JPH0678174A - Automatic time constant adjusting circuit - Google Patents

Abstract

PURPOSE:To simply change the time constant of a filter to be adjusted without being influenced by any broadcasting system. CONSTITUTION:A saw tooth wave generating circuit 1 generates a saw tooth wave in which the frequency of a leading waveform depends upon a frequency clock generated based upon horizontal osciilation frequency fH and amplitude is changed depending upon a bias current. A single output 90 deg. phase comparator 10 compares a signal outputted from the circuit 1 and sliced by a slicing circuit 9 with a clock phase and the detection output of the comparator 10 is smoothed by a capacitor 6. The voltage of the capacitor terminal is monitored to control the current of a bias current source 4 for the circuit 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a time constant automatic adjustment circuit for automatically adjusting the delay amount of an IC built-in delay line used in, for example, an image quality correction system of a TV signal processing IC.

[0002]

2. Description of the Related Art In recent years, due to demands for cost reduction and compatibility with any broadcasting system, circuits such as circuits that were considered difficult to embed before were highly integrated. , Built-in has advanced. Among them, the filter is one of the circuits that has been incorporated for a long time. However, when integrated into an IC, variations in time constant due to variations in internal resistance and capacitance of the IC cause a serious problem. Therefore, it is necessary to automatically adjust the time constant.

FIG. 3 shows a conventional time constant automatic adjustment circuit used for the above purpose, which will be described with reference to FIG. 4 showing the waveforms of the respective parts of FIG. A sawtooth wave generation circuit 1 that generates a sawtooth wave depending on a certain reference clock signal a outputs a signal b,
The output is input to the peak hold circuit 2 in the next stage. The peak hold circuit 2 holds the peak voltage of the sawtooth wave as shown by the signal c. The output signal c of the peak hold circuit 2 is input to the current output amplifier 3. In the current output amplifier 3, the difference between the reference voltage Vref and the output voltage of the peak hold circuit 2 is calculated, and the smoothing capacitance 6
To generate a predetermined DC voltage. The voltage generated in the capacitor 6 is monitored, the current value of the bias current source 4 of the sawtooth wave generation circuit 1 is changed, and the output signal b of the sawtooth wave generation circuit 1 is changed. By configuring the above loop,
A predetermined capacitance end voltage Vc shown by d is generated in the capacitance 6.

In this automatic adjustment circuit, the output voltage depending on the time constant of the sawtooth wave generation circuit 1 is compared with the reference voltage Vref. If the time constant is longer than a desired value, the sawtooth wave generation circuit 1
The peak of the output voltage becomes low, the current output amplifier 3 discharges a current, and the voltage generated across the capacitor 6 rises. Along with this, the current of the bias current source 4 increases, so that the peak voltage of the output of the sawtooth wave generating circuit 1 increases, and as a result, the capacitance end voltage is constant so that the peak voltage becomes the same voltage as the reference voltage Vref. It becomes a value. On the contrary, when the time constant of the sawtooth wave generation circuit is short, the process opposite to the above is followed and the capacitance end voltage Vc becomes a certain value.

Therefore, the capacitance end voltage Vc is a voltage at which the time constant of the sawtooth wave generating circuit 1 becomes a desired time constant. Here, by setting the time constant of the sawtooth wave generation circuit 1 and the time constant of the adjusted filter in another signal system at a certain ratio, the time constant of the adjusted filter 5 can be set at the capacitance end voltage Vc. It becomes possible to adjust.

FIG. 5 shows a circuit example of another conventional automatic time constant adjustment. In this example, the reference signal in the monitor filter 7 is input and the monitor filter 7
The 90 ° phase comparator 8 compares the phase of the output of 1 with the reference signal to generate a predetermined capacitance end voltage Vc in the capacitor 6 as in FIG. By monitoring this voltage Vc,
As in the case of FIG. 3, the current of the bias current source 4 is changed. Since the monitor filter 7 is configured such that the time constant is changed by the current of the bias current source 4,
As a result, the capacitance end voltage Vc is generated so that the output phase of the monitor filter 7 becomes 90 ° with the phase of the reference signal, and the time constant of the monitor filter 7 changes.

Therefore, if the difference between the phase determined by the desired filter time constant and the phase of the reference signal is set to 90 °, the capacitance end voltage Vc will generate a voltage having the desired time constant. Becomes Here, in the case of FIG. 5, since the time constant of the monitor filter 7 and the time constant of the adjusted filter 5 are the same, the time constant of the adjusted filter 5 is automatically adjusted to the desired time constant by the capacitance end voltage Vc. It will be possible.

By the way, when the time constant of the filter is adjusted by using the automatic adjustment circuit, generally, the time constant of the filter 5 to be adjusted cannot be directly changed from the outside. The reason is that if the time constant can be changed from the outside, the automatic adjustment circuit is unnecessary. Further, in order to adjust from the outside, the number of IC pins is increased. Increasing the number of pins leads to higher costs.

From the above, the user can use the time constant of the IC built-in filter only with the time constant determined by the circuit designer, or the time constant can be adjusted or changed from the outside by increasing the number of IC pins. Either settle down or settle down. For the above reasons, it is a conflicting requirement to adjust and change the time constant from the outside while adjusting the time constant of the built-in filter using the automatic time constant adjustment.

FIG. 5 is an example of satisfying this requirement. By adding a current offset to the smoothing capacitance end so that the operating range of the adjustment voltage Vc can be widened,
It is possible to change to a desired time constant from the outside, and automatic adjustment works to match the time constant changed from the outside. for that reason,
For example, there is an advantage that a circuit for removing the fluctuation of the time constant due to the temperature drift is unnecessary.

However, in the circuit shown in FIG. 5, the reference signal is 3.58 MHz which is the same as the burst signal of Japanese color broadcasting.
Since the CW (continuous wave) signal of
In the case of the signal processing IC, since the CW signal changes depending on the difference in broadcasting system, there is a drawback that it does not work properly as an automatic adjustment circuit.

Further, in the circuit shown in FIG. 3, in order to obtain the capacitance end voltage Vc required for adjusting the time constant,
The gain of the feedback loop composed of the sawtooth wave generation circuit 1, the peak hold circuit 2, the current output amplifier 3, and the bias current source 4 in FIG. 3 must be made extremely large. Because
This is because if the gain of the loop is small, the convergence point for determining the time constant, that is, the voltage at the smoothing capacitance end cannot be determined and adjustment becomes impossible.

Further, since the loop gain is increased, the operating range of the adjustment voltage Vc becomes very narrow as a result, as shown in FIG.
As described above, the time constant cannot be changed by applying a voltage to both ends of the capacitor 6.

[0014]

Of the above-mentioned conventional time constant automatic adjustment circuits, the one shown in FIG. 5 uses the CW signal as the reference signal used for automatic adjustment, so the time constant can be changed from the outside. However, it cannot be used as an automatic adjustment circuit used in a signal processing IC compatible with the whole world. Further, in the circuit shown in FIG. 3, the reference signal, that is, the clock CK can be arbitrarily set, but the time constant cannot be changed from the outside.

It is an object of the present invention to provide a time constant automatic adjustment circuit which operates stably regardless of any broadcasting system and can adjust / change the time constant from the outside.

[0016]

The present invention provides a sawtooth wave whose rising waveform frequency depends on a clock of a frequency generated based on the horizontal oscillation frequency fH and whose amplitude changes depending on a bias current. A sawtooth wave generating circuit to generate, a slice circuit to slice the output of this sawtooth wave generating circuit, a phase comparator for comparing the phases of the clock and the output of the slice circuit, and the detection output of the phase comparator is smoothed by a capacitance. , And a bias current source of the sawtooth wave generation circuit that changes the amount of current by monitoring the voltage at the capacitance end.

[0017]

The sawtooth wave output generated by the sawtooth wave generating circuit by the above means is sliced by the slicing circuit, the phase comparator compares the phase with the horizontal oscillation frequency multiplied by the constant, and the output of the phase comparator is obtained. A voltage is generated in the smoothing capacitor connected to and the voltage is monitored, and the sawtooth amplitude changes due to the current flowing in the bias current source that changes the current, resulting in a desired time constant. A voltage is generated at the capacity end.

[0018]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention. The same parts as those in FIGS. 3 and 5 are designated by the same reference numerals. 2 is a signal waveform of each part of FIG.

In FIG. 1, 32 fH is used as the reference signal input to the sawtooth wave generation circuit 1 and the single output 90 ° phase comparator 10. The output of the sawtooth wave generation circuit 1 is
This is the signal shown in the signal B synchronized with 32 fH of the signal A in FIG. The signal B is sliced to a certain level by the slice circuit 9 and output. This waveform becomes the signal C in FIG. The signal of C and the reference signal of 32 fH are input to the single output 90 ° phase comparator 10. The detection output of the single output 90 ° phase comparator 10 becomes the signal D of FIG. 2 before being smoothed by the capacitor 6.

Here, it is assumed that the time constant of the sawtooth wave generating circuit 1 is long. The output of the sawtooth wave generating circuit 1 at this time is as shown by the signal B ′ in FIG. As a result, the detection output of the single output 90 ° phase comparator 10 becomes as shown by the signal D ′ in FIG.
The signals B, C, and D in FIG. 2 are obtained when the time constant of the sawtooth wave generation circuit 1 is a desired time constant, and as described above, the signals B ′ and D ′ are the desired time constants. In the longer case. As can be seen by comparing the signals D and D ′ in FIG.
The areas of α and β are different for the signal D ′. That is, if the α side is in the direction of discharging the capacitor 6, it is obvious that the capacitor end voltage Vc becomes higher than the value at the desired time constant.

The bias current source 4 for changing the flowing current by monitoring the capacitance end voltage Vc is a capacitance end voltage Vc.
Since the current increases as the value of becomes higher, the output of the sawtooth wave generating circuit 1 changes in the direction of increasing. By constructing such a control loop, finally, the signals B,
The waveform is the same as the desired time constant shown in C and D. When the time constant of the sawtooth wave generation circuit 1 is shorter than the desired time constant,
By the reverse operation, the signal waveforms shown in B, C, and D finally become the same as the desired time constant. By such an operation, automatic adjustment with a desired time constant is performed.

Further, since the time constant of the sawtooth wave generating circuit 1 is constituted by a certain ratio with the time constant of the adjusted filter 5, the time constant of the adjusted filter 5 is automatically adjusted by the capacitance end voltage Vc. Will be done. Further, from the signal waveforms of C and D in FIG. 2, the circuit configuration is, so to speak, a configuration of a pulse delay circuit and a single output type 90 ° phase comparator, so that the automatic adjustment Vc is in a wide linear voltage range. Can be set.

For this reason, if the current source withstands the current offset through the resistor at the point D in FIG. 1, the DC voltage at the point D rises, and as a result, the time constant shifts to the higher side. Since the adjustment loop does not break and has the external offset and the detection waveform of D in FIG. 2 is obtained, the time constant can be changed from the outside, and the automatic adjustment circuit works so as to match the time constant. The reference signal is 32
Since fH is used, any broadcasting system can be supported.

[0024]

As described above, the time constant automatic adjustment circuit of the present invention can easily change the time constant of the filter to be adjusted regardless of any broadcasting system.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each part of FIG.

FIG. 3 is a circuit configuration diagram showing a conventional time constant automatic adjustment circuit.

FIG. 4 is a signal waveform diagram of each part of FIG.

FIG. 5 is a circuit configuration diagram showing another conventional time constant automatic adjustment circuit.

[Explanation of symbols]

1 ... Sawtooth wave generation circuit, 4 ... Bias current source, 5 ... Adjusted filter, 6 ... Capacitance, 9 ... Slice circuit, 10 ... Single output 90 ° phase comparison circuit.

Claims (3)

[Claims] 1. A sawtooth wave generation circuit for generating a sawtooth wave whose rising waveform frequency depends on a clock having a frequency generated based on a horizontal oscillation frequency fH and whose amplitude changes depending on a bias current, A slice circuit that slices the output of the sawtooth wave generation circuit, a phase comparator that compares the phases of the clock and the slice circuit output, a detection output of the phase comparator is smoothed by a capacitance, and the capacitance end voltage is monitored. And a bias current source for the sawtooth wave generating circuit for changing the amount of current by means of the sawtooth wave generating circuit. 2. A bias current source is used to change the output amplitude of the sawtooth wave generation circuit, and a loop is formed so that the capacitance end voltage becomes a voltage at which the phase difference between the clock and slice circuit outputs becomes 90 °. 2. The time constant automatic adjustment circuit according to claim 1, wherein the time constant of a filter in another signal system is adjusted to a desired time constant by the terminal voltage. 3. The time constant of the filter can be adjusted in any broadcasting system by constructing an adjustment loop with a sawtooth wave generation circuit, a single type phase comparison circuit and a pulse delay, and the convergence point of the loop can be adjusted from the outside. The time constant automatic adjustment circuit according to claim 1, wherein