Television – Receiver circuitry – Tuning
Reexamination Certificate
1999-12-02
2002-10-22
Lee, Michael H. (Department: 2714)
Television
Receiver circuitry
Tuning
C348S733000, C455S182300
Reexamination Certificate
active
06469750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic fine-tuning control voltage generating circuit. More particularly, the present invention relates to an automatic fine-tuning control voltage generating circuit having a level shift circuit for shifting, toward a lower level, a non-signal-time level of an automatic fine-tuning control voltage output from an integrated circuit for an intermediate frequency to which an intermediate video frequency signal which is filtered by a SAW (surface acoustic wave) filter is supplied.
2. Description of the Related Art
Hitherto, in a television receiver, etc., comprising an electronic tuner, when the electronic tuner is automatically preset-tuned or automatically search-tuned to a broadcasting signal of a predetermined receiving frequency by using a phase-locked loop (PLL), an automatic fine-tuning control (AFT) voltage which is output from an integrated circuit for an intermediate frequency (IF IC) is used. Also, when drift occurs at a tuning point due to ambient temperature variations, etc., an automatic fine-tuning control (AFT) voltage is also used in a similar manner.
FIGS. 5A
,
5
B, and
5
C are illustrations showing an example of a signal waveform, a voltage waveform, and a frequency variation state in each section of an electronic tuner when preset automatic tuning is performed in the electronic tuner.
FIG. 5A
shows a horizontal synchronization signal waveform detected from a received signal.
FIG. 5B
shows an AFT voice waveform output from an IF IC.
FIG. 5C
shows an oscillation frequency variation state of a voltage controlled oscillator (VCO), within a PLL, for generating a local oscillation signal.
When preset automatic tuning is performed in the electronic tuner, initially, as shown in
FIG. 5C
, sweeping of an oscillation frequency is performed by sequentially varying, in a stepped manner, the oscillation frequency of a VCO, within a PLL, for generating a local oscillation signal in units of two steps, for example, in units of (62.5×2) kHz, from lower frequencies (or higher frequencies) every fixed time period A (for example, 200 ms). During the sweeping of this oscillation frequency, as shown in
FIG. 5A
, a horizontal synchronization signal is detected from a received signal. Next, when the horizontal synchronization signal is detected, as shown in
FIG. 5B
, the lower level of an S-shaped curve of an AFT voltage output from the IF IC is detected while this horizontal synchronization signal is being monitored. When the lower level of the S-shaped curve of the AFT voltage is detected, the sweeping of the oscillation frequency of the VCO for generating a local oscillation signal is varied, in a direction opposite to that thus far, by one step, for example, by 62.5 kHz. Then, as shown in
FIG. 5C
, the oscillation frequency of the VCO for generating a local oscillation signal obtained at this time is taken to represent the preset automatic tuning point, and this is stored in a memory, terminating the preset automatic tuning.
While the electronic tuner is operating, if the horizontal synchronization signal cannot be detected, or if the higher level of the S-shaped curve of the AFT voltage which is output from the IF IC exceeds an upper-limit value, for example, 3.0 MHz, an operation for detecting the horizontal synchronization signal is performed again in the electronic tuner.
Next,
FIG. 3
is a block diagram showing an example of the construction of a known AFT voltage generating circuit used in an electronic tuner.
FIGS. 4A and 4B
are illustrations showing a signal waveform and a voltage waveform, respectively, of each section in the AFT voltage generating circuit shown in FIG.
3
.
FIG. 4A
is a waveform chart of a filtered intermediate video frequency signal.
FIG. 4B
is a waveform chart of an output AFT voltage.
As shown in
FIG. 3
, the AFT voltage generating circuit comprises a surface-acoustic-wave (SAW) filter
31
, an integrated circuit (IF IC)
32
for an intermediate frequency, a bias setting circuit
33
, an intermediate video frequency signal input terminal
34
, an AFT voltage using circuit (microcomputer)
35
, and a power-supply terminal
36
. The IF IC
32
comprises AFT voltage output transistors
32
1
and
32
2
, and an output buffer resistor
32
3
. The bias setting circuit
33
comprises series voltage-division resistors
33
1
and
33
2
having the same resistance values, and an output buffer resistor
33
3
.
The SAW filter
31
is connected at its input end to the intermediate video frequency signal input terminal
34
and is connected at its output end to the input end of the IF IC
32
. The IF IC
32
is connected at its AFT voltage output end to the input terminal of the bias setting circuit
33
. Inside the IF IC
32
, the AFT voltage output transistors
32
1
and
32
2
are connected in series between the power-supply terminal
36
and a ground, and the output buffer resistor
32
3
is connected between the connection point of the two output transistors
32
1
and
32
2
and the AFT voltage output end of the IF IC
32
. The bias setting circuit
33
is connected at its output end to the input end of the AFT voltage using circuit
35
. Inside the bias setting circuit
33
, the series voltage-division resistors
33
1
and
33
2
are connected between the power-supply terminal
36
and a ground, the connection point of the two resistors
33
1
and
33
2
is connected to the input end, and the output buffer resistor
33
3
is connected between the input end and the output end.
The known AFT voltage generating circuit constructed as described above operates in the following manner.
When an intermediate video frequency signal is input to the intermediate video frequency signal input terminal
34
, the intermediate video frequency signal is filtered by the SAW filter
31
, and a filtered intermediate video frequency signal, such as that shown in
FIG. 4A
, is output from the SAW filter
31
. The filtered intermediate video frequency signal is detected by the IF IC
32
and an AFT voltage is formed thereby, and the AFT voltage is taken out from the connection point of the two output transistors
32
1
and
32
2
through the output buffer resistor
32
3
. The taken-out AFT voltage is supplied to the bias setting circuit
33
whereby, as shown in
FIG. 4B
, the non-signal-time level is set to an intermediate level of a supplied DC voltage of the power-supply terminal
36
, for example, 2.5 V, after which the level is supplied to the using circuit. This AFT voltage is set so that the higher level is the supplied DC voltage of the power-supply terminal
36
, for example, a voltage of 3.5 V to 5 V, and the lower level a voltage of 1.5 V or closer to the ground voltage, 0 V.
In this case, as shown in
FIG. 4A
, the frequency characteristics of the intermediate video frequency signal which is filtered by the SAW filter
31
is such that, since a vestigial sideband system is employed in television broadcasting signals, the frequency band which is higher than the video carrier-wave frequency P is considerably narrower than the frequency band which is lower than the video carrier-wave frequency P. In correspondence with this, as shown in
FIG. 4B
, the S-shaped curve of the AFT voltage taken out from the bias setting circuit
33
reaches a high level for a relatively long period when the frequency of the frequency band which is lower than the video carrier-wave frequency P is swept. When, in contrast, the frequency of the frequency band higher than the video carrier-wave frequency P is swept, the S-shaped curve of the AFT voltage reaches a lower level for a short period.
In the known AFT voltage generating circuit, since the period at the lower level is much shorter than the period at the higher level in the S-shaped curve of the output AFT voltage, when preset automatic tuning is to be performed by sweeping a local oscillation signal frequency in the VCO within the PLL of the electronic tuner, problems arise in that detection of a lower level in which t
Fujita Ryuusuke
Kawamura Akira
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Lee Michael H.
LandOfFree
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