Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a tuner apparatus. More
specifically, the present inven~ion relates to a tuner apparatus
employing a voltage controlled variable reactance device such as
a voltage con-trolled variable capacitance diode for use in a
television receiver, an FM receiver and the like. In West
Germany, the FTZ (Fermnelde Technisches Zentralamt) has made the
following proposal in the draft of January, 1979. More
specifically, in West Germany the frequency range for the tele-
vision broadcasting has been determined such that the Band I
10 covers 47MHz to 68MHz, the Band III covers 174MHz to 230MHz and
the Bands IV and V cover 470MHz to 790MHz. A deviation allowance
outside the frequency range at each of the upper and lower limits
of the frequency range of each band has been determined in
principle as 300 kHz. By way of an exception, as for the receiv-
ing frequency band of 47 MHz to 870 MHz, a deviation allowance
outside the frequency range has been determined as 7 MHz at the
lower limit of the frequency range and as 8 MHz at the upper
limit o-- the frequency range.
An attempt has also been made to make similar restriction
in the case of the Canadian television broadcasting. According
to the Canadian television broadcasting standard, the VHF low
band comprises Channel Nos. 2 to 6, the VHF high band compris-s
Channel Nos. 7 to 13, and the UHF band comprises Channel Nos. 14
to 84. According to the draft of October, 1978 by the Canadian
DOC (Department of Communications) and the further development
thereof, the following restriction has been planed. More
specifically, according to the Canadian television broadcasting
--1--
.39
standard, the channels for the CATV have been allotted in the
region lower than Channel No. 7 and in the region higher than
Channel No. 13. Therefore, a restriction has been planned in
Canadian television receivers such that some of the CATV channels
allotted in the region lower than Channel No. 7 and in the
region higher than Channel No. 13 are made absolutely unreceiv-
able. More specifically, television receivers originally not
designed to receive such CATV broadcasting are sufficient enough
to be capable of surely receiving only the television signal of
Channel Nos. 2 to 6, Nos. 7 to 13, and Nos. 14 to 83 and there-
fore a restriction has been planned to make such receivers in-
capable of receiving a signal in Channels A to I of the CATV
channels in the region lower than Channel No. 7 and a signal in
CATV Channels A to W in the region higher than Channel No. 13.
In making such restriction, however, one channel, i.e. Channel I
in the region immediately lower than Channel No. 7 and one
channel, i.e. Channel ~ in the region immediately higher than
Channel No. 13 have been considered as allowable for a deviation
range.
As described in the foregoing, in some countries there have
been tendencies to a strict restriction to a deviation downward
or upward from the original receiving frequency band, for the
purpose of effective utilization of an electric wave and
observance of communication secrecy.
Under the circumstances, in order to cope with such strict
frequency restriction, one might think of restriction of the
upper and lower limits of the above described tuning voltage.
--2--
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However, generally tuner apparatuses involve a diversi-
fied tuning voltage versus receiving fre~uency character-
istic depending on each set. For example, as far as
the upper ends of the UHF band and the VHF high band
are concerned, it could happen that a given tuner in-
volves a relation in which t,he tuning voltage of thehigh end channel of the UHF band is lower than the tuning
voltage of the high end channel of the VHF high band.
The same is also true in a relation between the tuning
voltage of the low end channel of the VHF high band and
the tuning voltage of the low end channel of the VHF
low band. Thus, in the case where there is a diversi-
fication in a relation of which is higher,and lower be-
tween the tuning voltage of the high end channel and/or
the tuning voltage of the low end channel among the res-
pective bands, the above described restriction to thetuning voltage requires that: such tuning voltage re-
stricting means be provided for each of the receiving
bands. Accordingly, it becomes necessary to restrict
both Ihe upper and lower limits of the tuning voltage
for each of the receiving bands. Provision of such tun-
ing voltage restricting means for each of the receiving
bands, however, makes compli.cated the structure of the
tuner apparatus and makes e~pensive the cost of the
tuner apparatus.
According to the i.nvention there is provi.ded
a tuner apparatus which comprises a tuning circuit in-
cluding a voltage controllecl variable reactance means,
~ ~8~
the tuning circuit having a plurality of receiving bands,
tuning voltage generating means, coupled to the tuning
circuit for generating a tuning voltage and applying
the tuning voltage to the voltage controlled variable
reactance means, wherein the tuning voltage is within
a single predetermined range for all of the plurality
of receiving bands; and at least one of (a) and (b),
where (a) comprises upper li:mit restricting means, coupled
to the tuning voltage generating means, for restricting
the upper limit of the tuning voltage applied ~o the
voltage controlled variable reactance means; maximum
receivable frequency defining means including trimmer
loops and resonance conductors provided in the tuning
circuit for determining the maximum receivable frequency
of the tuning circuit with respect to the tuning voltage
applied to the tuning circuit for all of the receiving
bands such that the tuning voltage for the upper limit
of the receiving band which must be restricted is greater
than or equal to the tuning voltage for the upper limit
of the others of the receiving bands, thereby defining
the maximum receivable frequency of the plurality of
receiving bands; and ~b) comprises lower limit restric-
ing means including trimmer capacitors and resonance
conductors provided in the tuning voltage generating
means, for restricting the lower limit of the tuning
voltage applied to the voltage controlled variable re-
actance means; and minimum receivable frequency defining
means coupled to the tuning circuit for determining the
~' ~
39
minimum receivable frequency of the tuning c~rcuit with
respect to the tuning voltagP applied to the tuning
circuit for all of the receiving bands such that the
tuning voltage for the lower limit of the receiving band
which must be restricted is less than or equal to the
lower limit of the tuning voltage for the others of the
receiving bands, thereby defining the minimum receivable
frequency of the plurality of receiving bands.
In one embodiment of the present invention
the tuning voltage generating means may be of a voltage
synthesizer type, The tuning voltage generating means
of such voltage synthesizer type comprises a switching
device being rendered conductive responsive to a pulse
signal and a smoothing circuit for smoo-thing the output
of the switching device. In such embodiment, impedance
means is interposed in a current path of the switching
device for allowing a predetermined residual voltage
to be applied to the smoothing circuit even when the
switching device is rendered conductive. According to
the above described preferred embodiment, the lower limit
of the tuning voltage can be restricted with a very simple
structure.
In another embodiment of the present inven-
tion, the tuning voltage generating means comprises po-
tentiometers each provided for each channel, so thatthe source voltage is divided by each of the potentio-
meters to provide the tuning voltage corresponding to
each channel. A diode is connected to a sliding contact
~L.18(~
each potentiometer, the anode of each diode being commonly
connected to the input electrode of a transistor. The second
electrode of the transistor is connected to the voltage source
and the third electrode of the transistor is connected to the
tuning voltage withdrawing terminal. A predetermined output
resistor is connected between the tuning voltage withdrawing
terminal and the ground potential. The source voltage is
stabilized by a constant voltage diode, whereby the upper limit
of the tuning voltage obtained from the tuning voltage with-
drawing terminal is restricted. A predetermined resistor isconnected between the voltage source and the output resistor,
so that normally a constant current is caused to 10w through the
output resistor, whereby the lower limit of the voltage across
the output resistor or the output voltage (the tuning voltage)
obtained from the tuning voltage withdrawing terminal is restrict-
ed to a predetermined value. In the above described preferred
embodiment, the lower limit of the tuning voltage can be restrict-
ed only by adding one resistor to a conventional tuning voltage
generating means, while the upper limit of the tuning voltage
can also be restricted only by using a constant voltage diode,
with the result that a tuner apparatus adapted for restriction
of the tuning frequency can be provided with a very simple
structure.
Objects, features, aspects and advantages of the present
invention will become more apparent from the following detailed
description of exemplory emb~idments of the present invention
when taken in conjunction with the accompanying drawings.
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139
Fig. 1 is a block diagram showing one example of a
conventional television tuner wherein the present invention
can be advantageously employed;
Fig. 2 is a graph showing a relation between the tuning
voltage and the frequency (the receiving channels) of a
television system proposed in West Germany for explaining the
background of the present invention;
Fig. 3 is a graph showing the distribution of channels
of the television broadcasting standard in Canada;
Fig. 4 is a graph showing a relation between the tuning
voltage and the frequency (the receiving channels) for explain-
ing the principle of the present invention;
Figs. SA and 5B are schematic diagrams of a television
tuner taken as an example of the present invention, wherein Fig.
5A shows a UHF portion and Fig. 5B shows a VHF portion;
Fig. 6 is an outline view for explaining one embodiment
of the present invention;
Figs. 7, 8 and 9 are outline views for depicting different
embodiments of the present invention wherein different types of
channel selecting apparatuses, i.e. variable tuning voltage
generat~ing means are employed, respectively;
Figs. 10 to 13 are schematic diagrams of major portions of
further preferred embodiments of the tuning voltage generating
Cl rcult;
Figs. 14 and 15 are schematic diagrams showing examples
of impedance means;
Fig. 16 is a graph showing waveforms for explaining the
operation of the embodiments shown in Figs. 10 and 13;
~3
\
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Fig. 17 is a schematic diagram of a major portion of a
further preferred embodiment of the tuning voltage generating
circuit;
Fig. 18 is a graph depicting the operation of the Fig. 7
embodiment, wherein the abscissa indicates the number of
rotations of a fine tuning knob and the ordinate indicates the
tuning voltage;
Fig. 19 is a block diagram showing still a further embodi-
ment of the present invention; and
Fig. 20 is a graph showing a relation between the tuning
frequency (the receiving channel) and the tuning voltage forexplaining the Fig. 19 embodiment.
Fig. 1 is a block diagram showing one example of a tuner
apparatus of a television receiver of the superheterodyne system
wherein the present invention can be advantageously employed.
Since such television tuner is well-known to those skilled in
the art, only those portions associated with the present
invention will be briefly described. A tuner 100 comprises two
input terminals 117 and 119. The input terminal 117 is connected
to receive a television signal received by a VIIF antenna 1. The
input terminal 119 is connected to receive a television signal
received by a UHF antenna 2. The received signal from the VHF
antenna input terminal 117 is applied to a VHF high frequency
amplifier 103 and is amplified and the amplified output there-
25 from is applied to a VHF mixer 105.The tuner 100 also comprises a
VHF local oscillator 107. The oscillation output o' the VH~ local
oscillator 107 is applied to a VHF mixer 105. Accordingly, the
VHF mixer 105 serves to mix the V~IF television signal with the
; ~
oscillation output from the VHF local oscillator 107, thereby
to convert the VHF television signal into a VHF intermediate
frequency signal. On the other hand, the received signal
applied to the UHF antenna input terminal 119 is applied to a
UHF high frequency amplifier 109 and is amplified and the
amplified output therefrom is applied to a UHF mixer 111. The
tuner 100 also comprises a UHF local oscillator 113 and the
oscillation output therefrom is applied to a UHF mixer 111.
Accordingly, the UHF mixer 111 serves to mix the UHF television
signal with the oscillation output from the UHF local
oscillator 113, thereby to convert the UHF television signal
into a UHF intermediate frequency signal. The ou~put from the
UHF mixer 111, i.e. the UHF intermediate frequency signal is
amplified by a UHF intermediate frequency amplifier 115 and is
applied to a VHF mixer 105. On the occasion of reception of
the UHF signal, the VHF high frequency amplifier 103 and the VHF
local oscillator 107 are disabled, while the VHF mixer 105 is
kept enabled. Accordingly, on the occasion of reception of the
UHF signal, the VHF mixer 105 serves as a UHF intermediate
frequency amplifier for amplifying the UHF intermediate frequency
signal. Meanwhile, on the occasion of reception of the VHF
signal, those circuits 109, 111, 113 and 115 associated with the
UHF signal are all disabled, while only those circuits 103, 105
and 107 associated with the VHF signal are enabled. The VHF
intermediate frequency signal or the UHF intermediate frequency
signal obtained from the VHF mixer 105 is applied from the output
terminal 121 to the subsequent stage intermediate frequency
circuit, not shown. These circuits 103 to 115 are housed within
~ ~81)~.3~
a shield member 101 of such as a metallic casing or frame. There-
fore, any undesired radiation from those clrcuits housed within
the shield member 101 toward other wireless equipment is effect-
ively prevented, while any undesired electric wave or interfer-
ence electric wave from other wireless equipment to thosecircuits is also effectively prevented. The above described
antenna input terminals 117 and 119 and the intermediate
frequency output terminal 121 are formed at predetermined posit-
ions of the shleld member 101, while these terminals are electric-
ally isolated from the shield member 101.
The VHF high frequency amplifier 103, the VHF localoscillator 107, the U~F high frequency amplifier 109 and the
UHF local oscillator 113 each comprise a tuning circuit, not
shown, for varying the tuning frequency for selection of a
desired channel within a desired receiving frequency band. Each
of these tuning circuits comprises a voltage controlled variable
reactance device such as a voltage controlled variable
capacitance diode. To that end, the -tuner 100 housed in the
shield member 101 is also provided with a tuning voltage input
20 terminal 123, as electrically isolated from the shield member 101,
for supply of the tuning voltage Vt~ The tuning voltage Vt from
the terminal 123 is applied to the associated circuits 103, 107,
109 and 113. The shield member 101, i.e. the tuner 100, further
comprises a test point (TP~ terminal 127, as electrically
isolated from the shield member 101, for supply of the output
from the tuner 100 to alignment equipment, not shown,for align-
ment of the output waveform on the occasion of adjustment of the
tuner 100. In general, the VHF band comprises a VHF low band
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~.
~1~1)13~
(the first band) of a relatively low fre~uency range and a VHF
high band (the second band) of a relatively high frequency range.
On the other hand, the UHE band may be considered as the third
band of a frequency range higher than -that of the VHF high band.
Accordingly, the tuner 100 further comprises terminals 129, 131
and 133, as electrically isolated from the shield member 101~
for supply of voltage signals for selection of these frequency
bands. More specifically, the terminal 129 is aimed to supply
a band selection voltage BL for selection of the VHF low band,
the terminal 131 is aimed to provide a band selection voltage
VH for selection of the V~IF high band, and the terminal 133 is
aimed to provide a band selection voltage BU for selection of
the UHF band. The tuner 100 further comprises a terminal 125
for supply of an automatic gain control (AGC) voltage obtained
from the intermediate frequency circuit, not shown, and a
terminal 135 for supply of an automatic fine tuning (AFT)
voltage, both electrically isolated from the shield member 101.
Each of the terminals 129, 131 and 133 is supplied with the band
selection voltage BL, BH or BU of +15V, when the corresponding
receiving frequency band is to be selected. Each of the tuning
circuits included in the tuner 100 is structured to be responsive
to the given band selection voltage BL, BH or BU to change the
circuit constant or circuit connection of the tuning scheme so
as to be adaptable to the corresponding frequency band, as well-
known to those skilled in the art.
As described in the foregoing, the tuner 100 employs avoltage controlled variable capacitance diode as a tuning
element of each of the tuning circuits. In such conventional
--11--
3~
tuner, the tuning voltage Vt being supplied to the variable
capacitance diode was determined in accordance with a given
condition. In the following, therefore, such determination of
the tuning voltage will be described with reference to an
example of a television tuner in ~est Germany, as shown in Fig.
2. Referring to Fig. 2, the abscissa indicates the tuning
voltage and the ordinate indicates the respective channels in
the VHF low band, the VHF high band and the UHF band. In West
Germany, for example, the VHF low band (the first band) covers
channels E2 to E4, while the VHF high band (the second band)
covers channels E5 to E12. The UHF band (the third band)
covers channels E21 to E69. Such tuner has been designed such
that the lower limit frequency of the VHF low band may be
determined so that channel E2 can be received when the tuning
voltage Vt is 3V, for example. However, a television tuner must
be capable of surely selecting channel E2 even in any situation
and even in the worst condition. More specifically, in consider-
ation of a frequency drift due to a source voltage fluctuation,
an ambient temperature variation, a time dependent change and so
on, a frequency deviation due to a mechanical shock, and the like,
the television tuner must be designed to be capable of surely
receiving channel E2 even in the worst condition which seldom
occurs. Therefore, accordingly to a conventional approach, the
tuner was desi~ned such that the tuning voltage Vt which is as
low as 0.2 to 0.3V, for example, and is sufficiently lower than
the above described 3V, may be supplied from the channel select-
ing apparatus, not shown. As a result, with such a conventional
television tuner, the receivable frequency range extended over
L35~
the lower region beyond the necessary receivable frequency range
shown by the dotted line in Fig. 2 in a normal use condition.
For example t a conventional tuner was adapted such that in the
case of the VHF low band shown by the curve L in Fig. 2 the
signal can be received even when the frequenry becomes lower
than that of channel E2 by a frequency difference corresponding
to approximately one channel. A conventional tuner was further
adapted such that in the case of the VHF high band shown by the
curve H the signal can be received even when the frequency
becomes lower than the lower limit channel E5 by a frequency
difference corresponding to approximately three channels. A
conventional tuner was further adapted such that in the case of
the UHF band shown by the curve U the signal can be received
even when the frequency becomes lower than the lower limit
channel E21 by a frequency difference corresponding to approx-
imately ten channels. A conventional television tuner was
further adapted such that as for the upper limit of the respect-
ive bands as well the signal of any desired receiving frequency
band can be surely received with a sufficient margin in full
consideration of any imaginable worst condition.
However, for the purpose of effective utilization of the
electric wave and observance of secrecy of communication, in
some countries there have been tendencies to restriction of
reception by a tuner beyond the receivable frequency range in a
2~ television receiver, for example. More specifically, some
countries have shown tendencies to legislation to restrict the
receivable frequency range by a tuner in a television receiver
-13-
39
at the upper and lower limits of the respective receiving
frequency bands as shown in Fig. 2, with a margin frequency
corresponding to one channel, respectively.
Referring to Fig. 2, such diversification is shown by
ranges denoted as A to F in conjunction with the points a to f
of the lower and upper ends of the respective receiving bands
U, H and L. For example, in restricting the upper limit of the
receivable frequency of the UHF band so as to be conformable
to the FTZ standard, assuming such a relation as shown in Fig. 2
the upper limit of the tuning voltage ~rom the tuning voltage
generating means, not shown, cannot be uniformly set to the
minimum value, say 20 V,of a diversification range E. This can
be substantiated by an assumption that when the high end channel
of the VHF high band is receivable with the tuning voltage being
say 22 V then such high end channel becomes unreceivable by such
setting.
Examples of the present invention will be des~ribed by
taking several examples wherein the present invention is embodied
in a television tuner; however, it is pointed out that the
present invention can be applied not only to a television tuner
but also to an FM receiver and the like.
Fig. 4 is a graph in which the abscissa indicates the
tuning voltage and the ordinate indicates the tuning frequencies
tthe receiving channels) in the respective receiving bands of
the VHF low band, the VHF high band and the UHF band. In the
embodiment shown, it is assumed that the tuning voltage for
tuning to the upper limit frequency (the maximum receivable
frequency) of the UHF band is V6, the tuning voltage for tuning
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to the upper limit frequency in the VHF high band is V5, and the
tuning voltage for tuning to the upper limit frequency in the VHF
low band is V4. Further, it is assumed that the tuning voltage
for tuning to the lower limit frequency (the minimum receivable
frequency) in the VHF low band is Vl, the tuning voltage for
tuning to the lower limit frequency of the VHF high band is V2,
and the tuning voltage for tuning to the lower limit frequency
in the UHF band is V3. The relation of these voltages Vl to V6
is selected to be V6> V5, V6> V4, Vl~ V2 and Vl ~V3. More
specifically, adjustment
3~
is made such -that in the UHF band when the tuning voltage V6
is applied the high end channel E6g can be assuredly received
bu-t the channel E69 + 8 ~iHz cannot be exceeded as the upper
limit value. On the other hand, adjustment is made such
that in the VHF low band in the case of the tuniny voltage
Vl the low end channel E2 can be assuredly received but the
channel E2 - 7 MHz is e~ceeded. The lower limit ~requency
of the UHF band is restricted by the tuning voltage V3 and
the upper limit and the lower limit frequencies of the VHF
high band are restricted by the tuning voltages V5 and V2,
and the upper limit frequehcy of the VHF low band is restricted
by the tuning voltage V4. Alternatively, the tuning voltages
Vl to V6 may be set in a relation of V6 = V5 = V4 and Vl =
V2 = ~3. It is pointed out that the examples of the tuning
voltage generating means to be described subsequently are
structured to generate the tuning voltage in common to the
respective receiving bands in accordance with the last
mentioned relation.
Figs. 5A and 5B are schematic diagrams of an example of
a television tuner in accordance with the present invention.
Fig. 5A shows a UHF associated por~ion and Fig. SB shows a
VHF associated portion. These UHF and VHF portions are
implemented in a unitary tuner housed within a single shield
memb~r 101; however, these portions are shown as housed in
25 separate shield members 101 in Figs. 5A and 5B for simplicity
,~
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of illustration.
Referring to Fig. 5A, first the UHF portion will be
described. The shield member 101 is partitioned into suitable
cells by suitable shielf plates. A UHF high frequency
S amplifier 109 including an input tuning circui-t is provided
within the first cell. An inter stage tuning circuit 110 is
housed within the next cell and is disposed between the UHE'
high frequency ampllfier 109 and a UHF mixer 111. A mixer
diode Dm constituting a UHF mixer 111 is disposed within the
same cell as the interstage tuning circuit 110. The UHF
high frequency amplifier 109 comprises an input tuning
circuit, which comprises a resonance circui-t including a
first voltage controlled variable capacitance diode Dl and a
first resonance conductor Ll. The resonance circuit serves
to select a desired one of the broadcasting signals in the
UHF band fed from the UHF antenna input terminal 119. An
amplifying transistor Tl amplifies the selected UHF television
signal. The amplified television signal is applied to a
primary resonance circuit o~ the interstage tuning circuit
110. The primary resonance circuit comprises a second
voltage controlled variable capacitance diode D2 and a
second resonance conductor L2. The primary resonance circuit
is electromagnetically coupled to the secondary resonance
circuit. The secondary resonance circuit comprises a third
voltage controlled variable capacitance diode D3 and a third
~1
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resonance conductor L3. Accordingly, the television signal
amplified by the transistor Tl is fed through the coupling
between the primary resonance circuit and the secondary
resonance circuit to the anode of the mixer diode Dm. On
the other hand, the UHF local oscillator 113 comprises an
oscillation transistor T2, a fourth voltage controlled
variable capacitance diode D4 and a fourth resonance conductor
L4. The oscillation output from the UHF local oscillator
113 is applied to the cathode of the mixer diode Dm. Accordingly,
the mixer diode Dm serves to mix the two fed frequency
signals, thereby to provide an UHF intermediate frequency
signal, which is applied to an UElF intermediate frequency
amplifier 115. The UHF intermediate frequency amplifier 115
comprises an amplifying transistor T3, the output of which
is applied to a VHF mixer 105 shown in Fig. 5B. The tuning
voltage Vt obtained from the tuning voltage terminal 123 of
the tuner 100 is commonly applied to the first, second,
third and fourth voltage controlled variable capacitance
diodes Dl, D2, D3 and D4 conskituting the respective resonance
circuits. The tuning voltage Vt is also applied to the VHF
portion shown in Fig. 5B. An UHF band selecting voltage BU
obtained from a terminal 133 is applied to the transistors
Tl, T2 and T3. Accordingly, these transistors Tl to T3 are
enabled upon application of the voltage BU from the terminal
25 133.
. ~ .
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Frequency adjustment of the UHF portion will be made in
the following manner. For the purpose of ajusting the
tuning frequency in the UHF portion, first the voltage BU is
applied to the terminal 133. As the tuning voltage V-t being
applied to the terminal 123, the tuning voltage V6 shown in
Fig. 4, for example, is determined. Then the local oscillation
frequency of the UHF local oscillator 113 is adjusted. The
frequency adjustment is made with the trimmer loop TL4
coupled to the fourth resonance conductor L4. The local
oscillation frequency at that time is adjusted such that the
same is lower than the normal local oscillation frequency of
the high end channel of the UHF band by several MHz and the
same may be the frequency corresponding to one channel at
the highest. Then the tuning voltage Vt obtained from the
lS terminal 123 is applied as the voltage V3. The trimmer
capacitor TC4 included in the UHF local oscillator 113 is
adjusted at that time, so that the normal local oscillation
frequency of the low end channel of the UHF band is attained
with the voltage V3. After the frequenc~ of the UHP local
oscillator 113 i9 thus adjusted, the input tuning circuit of
the UHF high frequency amplifier 109 and the interstage
tuning circuit 110 are adjusted so that the ou-tput of -the
UHF mixer 111 may be the normal intermediate frequency
signal. More specifically, if and when the -tuning voltage
Vt is the upper limit voltage restricted by the zener diode
_ ~ _
1~
~8(~39
ZD, trimmer capacitors Tc5, Tc6 and Tc7 and the corresponding
trimmer loops TLl, TL2 and TL3 are adjusted so that the
normal intermediate frequency may be attained at the highest
receiving frequency of the UHF band. Then the trimmer
capacitors Tcl, Tc2 and Tc3 are adjusted so tha-t when the
tuning voltage Vt is brought to the lower limit the normal
intermediate frequency may be attained at the receiving
lowest frequency of the UHF band. In general, adjustment of
the diference between the local oscillation frequency and
the input tuing and interstage tuning resonance frequencies
to be the normal intermediate frequency is referred to as
tracking adjustment. Such tracking adiustment should be
made not only in the UHF high end and the UHF low end band
but also in the region therebetween. Such tracking adjustment
in the intermediate region is made by the trimmer loops TLl,
TL2 and TL3. The characteristic of the receiving frequency
with respect to the tuning voltage Vt is thus determined as
shown by the curve U in Fig. 2, for example. Then the
characteristic of the receiving frequency in the UHF band
with respect to the tuning voltage Vt is determined through
adjustment of the UHF portion.
Referring to Fig. 5B, the VHF portion of the tuner 100
is shown. The VHF portion comprises a VHF high frequency
amplifier 103. The V~IF high frequency amplifier 103 comprises
an input tuning circuit, which receives a VHF -television
B
2--Z --
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signal from the VHF antenna input terminal 117. The input
tuning circuit comprised inductors L5 and L6, and a voltaye
controlled variable capacitance diode D5 cooperating wi-th
these inductors for determining the tuning frequency of the
resonance circuit. Furthermore, the VHF high frequency
ampllfier 103 comprises an ampli~ying transistor T4, the
output of which is applied to the primary resonance circuit
constituting an interstage tuning circuit 104. The primary
resonance circuit comprises a voltage controlled variable
capacitance diode D6 and inductors L7 and L8 which are
coupled to a secondary resonance circuit. The secondary
resonance circuit comprises inductors L9 and L10 and a
voltage controlled variable capacitance diode D7. Accordingly,
the VHF television signal selected by the input tuning
circuit of the VHF high frequency amplifier 103 is amplified
by the transistor T4 and is applied through the primary
resonance circuit and the secondary resonance circuit of the
interstage tuning circuit 104 to a t.ransistor T5 constituting
a VHF mixer 105. On the other hand, the VHF local oscillator
107 comprises an oscillation transistor T6, a voltage controlled
variable capacitance diode D8, and inductors Lll and L12.
Switching diodes SDll, SD12, SD13 and SD14 are coupled to
the input tuning circuit included in the VHF high frequency
amplifier 103, the primary resonance circuit and the secondary
resonance circuit of the interstage tuning circuit 104 and
.~.--~
, ~ _ ,~ _
the VHF local oscillator 107. A V~IF low band selecting
voltage BL is applied from a terminal 129 to the cathodes of
these switching diodes SDll to SD14, and a VHF high band
selecting voltage BH is applied from a terminal 131 to the
anodes of these switching diodes SDll to SD14. Accordingly,
when the VHF high band is to be selected, the inductors L6,
L8, L10 and L12 are removed from the respective resonance
circuits, because the corresponding switching diodes S~ll,
SD12, SD13 and SD14 are rendered conductive by the band
selecting voltage BH. Meanwhile, the transistor T5 of the
VHF mizer 105 is supplied with the operation voltage not
only on the occasion of VHF reception but also on the occasion
of UHF reception, whereby the transistor T5 serves as a UHF
intermediate frequency amplifier on the occasion of UHF
reception. The otput of the VHF mixer 105 is applied to the
terminal 121 as the intermediate frequency signal. Meanwhile,
a terminal 127 serving as a test point is connected to -the
output of the secondary resonance circuit of the interstage
tuning circuit 104. A terminal 135 for an automatic fine
tuning voltage is provided in association with the VHF local
oscillator 107 and the UHF local oscillator 113, although
not shown. A terminal 125 for an automatic gain control
volgate is provided to supply an automatic gain control
voltage to the transistor Tl shown in Fig. 5A and the transistor
T4 shown in Fig. 5B.
,, -- ,;~ --
~-æ
Frequency adjustment of the V~IF portion is made after
the frequency adjustment of the UHF portion described previously
is made. In the frequency adjustmen-t of the VHF portion,
first the VHF high band is adjusted, which is followed by
adjustment of the VHF low band. ~lowever, the sequential
order of adjustment of the VHF high band and the VHF low
band may be reversed. First the voltage BH is applied to
the terminal 131, whereby the VHF portion is placed in the
VHF high band mode. Then, as the tuning voltage Vt the
voltage V5 shown in Fig. 4, for example, is applied. The
inductor Lll included in the VHF local oscillator 107 is
adjusted, whereby adjustment is made for tuning to the high
end channel E12 of the VHF high band. Thereaf-ter, as the
tuning voltage Vt the voltage V2 of Fig. 4 is applied and
the inductor Lll is adjusted, so that the frequency is tuned
to the low end channel of the VHF high band. Thereafter the
inductors L5, L7 and L9 are adjusted to perform tracking
adjustment.
Then, for the purpose of adjusting the VHF low band,
the voltage BL is applied to the terminal 129. Then as the
tuning voltage Vt the voltages Vl and V4 shown in Fig. 4 are
applied and the inductance L12 is adjusted~ so that the
frequency may be tuned to the low end channel and the high
end channel of the VHF low band. At that time, adjustment
i~ made such that the lower limit of the local oscillation
B
~3
~8~39
frequency may be lower than the normal local oscillation
frequency of the low end channel E2 by several MHz, say
within 7 ~Iz. On the other hand, tracking adjustment is
made by adjusting the inductors L6, L8 and L10. After the
above described adjustment is made in each of the frequency
bands, adjustment of the VHF high band is made again to
correct an influence caused by adjustment of the VHF low
band.
As a matter of practice, the above described frequency
adjustment is determined in consideration of a temperature
drift of the locaL oscillation frequency of -the UHF local
oscillator 113 and the VHF local oscillator 107, a -time
dependent drift, and pull-in frequency and a holding range
of an automatic frequency tuning operation.
In making frequency adjustment, consideration is given
to a temperature drift and a time dependent drift of the
local oscillation frequency of the UHF local oscillator 113
and a pùll-in frequency of an automatic fine tuning operation.
More specifically, the temperature drift of the local oscillation
frequency of the UHF local oscillator 113 in a tuner presently
available is within +1.5 MHz in the temperature variation
range of -10C to +60C and a time dependent dif-t of the
local oscillation frequency of the UH~ local oscillator is
within +2 MUIz, while the pull-in range of the automatic fine
tuning operation is +1.5 MHz. Accordingly, the highest
~80~39
receivable frequency in the UHF band rnay be determined to a
frequency higher than the highest channel (E69) by 2 MHz.
By selecting the highest frequency in the above described
manner, the total sum (+7 MHz) of the temperature drift
(+1.5 MHz) + the time dependent drift (+2 MHz) + the pull-in
range of the automatic fine tuing operation (+1.5 MHz) and
the above described 2 MHz would be a frequency range which
involves a possibility of deviation toward a higher frequency
exceeding the above described highest receiving channel.
Since the frequency range allowed for the upper limit of the
UHF channel in West Germany is 8 ~z, it follows that there
is still a margin of 1 MHz even in the worst situation in
consideration of the above described various drifts and the
pull-in range and accordingly ~he tuner thus implemented
still suffices to meet the requirement of the FTZ standard.
Even when the temperature drift and the time dependent drift
have exerted an influence upon a lower frequency, such
drifts would be -1.5 MHz and -2 MHz, respectively. Since
adjustment has been made to a frequency higher than the
normal frequency of the highest channel by 2 M~lz in making
the above described adjustment, no problem is caused in
reception of the highest channel.
On the other hand, the minimum receivable frequency of
the VHF low band may be determined to thè frequency lower
than the minimum channel (E2) by 1.5 MHz. For example, a
. . .~
r-
~s
~1~013~
temperature drift of the oscillation frequency of the V~F
local oscillator 107 is within +0.4 MHz in a temperature
variation range of -10C to -~60C and a time dependen-t drif-t
is within +1.0 MHz. A pull-in range of the automatic frequency
tuning is +1.5 MHz. Accordingly, in consideration of the
above described minimum receivable frequency, the total (-
4.4 MHz) of the temperature drift (-0.4 MHz) + the time
dependent drift (-1 MHz) + the pull-in range of the automatic
fine tuning (-1.5 MHz) + (-1.5 MHz) is a frequency range in
which the frequency could drift in the lower direction to be
lower than the lowest receiving channel. Since the frequency
range allowed for the lower limit of the VHF low band in
West Germany is 7 MHz, there is still a margin of 2.6 MHz
even in consideration of the above described drifts and the
pull-in range and even in the worst situation. Accordingly,
the inventive tuner thus implemented can fully meet the
requirement of the FTZ Standard. Even when the frequency is
changed to be hiyher due to the temperature drift and the
time dependent drift, such drifts are +0.4 MHz and +1.5 MHz
and~ since adiustment has been made such that the frequency
may be lower by 1.5 ~Hz than the normal frequency of -the
lowest channel, no influence is exerted upon reception of
such lowest channel (-the low end channel).
Fig. 6 is a schematic diagram showing one embodiment of
the present invention. Referring to Fig. 6, the tuning
B
~1~3(3~3S~
voltage generating circuit 200 is connected to provide the
tuning voltage Vt to the tuner 100 described previously. It
is pointed out that the circuit path including the -terrninal
137 and zener diode ZD shown by the dotted line in Fig. 6
S may be applied to the Fig. 8 embodiment to be described
subsequently. Now several preferred embodiments of the
tuning voltage generating circuit 200 shown in Fig. 6 will
be described in the following with reference to Figs. 7 to
18.
Fig. 7 is a schematic diagram of a major por-tion of a
preferred embodiment of the tuning voltage generating
circuit. In the Fig. 7 embodiment the zener diode ZD is
used to restrict the upper limit of the tuning voltage Vt.
The voltage being applied through the resistor R from the
voltage source Vcc is made to a constan~ voltage by means of
the zener diode ZD. The constant voltage thus attained by
the zener diode is adjusted by means of a semifixed resistor
VRl so as to attain the tuning voltage V6 only for tuning to
the high end channel E69 of the UHF band. On the other
hand, the tuning voltage Vl tunable to the low end channel
E2 of the VHE` low band is applied through the variable
resistor VR2 from the terminal 129 of the VHF low band
setting voltage BL of the tuner 100. Po-tentiome-ters or
variable resistors Pl to Pn are provided in parallel between
the tuning voltage supplying terminal 123 and the output of
~`
~ . ~ , .
the semifixed resistor VRl so as to provide a predetermined
tuning voltage in associa-tion with the respective channels.
sy closing one of the switches SWl to SWn connected to the
respective sliding contacts o~-the potentiometers, the
voltage of the potentiometer corresponding to the closed
switch is applied from the terminal 123 through the diode
D10 to the tuner 100. Even if any of the potentiometers i5
adjusted to provide a voltage lower than the lower limit Vl
of the tuning voltage, since the voltage set by the variable
resistor VR2 is higher than that voltage, the diode D10 is
reverse biased to be turned off, so that the voltage lower
than the voltage Vt is prevented from being supplied to the
tuner 100. In such a case, the lower limit voltage Vl
adjusted by the variable resistor VR2 is applied through the
diode D9 to the tuner 100. According to the Fig. 7 embodiment,
the upper limit oE the tuning voltage Vt is restricted as
the voltage V6 and the lower limit of the tuning voltage Vt
is restricted as the voltage Vl. Accordingly, as previously
shown in Fig. 4, the maximum receivable frequency of the UHF
band and the minimum receivable frequency of the VHF low
band are restricted. Meanwhile, the tuning voltage generating
circuit 200 has been adapted to provide the voltage of V6 =
V5 = V4, and Vl = V2 = V3 to the -tuner 100 and the same
applies to various embodiments to be described suhsequently
as well as -the Fig. 7 embodiment. By doing so, an advan-tage
~ . ,
3~
is brought about that the restricting means of the tuniny
voltage may be adap-ted to determine only the upper limit and
the lower limit. Accordingly, it is not necessary to
determine the upper limit and the lower limit of the tuning
voltage Vt individually for each of the receivlng bands.
Therefore, a circuit configuration may be very simple.
Furthermore~ the Fig. 7 embodiment is adapted such that
the voltage BL for setting the VHF low band is used to
obtain the voltage Vl. The purpose is that, since it is
sufficient to define the minimum receivable frequency of at
least the VHF low band with the voltage Vl, utilization of
the voltage BL makes simple the structure. Accordingly, the
band setting voltage BL need not be utilized for the purpose
of obtaining the voltage Vl, as done in the Fig. 7 embodiment,
and the voltage may be withdrawn from any other arbitrary
portion. Therefore, the tuner may be structured such that
the ~oltage Vl is always obtained in any of the receiving
bands. Meanwhile, the semifixed resistor VRl is provided
for the purpose of fine tuning and ls not necessarily required.
Fig. 8 shows a modification of the Fig. 7 embodiment.
The Fig. 8 embodiment employs the path shown by the dotted
line in Fig. 6, ~or the purpose of setting the upper limi-t
voltage V6 of the tuning voltage Vt. The variable resistor
VR2 for the purpose of setting the lower limit voltage Vl is
provided inside the tuner 100.
- ~r-
~q
~01~
Fig. 9 shows a schematic diagram of a major portion of
a further preferred embodiment of the tuning voltage generating
circuit. Fig. 9 is of a simplest example. More specifically,
the embodiment shown employs a jumper wire 201. In -the
embodiment, first the jumper wire 201 is removed and the
voltage of +30 V, for example, is applied to the junction
204 from an external voltage source, not shown. The minimum
voltage Vl of the tuning voltage Vt is determined by adjusting
the variable semifixed resistor VR3 connected between one
end of each of the potentiometers Pl to Pn and the ground
potential. Then the jumper wire 201 is connected between
the terminals 202 and 203 and a uoltmeter, not shown, is
connected to the junction 204. By looking at the voltmeter,
the semifixed resistor VRl is adjusted so that the indication
of the voltmeter may be +30 V, whereby the maximum voltage
V6 of the tuning voltage Vt is determined. The jumper wire
201 is kept connected between the terminals 202 and 203 even
when the tuner is built in a television receiver.
Figs. 10 to 13 show further preferred embodiments of
the tuning voltage generating circuit and Figs. 14 and 15
are views showing one example of the impedance means for use
in the above described embodiments. The embodiments shown
in Figs. 10 to 13 are the tuning voltage generating circuit
of the so-called voltage synthesizer type. A channel selec-ting
apparatus of a voltag~ synthesizer type is disclosed in, for
_ ~ _
~,0
~18~
example, United States Patent No. 3,968,440 issued July 6,
1976 to George John Ehni, III.
Referring to the Fig. 10 embodiment, a pulse signal as
shown in Fig. 16~ is applied from the -terminal 207. The
pulse width of such pulse slgnal is controlled by a control
circuit, not shown, so that the same may be that associated
with the designated channel. The conduction period of the
switching transistor Tr included in the switching circui-t
205 is controlled as a function of the pulse signal applied
from the terminal 207 and during the conduction period of
the transistor Tr a current flows from the voltage source of
+120 V through the resistor R3 to the transistor Tr. Accordingly,
a voltage of the magnitude associated with the conduction
period of the transistor Tr, i.e. the pulse width of the
pulse signal thus applied appears at the junction or at the
input of the smoothing circuit 206. The voltage appearing
at the junction 208 is smoothed ~y the smoothing circuit 206
and is applied to the terminal 123 of the tuner 100 as the
tuning voltage Vt. ~In the embodiment shown the impedance
means Z is interposed in the current path of the switching
circuit 205, i.e, the emitter circuit of the transistor Tr.
Furthermore, the zener diode ZD is connected between the
junction ~08 and the ground. The zener diode ZD serves to
restrict the input voltage of the smoothing circuit 206 and
thus the upper limit of the tuning voltage Vt to the constant
B
V~39
voltage (V6). On the other hand, the impedance means Z
serves to restrict the lower limit (Vl) of the tuning voltage
Vt. More specifically, the voltage V on the occasion of
conduction of the transistor Tr, at the junction 208 becomes
V = Z X (120 - VCE)/(R3 + z) + VCE by means of the impedance
means Z, where the voltage VcE ls a saturated voltage between the
collector and emitter electrodes on the occasion of conduction
of the transistor ~r. On the other hand, in the absence of
the impedance means Z, the voltage V is equal to the voltage
VcE and accordingly the voltage V in the presence of the
impedance means Z is higher than that in the absence of the
impedance means Z by the value (120 - VcE) x R3Z~ z Accordingly,
the lower limit of the tuning voltage Vt is restricted to
the constant voltage Vl (say 1.25 V). Thus, according to
the Fig. 10 embodiment, the impedance means Z is in-terposed
so that a given residual voltage may appear at the output
point 208 even when the switching element is rendered
conductive and therefore the lower limit of the tuning
voltage Vt is prevented from undesirably becoming lower than
the voltage Vl.
Fig. 11 shows an example in which the impedance means Z
is not connected between the emitter electrode of the switching
transistor Tr and ground and the junction 208 but between the
collector electrode of the switching transistor Tr and the junc-
tion 208. In this case as well, a residual impedance exists in
-- 34 --
~>''
B
~8(~
the current path on the occasion of conduction of the switchingtransistor Tr and accordingly a given residual voltage
arises at the junction 208. Accordingly, the tuning voltage
Vt is restricted so tha-t the same may not become lower than
the constant voltage V1.
Figs. 12 and 13 show examples wherein a field effect
transistor is used as the switching transistor Tr, while the
remaining portions thereof are the same as those in the
emboidments shown in Figs. 10 and 11.
Fig. 14 shows one example of the impedance means Z.
The Fig. 14 embodiment employs a resistor as the impedance
means Z.
Fig. 15 shows another example of the impedance means Z.
The Fig. 15 embodiment employs a series connection of a
plurality of diodes Dll to Dln as the impedance means Z.
Meanwhile, the connecting direction of the series connection
of the diodes is selected such that ~he current flowing
direction in the current path of the switching element 205
may be the forward direction of the series connection. In
employing the series connection of the diodes Dll to Dln as
the impedance means Z, the voltage V' of the junction 20~
becomes V' = VcE + nVak, where Vak is a forward drop vol-taye
for each of the diodes Dl to Dn. Accordinyly, in the case
where the impedance means Z is implemented by a series
connection of the diodes, the voltage V is increased by the
3~
~ ~ ~(3~39
total nVak of -the drop vol-tages of the diodes and accordingly
the tuning voltage Vt ls restricted to the constant voltage
Vl.
Now representing the o-thers the operation of the Fig.
10 embodiment in combination with the Fig. 14 embodiment
will be described in more detail with reference to Fig. 16.
A pulse signal (Fig-. 16A) having the pulse width corresponding
to the designated channel is applied from the pulse signal
circuit, not shown, to the terminal 207. The transis-tor Tr
is rendered conductive as a function of the pulse width of
the pulse signal. At that time, the voltage at the junction
208 remains at the voltage Vl described previously even on
the occasion of conduction of the transistor Tr by means of
the impedance means Z, i~e. the resistor, without becoming
O(VcE). Accordingly, the voltage appearing at the junction
208 as shown in ~ig. 16B is smoothed by the smoothing circuit
206 to a direct current without including a ripple component
as shown in Figs. 16C and 16D. ~he pulse width of the pulse
signal at that time is ex~remely large and, even if the
transistor Tr had been rendered conductive for a substantial
portion of the period, the lower limit is restricted by the
resistor connected to the emitter electrode of the transistor
Tr and the direct current voltage, i.e. the tuning voltage
Vt will neither become lower than the constant volta~e Vl
25 (say 1 25 V).
, ~
Fig. 17 is a schematic diacJram of a major portion of a
further embodimen-t of the tuning voltage generating circuit
and Fig. 18 is a graph explaining the operation thereof. A
characteristic feature of the embodiment shown is that the
resistor R9 is connected between the collector and emitter
electrodes of the transistor Trl.
In operation, a channel selecting signal is applied to
any one of the input terminals al to an of -the channel
setting portion 209 implemented by an integrated circuit,
for example. Then one of transistors Ql to Qn corresponding
to the channel selecting signal is rendered conductive and a
current flows from the voltage source Vcc to the corresponding
one of the variable resistors VRll to VRln corresponding to
the transistor now in conduction. Accordingly, one of
diodes D21 to D2n connected to the sliding contact of -the
variable resistor corresponding to the transistor now in
conduction is also rendered conductive and the voltage being
set by the corresponding variable resistor is applied to the
base electrode of the transistor Trl. The transistor Trl
has the base electrode commonly connected to the anodes of
the respective diodes D21 to D2n, as described previously,
and also connected through the resistors R8 and R10 to the
voltage source Vcc and has -the emitter electrode connec-ted
to the output resistor R7 and the ripple removing capacitor
C4. Meanwhile, the diode D~l is connectecl in parallel in a
~ 35
~8~:~13~3
reverse direction between the base and emitter electrodes of
the transistor Trl. The diode D~l is rendered conductive in
the case where the tuning voltage Vt from the output terminal
210 changes from a high value to a low value, thereby to
help a discharge of the capacitor C4. The transistor Trl is
provided to compensate for a variation of the tuning voltage
Vt caused by a variation of the drop voltages by the diodes
D21 to D2n depedning on the temperature and such compensation
is achieved by a PN junction interposed in the reverse
direction with respect to these diodes D21 to D2n. The
zener diode ZD is aimed to restrict the upper limit of the
tuning voltage Vt at the predetermined value V6. The input
terminals 211, 212 and 213 are the input terminals for the
band setting voltages BL, BH and BU, respectively, and are
selectively connected to the terminals bl to bn of the
channel setting portion 209 by means of the selecting switches
Sl to Sn. Meanwhile, the diodes D31 to D3n are aimed -to
connect the terminals 211 to 213 and the terminals bl to bn
only with respec-t to the selected channel and not to connect
them with respect to the other channels.
In the embodiment shown the resistor R9 is connected
between the voltage source line 214 and the output resistor
R7. Accordingly a constant current normally flows in the
output resistor R7 through the resistor R9 irrespective of
conduction or non-conduction of the transistor Trl. Therefore,
13~
a constant drop voltage is obtained due to such constant
current at both ends of the output resistor R7, i.e. at the
output terminal 210. Accordingly, by properly selecting the
ratio of the resistors R9 and R7, the lower limit of the
tuning voltage Vt can be restricted to the constant voltage
Vl. For example, if and when the voltage of the voltage
source line 214 is 32 V, the resistor R7 is selected to be
100 kQ and the resistor R9 is selected to be approximately
2.2 MQ. Then, the lower limit of the tuning voltage Vt is
restricted with the constant voltage Vl (say 1.25 V).
More specifically, referring to the Fig. 17 diagram,
the voltage obtained from any one of the variable resistors
VRll to VRln designated as a function of the selecting
signal is applied to the base electrode of the transis-tor
Trl. Then the magnitude of the voltage and thus the conductivity
of the transistor Trl is determined and accordingly the
capacitor C4 is charged, while the voltage at the output
terminal 210, i.e. the tuning voltage Vt increases to become
the voltage value of the channel associated with the channel
selecting signal. If and when the tuning voltage Vt is
switched from that of a higher channel to that of a lower
channel, the electric charge stored in the capacitor C4 is
discharged through the diode D41 and accordingly the voltage
at the output terminal 210 decreases, as described previously.
However, in such a case, even if the voltage lower than the
3'1
~ ~013~
set voltage Vl (say 1.25 V is applied from any one of the
variable resistors VRl to VRn to the base electrode of the
transistor Trl, no forward bias is applied between the base
and emitter electrodes of the transistor Trl and therefore
the transistor Trl is not rendered conductive, with the
result that the tuning voltage Vt lower than the voltage Vt
(say 1.25 V~ will not appear at the output terminal 210.
On the other hand, the upper limit V6 of the tuning
voltage Vt is restricted to a value lower than the voltage
of the voltage source line 204 by the drop voltage between
the collector and emitter electrodes of the transistor Trl.
Fig. 18 shows a relation oE the tuning voltage Vt with
respect to the number of rotations of a fine tuning knob,
not shown, in the case where the sliding contact of any one
lS of the variable resistors VRl to VRln is moved by means of
the fine tuning knob in the Fig. 17 embodiment. As seen
from Fig. 18, even in case where the numer of rotations of
the fine tuning knob, not shown, is extremely smallr i.e.
the voltage obtained from the variable resistor is small,
the minimum value of the tuning voltage Vt will not become
lower than the voltage Vl (say 1.25 V). In the absence of
the resistor R9, when the number of rotations of the fine
tuning knob is sm~ll, as shown by the dotted line in Fig.
18, the tuning voltage Vt would become approximately 0 V,
with the result -that the minimum receivable frequency of the
;`' '`' -- 4er _
V~IF low band could be off the restrlction by the FTZ standard,
for example. ~nder the circumstances, the embodiments shown
in Figs. 7 to 18 are adapted such that the lower limit of
the tuning voltage Vt is restricted with the constant voltage
Vl. On the other hand, a conventional tuner does not comprise
such restriction of the lower limit of the tuning voltage Vt
and accordingly the voltage could become lower than 0.3 V.
~ccordingly, in order to restrict the minimum receivable
frequency in the VHF low band so as to satisfy the requirements
of the FTZ standard, it is necessary to design a tuner such
that the minimum channel (E2) can be received with the
tuning voltage o~ approximately 0.5 V. However, reception
performance of the channel E2 is degraded as compared with
that of the other channels. The reason is that the lower
the tuning voltage being applied the worse the quality
factor of the voltage controlled variable capacitance diode.
By contrast, by restricting the lower limit of the tuning
voltage Vt to the constant voltage Vl (say 1.25 V), as done
in the present invention, it is sufficient to design the
2Q tuner such that the channel E2 can be received with such a
relatively high tuning voltage and performance of the tuner
is enhanced.
It goes without saying that the embodiments shown in
Figs. 7 to 18 can be employed not only for adap-tation -to the
FTZ standard but also for adaptation to other standards such
.~ ~9
1~8V~ 39
as the DOC standard.
Fig. 19 is a modification of a television tuner of a
double conversion type. A television tuner of such double
conversion type is disclosed in, for example, United States
Patent No. 3,639,840 issued February 1, 1972 to Jacob Shekel
et al. In such television tuner of a double conversion
type, it is necessary to change the tuning frequencyin a
wide range from the VHF low band to the UHF band by the use
of a single variable local oscillator. However, in achieving
such a wide range of the -tuning frequency using only a
single voltage controlled variable local oscillator, a
difficulty is caused in the local oscillator 504 and in
order to prevent such difficulty two voltage controlled
variable local oscillators 504V and 504U have been employed
in the Fig. 19 embodiment. More specifically, the variable
local oscillator 504V is used for the VHF band and is adapted
to be variable over the frequency range from 2,000 to 2,350
MHz. On the other hand, the voltage controlled variable
local oscillator 504U is provided for the UHF band and is
adapted to be variable over the frequency range from 2,500
to 2,900 MHz, for example. The oscillation outputs of these
two variable local oscillators 504V and 504U are applied to
the contacts 511V and 511U of a band selsecting swi-tch 511.
The switch 511 is switched responsive to the band selecting
voltages BV and BU being applied to band selecting voltaye
~î8{~3~
terminals 531 and 533 provided in the tuner 500. More
specifically, if and when the VHF band selecting voltage BV
for selecting the VHF band is applied from the channel
selecting apparatus, not shown, to the -terminal 531, the
switch 511 is turned to the contact 511V On the other
hand, if and when the UHF band selecting voltage BU is
applied from the terminal 533, the switch Sll is turned to
the contact 511u. Accordingly, when the VHF band is to be
selected, the oscillation output from the variable local
oscillator 504V is applied to the first mixer 503. Conversely,
if the UHF band is to be selected, the oscillation output
from the variable local oscillator 504U is applied to the
first mixer.
Fig. 20 is a graph showing a relation between the
broadcasting channels (frequencies) and the tuning voltage
in the case where the Fig. 10 embodiment is employed as a
television tuner for the West Germany standard. Even in
case of scuh modified embodiment of double conversion type,
it is sufficient to supply the tuning voltage Vt having the
upper limit and the lower limit restricted as shown in Figs.
7 to 18 to the terminal 523. By doing so, the maximum
receivable frequency in the UHF band and the minimum receivable
frequency in the UHF band are restricted with -the constant
tuning voltages V6 and Vl, respectively.
Although the present invention has been described and
- ~3' -
~V~3~
illustrated in detall, it is clearly unders-tood that the
same is by way of illustratiorl and example only and is not
to be taken by way of limitation, the spirit and scope of
the present invention being limi-ted only by the terms of the
S appended claims.
~f~
