Circuit for generating a reference signal

Details Circuit for generating a reference signal Circuit for generating a reference signal

1999-05-11

2001-06-12

Kostak, Victor R. (Department: 2611)

Television

Synchronization

Phase locking regenerated subcarrier to color burst

C348S549000

Reexamination Certificate

active

06246440

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for generating a reference signal synchronized with an input burst signal. In particular, the present invention relates to a circuit for generating various reference signals of different frequencies that are adapted to different television systems, that is, different reference subcarriers for chrominance demodulation.
2. Description of the Prior Art
There are currently three well-known television systems, namely NTSC, PAL and SECAM, and especially NTSC and PAL are widely used around the world. Moreover, in addition to PAL, there are the PAL-N and the PAL-M formats. The frequencies of the reference subcarriers used for NTSC, PAL-N, PAL-M and PAL are 3.579545 MHz, 3.582056 MHz, 3.575611 MHz and 4.433619 MHz, respectively.
Conventional circuits for generating reference carriers often use automatic phase control (APC) with a phase-locked loop (PLL), as shown in FIG.
8
.
In
FIG. 8
, a voltage-controlled oscillator (VCXO)
1
outputs an oscillation signal with the stabilized frequency f
ref
that is substantially determined by the resonance frequency (for example 3.58 MHz) of an external quartz oscillator
2
. For simplicity, the oscillation signal with the frequency f
ref
is sometimes referred to as the “oscillation signal f
ref
” in the following. The same is true for other signals as well, where sometimes the frequency is used as the name of the signal, especially when there is no need to distinguish between the two.
A burst gate circuit
3
extracts a burst signal f
SC
from an input chrominance signal C
in
. The oscillation signal f
ref
and the burst signal f
SC
are provided as input to a phase comparator
4
a
of an APC circuit
4
, whose output is a phase comparison signal &Dgr;f
SC
. After the phase comparison signal &Dgr;f
SC
is smoothed with a low-pass filter (LPF)
4
b
, it is returned as a feedback signal to the control terminal of the voltage-controlled oscillator
1
.
The voltage-controlled oscillator
1
is controlled by this voltage feedback signal. As a result, the oscillation signal f
ref
of the voltage-controlled oscillator
1
is controlled in a manner that f
ref
has the same frequency and the same phase as the burst signal f
SC
. Thus, the continuous oscillation signal f
ref
, which has the same stability as the burst signal f
ref
can be used as the reference subcarrier signal f
ref
.
Since such an APC-type circuit for generating a reference carrier uses neither coils nor capacitors, it has not only mechanical stability but also stability toward temperature changes, and is suitable for integrated circuits. When a quartz is used, the free-running frequency of the oscillation signal of such a circuit for generating a reference carrier shows only small variations, and the range over which the frequency can be varied with the voltage control terminal is about ±500 Hz. Thus, a continuous oscillation signal with the same frequency and the same phase as the burst signal can be obtained instantly, when the chrominance signal is input.
However, because of the previously mentioned small variable frequency range, one separate quartz oscillator has to be provided for each frequency to generate several reference carriers with different frequencies.
On the other hand, if a voltage-controlled oscillator with a broad variable range (for example an RC oscillator) is used for generating several reference carriers with different frequencies, the time from free running until an oscillation signal with the same frequency and the same phase as the burst signal can be obtained—that is, until the APC locks—is long. Moreover, so-called “side-locking” can occur, where a frequency near the targeted oscillation frequency, but one line frequency away, is locked.
In order to avoid side-locking, variations in the oscillation frequency have to be suppressed and the variable frequency range has to be narrowed, but then it becomes difficult to generate several reference carriers with different frequencies. Moreover, it is difficult to include RC oscillators on integrated circuits, because in ICs, the variations in the resistance R and the capacitance C are large.
To generate a plurality of reference carriers with different frequencies adapted to a plurality of television systems as described above, Publication of Examined Japanese Patent Application No. Sho 63-28521 discloses the use of several different external quartz oscillators, one for each frequency, which can be switched accordingly.
Furthermore, when the passband of the lowpass filter
4
b
in the APC circuit using the PLL is narrow, the time until synchronization is long, resulting in deterioration of stability. Publication of Unexamined Japanese Patent Application No. Sho 63-82084 discloses a configuration for increasing the precision of the oscillation frequency while increasing the stability by widening the passband of the lowpass filter. This conventional example uses one quartz oscillator each for two voltage-controlled oscillator circuits, and increases the stability of the oscillation frequency by using a dual loop PLL.
However, in these conventional technologies the circuits become rather complicated, resulting in the problem of increasing size and costs. Thus, it has been desired strongly to solve those problem, particularly in a field of appliances where the demand for lower costs and miniaturization is high.
SUMMARY OF THE INVENTION
The present invention has been developed in consideration of these shortcomings of the prior art. It is an object of the present invention to provide a comparatively simple, compact and inexpensive circuit for generating a plurality of stable reference signals from a plurality of burst signals with different frequencies, selectively.
A circuit of a first configuration according to the present invention includes, as main elements, a first and a second voltage-controlled oscillators having a control terminal, respectively, with oscillation frequencies being variable in response to signals supplied to the control terminals. In addition to the main elements, the circuit further includes: a first frequency divider for dividing a frequency of an output signal of the first voltage-controlled oscillator by j; a second frequency divider for dividing a frequency of an output signal of the second voltage-controlled oscillator by k; a first phase comparator for comparing a phase of an output signal of the first frequency divider with a phase of an output signal of the second frequency divider; a first lowpass filter for smoothing an output signal of the first phase comparator and supplying the smoothed signal to a control terminal of the second voltage-controlled oscillator so as to form a feedback control path; a second phase comparator for comparing a phase of an output signal of the second voltage-controlled oscillator with a phase of the burst signal; and a second lowpass filter for smoothing an output signal of the second phase comparator and supplying the smoothed signal to a control terminal of the first voltage-controlled oscillator so as to form a feedback control path. The reference signal is obtained from the output signal of the second voltage-controlled oscillator.
It is preferable that the first configuration of the circuit further comprises means for activating the feedback from the second phase comparator to the first voltage-controlled oscillator only when the burst signal is active, and means for activating the feedback from the first phase comparator to the second voltage-controlled oscillator only when the burst signal is not active. This stabilizes the control of the two feedback loops.
It is preferable that the feedback from the first phase comparator to the second voltage-controlled oscillator is only active for a predetermined period, and a duty ratio which is a ratio of the predetermined period to the other period is variable. Thereby the predetermined time can be long enough (i.e. the duty ratio is high) until the PLL locks and the variable frequency range of the voltage-

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