inductance thereof is specified in advance such that the double-tuned circuit
51
has a predetermined transfer characteristic. Therefore, the tuning coil
74
in the primary tuning circuit
51
c
and the tuning coil
75
in the secondary tuning circuit
51
d
do not cause direct inductive coupling.
On the other hand, to switch the double-tuned circuit
51
shown in
FIG. 5
to a state in which a high-band television signal is received, a band switching voltage of, for example, 5 V is applied to the high-band switching terminal
69
. A voltage is applied to the switch diode
54
and the switch diode
60
in the forward directions and both switch diodes
54
and
60
become a continuity state.
As a result, the connection point of the high-band tuning coil
52
and the low-band tuning coil
53
in the primary tuning circuit
51
c,
and the connection point of the high-band tuning coil
58
and the low-band tuning coil
59
in the secondary tuning circuit
51
d
are ground in high frequencies. Since the low-band tuning coil
53
in the primary tuning circuit
51
c,
the low-band tuning coil
59
in the secondary tuning circuit
51
d,
or the coupling coil
64
does not function, the double-tuned circuit
51
shown in
FIG. 5
works as an equivalent circuit shown in FIG.
7
.
Therefore, the high-band tuning coil
52
in the primary tuning circuit
51
c
and the high-band tuning coil
58
in the secondary tuning circuit
51
d
are disposed at a predetermined distance to be coupled to obtain a coupling state between the primary tuning circuit
51
c
and the secondary tuning circuit
51
d.
In the conventional double-tuned circuit
51
, however, the positional relationship (layout) between the high-band tuning coil
52
in the primary tuning circuit
51
c
and the high-band tuning coil
58
in the secondary tuning circuit
51
d
needs to be specified in advance to obtain an appropriate coupling state between the primary tuning circuit
51
c
and the secondary tuning circuit
51
d
in a state in which a high-band television signal is received. Therefore, the degree of freedom in designing the layout is limited.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to increase the degree of freedom in designing the layout of the high-band tuning coil by independently providing coupling coils for a state in which a low-band television signal is received and for a state in which a high-band television signal is received.
The foregoing object is achieved according to the present invention

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