CA1245309A - Low pass filter circuit - Google Patents
Low pass filter circuitInfo
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- CA1245309A CA1245309A CA000548540A CA548540A CA1245309A CA 1245309 A CA1245309 A CA 1245309A CA 000548540 A CA000548540 A CA 000548540A CA 548540 A CA548540 A CA 548540A CA 1245309 A CA1245309 A CA 1245309A
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- pass filter
- voltage
- time constant
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Abstract
ABSTRACT OF THE DISCLOSURE
A tuning control apparatus is provided with a phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter. The low-pass filter includes a series resistor in a signal path for the phase comparator signal and a capacitor connected between the series resistor and ground. The time constant of the filter can be changed in response to the signal through the series resistor by a further resistor series connected with said series resistor, a diode which turns on in response to a predetermined voltage and off in response to a voltage below the predetermined value. The diode is connected across the further resistor for changing the time constant filter.
A tuning control apparatus is provided with a phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter. The low-pass filter includes a series resistor in a signal path for the phase comparator signal and a capacitor connected between the series resistor and ground. The time constant of the filter can be changed in response to the signal through the series resistor by a further resistor series connected with said series resistor, a diode which turns on in response to a predetermined voltage and off in response to a voltage below the predetermined value. The diode is connected across the further resistor for changing the time constant filter.
Description
~L29~53~9 This is a division of Patent Application No.
482,319, filed May 24, 1985.
The present invention relates to a low-pass filter circuit. More specifically, the present invention relates to a low-pass filter circuit for use in a system for controlling a tuning apparatus employing a phase locked loop.
In the accompanying drawings:-Figure 1 is a block diagram of a system for controlling a tuning apparatus employing a phase lockedloop;
Figure 2 is a schematic diagram of a conventional low-pass filter circuit employed in the system shown in Figure l;
Figure 3 is a schematic diagram of one embodiment of the inventive low-pass filter circuit;
Figure 4 is a schematic diagram of another embodiment of the inventive low-pass filter circuit;
Figure 5 is a graph showing a comparison of the response characteristic of the conventional and inventive low-pass filter circuits; and Figure 6 is a schematic diagram of a further embodiment of the inventive low-pass filter circuit.
Figure 1 shows a block diagram of a system for controlling a tuning apparatus employing a phase locked loop. Reerring to Figure 1, the system comprises a receiving antenna 1, a high frequency amplifier 2 coupled to the antenna 1, and a local oscillator 3 coupled to the high frequency amplifier 2. The local oscillator 3 comprises a tuning circuit including a coil and a voltage controlled variable capacitance device 12. Such a voltage controlled variable capacitance device may comprise a voltage controlled variable capacitance diode which is commercially available and referred to as a "Varicap cap".
The voltage controlled variable capacitance device 12 is connected to receive an output of a low-pass filter circuit 4. The input of the low-pass filter circuit 4 is connected from a phase comparator 5. The phase comparator ,~
~... ' 53~9 5 is connected to receive one input from a reference frequency signal terminal 11 and the other input from a programmable frequency divider 7. The programmable frequency divider 7 is connected to receive a frequency siynal to be divided from a prescaler 8 and a frequency division ratio controlling signal from a central processing unit 6. The prescaler 8 comprises a frequency divider for frequency dividing the output from the local oscillator 3. Thus, the local oscillator 3, the prescaler 8, the programmable frequency divider 7, the phase comparator 5 and the low-pass filter 4 constitute a phase locked loop, as well known. The frequency converted output obtained from the local oscillator 3 is supplied to a television signal processing circuit 9 and the output lS from the television signal processing circuit is supplied to a pictuxe tube lO, which may comprise a cathode ray tube.
An oscillation output from the local oscillator 3 is first frequency divided by the prescaler 8 and the frequency divided output thus obtained from the prescaler 8 is supplied to the programmable frequency divider 7.
The programmable frequency divider 7 serves to frequency divide the output from the prescaler 8 as a function of the data supplied from the central processing unit 6. The phase comparator 5 compares the phases of the frequency divided output from the programmable frequency divider 7 and the reference frequency signal from the output frequency signal terminal 11, whereupon the output signal is supplied to the low-pass filter circuit 4. The output from the low-pass filter circuit 4 is supplied, as a tuning voltage, to the voltage controlled variable capacitance device 12 included in the local oscillator 3.
As a result, the oscillation frequency of the local oscillator 3 is controlled to be a value determined by the data obtained from the central processing unit 6.
Figure 2 is a schematic diagram of one example of a conventional low-pass filter circuit 4 for use in the system shown in Figure l. The low-pass filter circuit 4 S36~
shown in Figure 2 comprises a resistor 13 having one end connected to an input terminal IN for receiving an input voltage Ei and the other end connected to an input 15a of a phase inverting amplifier 15 operable to be switchable and having a high input impedance, and a capacitor 14 coupled between the other end of the resistor 13 and the ground. The resistor 13 and the capacitor 14 constitute a low-pass filter. The inverting amplifying circuit 15 has its input 15a connected to the other end of the resistor 13 and to the end of the capacitor 14 and its output lSb connected to the output terminal OUT. The phase inverting amplifier 15 is shunted by a feedback circuit 16 having a predetermined time constant and coupled between the input 15a and the output 15b of the amplifier and further has a load resistor 17 coupled to the output 15b of the amplifier 15. The output voltage appearing at the output terminal OUT is denoted as Eo.
In operation, a response rate in which the output voltage Eo is switched from a lower voltage to a higher voltage as a function of the data supplied from the central processing unit 6 and thus as a function of the input voltage Ei supplied to the input terminal IN is substantially determined by the time constant set in the feedback circuit 16. Therefore, when the time constant of the feedback circuit 16 is decreased to increase the response rate, a ripple component included in the output voltage Eo appearing at the output terminal OUT is increased, which adversely affects a first reception state, in which the output terminal OUT is to be brought to a constant potential. On the other hand, when the time constant of the feedback circuit 16 is increased to decrease the response rate, while the ripple of the output voltage is decreased and any problem in first reception is eliminated, a disadvantage is caused during transition in reception where the output potential appearing at the output terminal OU~ is changed.
In order to solve such dilemma, a conventional approach was to employ a value of the time constant of the S3~9 feedback circuit which is determined as a compromise of the above-described two requirements. As a result, there existed limits in simultaneousl~ increasing a response rate and decreasing a ripple.
Accordingly, a principal object of the present invention is to provide a low-pass filter circuit wherein the rate of change of an output voltage at an output terminal -from a low voltage to a high voltage is increased while a ripple in an output potential appearing at the output terminal is decreased.
The present invention provides a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal var~ing at a frequency determined by a voltage variable capacitor a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter, the. low-pass filter including a series resistor in a signal path for the signal provided by the phase comparator and a capacitor connected between the series resistor and ground, the improvement comprising time constant changing means for changing a time constant oE the low-pass filter between a plurality of values in response to a magnitude of the signal passing through the series resistor, the time constant changing means including a further resistor series connected with the series resistor, and diode means 3~ operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below the predetermined value, the diode means connected across the further resistor for changing the time constant of the low-pass filter between a firs~, high, value proportional to a sum of the series resistor and the further resistor when the voltage across the diode means is below the predetermined value and a second, low, value proportional ~2~53~9 to the series resistor when the voltage across the diode means is above the predetermined value.
The present invention further provides a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass ~ilter, the low-pass filter including a series resistor in a signal path for the signal provided by the phase comparator and a capacitor connected between the series resistor and ground, the low-pass filter further including an inverting amplifying means connected in the signal path, and feedback means connecting an output of the inverting amplifying means to an input thereof, the feedback means including means for setting a time constant thereof within a range from a low value which permits an increased rate of response to a changing signal from the phase comparator, but which passes signal ripple, to a high value whic~ decreases signal ripple but which 2S provides a reduced rate of response to a changing signal from the phase comparator, the improvement comprising, compensating means for compensating for the time constant of the feedback means and for permitting setting the feedback means time constant to the high value while maintaining a high response rate to voltage signals including, time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to the signal passing through the series resistor.
The present invention will become more apparent from the following detailed description of embodiments of the present invention when taken in conjunction with Figures 3 to 6 of the accompanying drawingsu ~%~S3~39 Referring to Figure 3, a series connection of a first resistor 18 and a second resistor 20 is connected between an input terminal IN and a input 22a of a phase inverting amplifier 22. A capacitor 21 is connected 5 between the input 22a of the phase inverting amplifier 22 and the ground. It is to be noted that the series connected resistors 18 and 20 and the capacitor 21 constitute a low-pass filter. In accordance with the embodiment shown in Figure 3, the first resistor 18 is 10 shunted by a diode 19, with the cathode of the diode 19 connected to the input terminal IN. The phase inverting amplifier 22 may comprise a phase inverting amplifier operable to be switchable and having a high input impedance with its input 22a connected to the above-15 described series connection and to the capacitor 21 andwith its output 22b connected to the output terminal OUT.
The phase inverting amplifier 22 is shunted by a feedback circuit 23 having a predetermined time constant selected to decrease a ripple in the output voltage Eo in a normal 20 reception state. A load resistor 24 is also connected to the output of the phase inverting amplifier 22. The input voltage is denoted as Ei and the output voltage is denoted as Eo.
With reference to the diagram shown in Figure 3, 25 it is pointed out that since the diode 19 is connected in parallel with the first resistor 18 constituting a part of the low-pass filter to be connected to the input of the phase inverting amplifier 22, whereby a discharging circuit of a smaller time constant is formed, the electric 30 charge in the feedback circuit 23 can be quickly discharged through the diode 19. ~ccordingly, although the time constant of the Eeedback circuit 23 is selected such that the ripple in the output voltage Eo at the output terminal OUT is decreased, a response rate in a 35 change of the output voltage Eo in an increasing direction can be increased. Meanwhile, since the voltages at both ends of the first resistor 18 are approximately equal on the occasion of a first reception state, the diode 19 is 53~9 turned off, with the result that the diode 19 does not e~ert any adverse influence upon the low-pass filter circuit on the occasion of a first reception state, i.e.
on the occasion of a first operational state when the output voltage Eo appearing at the output terminal OUT is maintained constant.
Figure 4 is a schematic diagram of another embodiment of the inventive low-pass filter circuit. A
difference between the embodiment shown in Figure ~ as compared with the embodiment shown in Figure 3 is that a second diode 25 is further connected in parallel with the first resistor 18 and the first diode 19, the polarity of the diode 25 being opposite to that of the first diode 19, so that the electric charge of the feedback circuit 23 may be discharged by the first diode l9 while the electric charge may he charged to the feedback circuit 23 by the second diode 25. Since the remaining portions oE the embodiment shown in Figure 4 are substantially the same as those in the embodiment shown in Figure 3, it is not believed necessary to describe the same again.
It is pointed out that in the embodiment shown in Figure 4 the time constant of the feedback circuit 23 is selected so that the ripple in the output voltage Eo on the occasion of a first reception state may be small.
Since the voltages at both ends of the first resistor 18 at that time are substantially equal to each other, both diodes l9 and 25 are turned off and then the low-pass filter circuit per se operates as if these diodes l9 and 25 had been removed. This means that these diodes do not adversely affect the low-pass filter circuit.
Now let it be assumed that a pu]se for causing a change in the output voltage Eo is applied to the input terminal IN from the phase comparator 5 shown in Figure 1.
As a result, when a change is about to occur in the output voltage Eo from a high voltage to a low voltage, the second diode 25 is biased in a forward direction, so that the same is turned on, with the result that the electric charge is quickly fed to the feedback circuit 23. On the .
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other hand, when a change is about to occur in the output voltage Eo from a low voltage to a high voltage, the first diode 19 is biased in a forward direction so that the same is turned on, with the result that the electric charge is quickly discharged from the feedback circuit 23. ~hus, even when the output voltage Eo i9 about to change in either di~ection, a ripple can be decreased and simultaneously the response rate o~ the output voltage Eo can be increased.
Figure 5 is a graph showing a comparison of the response characteristics of the inventive and conventional low-pass filter circuits. Referring to Figure 5, a broken line curve A shows the response characteristic of the conventional low-pass filter circuit shown in Figure 2, another broken line curve B shows the response characteristic of one embodiment of the inventive low-pass filter circuit shown in Figure 3, and a solid line curve C
shows the response characteristic of another embodiment of the inventive low-pass Eilter circuit shown in Figure 4.
As can be appreciated from these curves in Figure 5, the response characteristics of the embodiments of the inventive low-pass filter circuit shown in Figures 3 and 4 are improved as compared with the conventional low-pass filter circuit shown in Figure 2.
Figure 6 is similar to Figure 4 and shows a schematic diagram of a further embodiment of the inventive low-pass filter circuit. A difference of the embodiment shown in Figure 6 from that shown in Figure 4 is that the first diode 19 is omitted and only the second diode 25 is maintained. The embodiment shown in Figure 6 aims to provide a low-pass filter circuit wherein a response rate is increased only in case where the output voltage Eo at the output terminal OUT is about to change from a high voltage to a low voltage, in which case provision of only the second diode 25 is sufficient.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and ~2~53~5~
example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
482,319, filed May 24, 1985.
The present invention relates to a low-pass filter circuit. More specifically, the present invention relates to a low-pass filter circuit for use in a system for controlling a tuning apparatus employing a phase locked loop.
In the accompanying drawings:-Figure 1 is a block diagram of a system for controlling a tuning apparatus employing a phase lockedloop;
Figure 2 is a schematic diagram of a conventional low-pass filter circuit employed in the system shown in Figure l;
Figure 3 is a schematic diagram of one embodiment of the inventive low-pass filter circuit;
Figure 4 is a schematic diagram of another embodiment of the inventive low-pass filter circuit;
Figure 5 is a graph showing a comparison of the response characteristic of the conventional and inventive low-pass filter circuits; and Figure 6 is a schematic diagram of a further embodiment of the inventive low-pass filter circuit.
Figure 1 shows a block diagram of a system for controlling a tuning apparatus employing a phase locked loop. Reerring to Figure 1, the system comprises a receiving antenna 1, a high frequency amplifier 2 coupled to the antenna 1, and a local oscillator 3 coupled to the high frequency amplifier 2. The local oscillator 3 comprises a tuning circuit including a coil and a voltage controlled variable capacitance device 12. Such a voltage controlled variable capacitance device may comprise a voltage controlled variable capacitance diode which is commercially available and referred to as a "Varicap cap".
The voltage controlled variable capacitance device 12 is connected to receive an output of a low-pass filter circuit 4. The input of the low-pass filter circuit 4 is connected from a phase comparator 5. The phase comparator ,~
~... ' 53~9 5 is connected to receive one input from a reference frequency signal terminal 11 and the other input from a programmable frequency divider 7. The programmable frequency divider 7 is connected to receive a frequency siynal to be divided from a prescaler 8 and a frequency division ratio controlling signal from a central processing unit 6. The prescaler 8 comprises a frequency divider for frequency dividing the output from the local oscillator 3. Thus, the local oscillator 3, the prescaler 8, the programmable frequency divider 7, the phase comparator 5 and the low-pass filter 4 constitute a phase locked loop, as well known. The frequency converted output obtained from the local oscillator 3 is supplied to a television signal processing circuit 9 and the output lS from the television signal processing circuit is supplied to a pictuxe tube lO, which may comprise a cathode ray tube.
An oscillation output from the local oscillator 3 is first frequency divided by the prescaler 8 and the frequency divided output thus obtained from the prescaler 8 is supplied to the programmable frequency divider 7.
The programmable frequency divider 7 serves to frequency divide the output from the prescaler 8 as a function of the data supplied from the central processing unit 6. The phase comparator 5 compares the phases of the frequency divided output from the programmable frequency divider 7 and the reference frequency signal from the output frequency signal terminal 11, whereupon the output signal is supplied to the low-pass filter circuit 4. The output from the low-pass filter circuit 4 is supplied, as a tuning voltage, to the voltage controlled variable capacitance device 12 included in the local oscillator 3.
As a result, the oscillation frequency of the local oscillator 3 is controlled to be a value determined by the data obtained from the central processing unit 6.
Figure 2 is a schematic diagram of one example of a conventional low-pass filter circuit 4 for use in the system shown in Figure l. The low-pass filter circuit 4 S36~
shown in Figure 2 comprises a resistor 13 having one end connected to an input terminal IN for receiving an input voltage Ei and the other end connected to an input 15a of a phase inverting amplifier 15 operable to be switchable and having a high input impedance, and a capacitor 14 coupled between the other end of the resistor 13 and the ground. The resistor 13 and the capacitor 14 constitute a low-pass filter. The inverting amplifying circuit 15 has its input 15a connected to the other end of the resistor 13 and to the end of the capacitor 14 and its output lSb connected to the output terminal OUT. The phase inverting amplifier 15 is shunted by a feedback circuit 16 having a predetermined time constant and coupled between the input 15a and the output 15b of the amplifier and further has a load resistor 17 coupled to the output 15b of the amplifier 15. The output voltage appearing at the output terminal OUT is denoted as Eo.
In operation, a response rate in which the output voltage Eo is switched from a lower voltage to a higher voltage as a function of the data supplied from the central processing unit 6 and thus as a function of the input voltage Ei supplied to the input terminal IN is substantially determined by the time constant set in the feedback circuit 16. Therefore, when the time constant of the feedback circuit 16 is decreased to increase the response rate, a ripple component included in the output voltage Eo appearing at the output terminal OUT is increased, which adversely affects a first reception state, in which the output terminal OUT is to be brought to a constant potential. On the other hand, when the time constant of the feedback circuit 16 is increased to decrease the response rate, while the ripple of the output voltage is decreased and any problem in first reception is eliminated, a disadvantage is caused during transition in reception where the output potential appearing at the output terminal OU~ is changed.
In order to solve such dilemma, a conventional approach was to employ a value of the time constant of the S3~9 feedback circuit which is determined as a compromise of the above-described two requirements. As a result, there existed limits in simultaneousl~ increasing a response rate and decreasing a ripple.
Accordingly, a principal object of the present invention is to provide a low-pass filter circuit wherein the rate of change of an output voltage at an output terminal -from a low voltage to a high voltage is increased while a ripple in an output potential appearing at the output terminal is decreased.
The present invention provides a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal var~ing at a frequency determined by a voltage variable capacitor a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter, the. low-pass filter including a series resistor in a signal path for the signal provided by the phase comparator and a capacitor connected between the series resistor and ground, the improvement comprising time constant changing means for changing a time constant oE the low-pass filter between a plurality of values in response to a magnitude of the signal passing through the series resistor, the time constant changing means including a further resistor series connected with the series resistor, and diode means 3~ operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below the predetermined value, the diode means connected across the further resistor for changing the time constant of the low-pass filter between a firs~, high, value proportional to a sum of the series resistor and the further resistor when the voltage across the diode means is below the predetermined value and a second, low, value proportional ~2~53~9 to the series resistor when the voltage across the diode means is above the predetermined value.
The present invention further provides a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass ~ilter, the low-pass filter including a series resistor in a signal path for the signal provided by the phase comparator and a capacitor connected between the series resistor and ground, the low-pass filter further including an inverting amplifying means connected in the signal path, and feedback means connecting an output of the inverting amplifying means to an input thereof, the feedback means including means for setting a time constant thereof within a range from a low value which permits an increased rate of response to a changing signal from the phase comparator, but which passes signal ripple, to a high value whic~ decreases signal ripple but which 2S provides a reduced rate of response to a changing signal from the phase comparator, the improvement comprising, compensating means for compensating for the time constant of the feedback means and for permitting setting the feedback means time constant to the high value while maintaining a high response rate to voltage signals including, time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to the signal passing through the series resistor.
The present invention will become more apparent from the following detailed description of embodiments of the present invention when taken in conjunction with Figures 3 to 6 of the accompanying drawingsu ~%~S3~39 Referring to Figure 3, a series connection of a first resistor 18 and a second resistor 20 is connected between an input terminal IN and a input 22a of a phase inverting amplifier 22. A capacitor 21 is connected 5 between the input 22a of the phase inverting amplifier 22 and the ground. It is to be noted that the series connected resistors 18 and 20 and the capacitor 21 constitute a low-pass filter. In accordance with the embodiment shown in Figure 3, the first resistor 18 is 10 shunted by a diode 19, with the cathode of the diode 19 connected to the input terminal IN. The phase inverting amplifier 22 may comprise a phase inverting amplifier operable to be switchable and having a high input impedance with its input 22a connected to the above-15 described series connection and to the capacitor 21 andwith its output 22b connected to the output terminal OUT.
The phase inverting amplifier 22 is shunted by a feedback circuit 23 having a predetermined time constant selected to decrease a ripple in the output voltage Eo in a normal 20 reception state. A load resistor 24 is also connected to the output of the phase inverting amplifier 22. The input voltage is denoted as Ei and the output voltage is denoted as Eo.
With reference to the diagram shown in Figure 3, 25 it is pointed out that since the diode 19 is connected in parallel with the first resistor 18 constituting a part of the low-pass filter to be connected to the input of the phase inverting amplifier 22, whereby a discharging circuit of a smaller time constant is formed, the electric 30 charge in the feedback circuit 23 can be quickly discharged through the diode 19. ~ccordingly, although the time constant of the Eeedback circuit 23 is selected such that the ripple in the output voltage Eo at the output terminal OUT is decreased, a response rate in a 35 change of the output voltage Eo in an increasing direction can be increased. Meanwhile, since the voltages at both ends of the first resistor 18 are approximately equal on the occasion of a first reception state, the diode 19 is 53~9 turned off, with the result that the diode 19 does not e~ert any adverse influence upon the low-pass filter circuit on the occasion of a first reception state, i.e.
on the occasion of a first operational state when the output voltage Eo appearing at the output terminal OUT is maintained constant.
Figure 4 is a schematic diagram of another embodiment of the inventive low-pass filter circuit. A
difference between the embodiment shown in Figure ~ as compared with the embodiment shown in Figure 3 is that a second diode 25 is further connected in parallel with the first resistor 18 and the first diode 19, the polarity of the diode 25 being opposite to that of the first diode 19, so that the electric charge of the feedback circuit 23 may be discharged by the first diode l9 while the electric charge may he charged to the feedback circuit 23 by the second diode 25. Since the remaining portions oE the embodiment shown in Figure 4 are substantially the same as those in the embodiment shown in Figure 3, it is not believed necessary to describe the same again.
It is pointed out that in the embodiment shown in Figure 4 the time constant of the feedback circuit 23 is selected so that the ripple in the output voltage Eo on the occasion of a first reception state may be small.
Since the voltages at both ends of the first resistor 18 at that time are substantially equal to each other, both diodes l9 and 25 are turned off and then the low-pass filter circuit per se operates as if these diodes l9 and 25 had been removed. This means that these diodes do not adversely affect the low-pass filter circuit.
Now let it be assumed that a pu]se for causing a change in the output voltage Eo is applied to the input terminal IN from the phase comparator 5 shown in Figure 1.
As a result, when a change is about to occur in the output voltage Eo from a high voltage to a low voltage, the second diode 25 is biased in a forward direction, so that the same is turned on, with the result that the electric charge is quickly fed to the feedback circuit 23. On the .
~Z453~
other hand, when a change is about to occur in the output voltage Eo from a low voltage to a high voltage, the first diode 19 is biased in a forward direction so that the same is turned on, with the result that the electric charge is quickly discharged from the feedback circuit 23. ~hus, even when the output voltage Eo i9 about to change in either di~ection, a ripple can be decreased and simultaneously the response rate o~ the output voltage Eo can be increased.
Figure 5 is a graph showing a comparison of the response characteristics of the inventive and conventional low-pass filter circuits. Referring to Figure 5, a broken line curve A shows the response characteristic of the conventional low-pass filter circuit shown in Figure 2, another broken line curve B shows the response characteristic of one embodiment of the inventive low-pass filter circuit shown in Figure 3, and a solid line curve C
shows the response characteristic of another embodiment of the inventive low-pass Eilter circuit shown in Figure 4.
As can be appreciated from these curves in Figure 5, the response characteristics of the embodiments of the inventive low-pass filter circuit shown in Figures 3 and 4 are improved as compared with the conventional low-pass filter circuit shown in Figure 2.
Figure 6 is similar to Figure 4 and shows a schematic diagram of a further embodiment of the inventive low-pass filter circuit. A difference of the embodiment shown in Figure 6 from that shown in Figure 4 is that the first diode 19 is omitted and only the second diode 25 is maintained. The embodiment shown in Figure 6 aims to provide a low-pass filter circuit wherein a response rate is increased only in case where the output voltage Eo at the output terminal OUT is about to change from a high voltage to a low voltage, in which case provision of only the second diode 25 is sufficient.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and ~2~53~5~
example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter, the low-pass filter including a series resistor in a signal path for said signal provided by the phase comparator and a capacitor connected between the series resistor and ground, the improvement comprising:
time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to a magnitude of the signal passing through the series resistor;
said time constant changing means including a further resistor series connected with said series resistor; and diode means operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below said predetermined value;
said diode means connected across said further resistor for changing said time constant of said low-pass filter between a first, high, value proportional to a sum of said series resistor and said further resistor when said voltage across said diode means is below said predetermined value and a second, low, value proportional to said series resistor when said voltage across said diode means is above said predetermined value.
time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to a magnitude of the signal passing through the series resistor;
said time constant changing means including a further resistor series connected with said series resistor; and diode means operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below said predetermined value;
said diode means connected across said further resistor for changing said time constant of said low-pass filter between a first, high, value proportional to a sum of said series resistor and said further resistor when said voltage across said diode means is below said predetermined value and a second, low, value proportional to said series resistor when said voltage across said diode means is above said predetermined value.
2. An improved tuning control apparatus as recited in claim 1, wherein said low-pass filter includes an inverting amplifying means connected in said signal path, and feedback means connecting an output of said inverting amplifying means to an input thereof, said feedback means including means for setting a time constant thereof within a range from a first value which permits an increased rate of response to a changing signal from said phase comparator, but which passes signal ripple, to a second value which decreases signal ripple but which provides a reduced rate of response to a changing signal from said phase comparator;
said time constant changing means operable for enabling the time constant of said feedback means to be set to said second value thereby to reduce signal ripple and for compensating for said reduced rate of response by reducing the time constant of said low-pass filter thereby to increase responsiveness thereof when said voltage provided by said phase comparator increases.
said time constant changing means operable for enabling the time constant of said feedback means to be set to said second value thereby to reduce signal ripple and for compensating for said reduced rate of response by reducing the time constant of said low-pass filter thereby to increase responsiveness thereof when said voltage provided by said phase comparator increases.
3. In a tuning control apparatus including a phase lock loop, the phase lock loop including a local oscillator providing a signal varying at a frequency determined by a voltage variable capacitor, a low-pass filter providing a control voltage to the voltage variable capacitor, a frequency dividing circuit for dividing the frequency of the signal provided by, the local oscillator and a phase comparator for comparing the phase of the divided frequency signal with a reference and for providing a signal to the low-pass filter;
the low-pass filter including a series resistor in a signal path for said signal provided by the phase comparator and a capacitor connected between the series resistor and ground;
the low-pass filter further including an inverting amplifying means connected in said signal path, and feedback means connecting an output of said inverting amplifying means to an input thereof, said feedback means including means for setting a time constant thereof within a range from a low value which permits an increased rate of response to a changing signal from said phase comparator, but which passes signal ripple, to a high value which decreases signal ripple but which provides a reduced rate of response to a changing signal from said phase comparator, the improvement comprising:
compensating means for compensating for the time constant of said feedback means and for permitting setting the feedback means time constant to said high value while maintaining a high response rate to voltage signals including:
time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to the signal passing through the series resistor.
the low-pass filter including a series resistor in a signal path for said signal provided by the phase comparator and a capacitor connected between the series resistor and ground;
the low-pass filter further including an inverting amplifying means connected in said signal path, and feedback means connecting an output of said inverting amplifying means to an input thereof, said feedback means including means for setting a time constant thereof within a range from a low value which permits an increased rate of response to a changing signal from said phase comparator, but which passes signal ripple, to a high value which decreases signal ripple but which provides a reduced rate of response to a changing signal from said phase comparator, the improvement comprising:
compensating means for compensating for the time constant of said feedback means and for permitting setting the feedback means time constant to said high value while maintaining a high response rate to voltage signals including:
time constant changing means for changing a time constant of the low-pass filter between a plurality of values in response to the signal passing through the series resistor.
4. An improved tuning control apparatus as recited in Claim 3 wherein:
said time constant changing means for the low-pass filter includes a further resistor series connected with said series resistor; and diode means operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below said predetermined value;
said diode means connected across said further resistor for changing said time constant of said low-pass filter between a first, high, value proportional to a sum of said series resistor and said further resistor when said voltage across said diode means is below said predetermined value and a second, low, value proportional to said series resistor when said voltage across said diode means is above said predetermined value.
said time constant changing means for the low-pass filter includes a further resistor series connected with said series resistor; and diode means operable for turning on in response to a voltage thereacross in excess of a predetermined value and for turning off in response to a voltage thereacross below said predetermined value;
said diode means connected across said further resistor for changing said time constant of said low-pass filter between a first, high, value proportional to a sum of said series resistor and said further resistor when said voltage across said diode means is below said predetermined value and a second, low, value proportional to said series resistor when said voltage across said diode means is above said predetermined value.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000548540A CA1245309A (en) | 1984-06-13 | 1987-10-02 | Low pass filter circuit |
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP124094/1984 | 1984-06-13 | ||
| JP59124094A JPS611107A (en) | 1984-06-13 | 1984-06-13 | Low-pass filter circuit |
| JP59174545A JPS6152016A (en) | 1984-08-20 | 1984-08-20 | Low pass filter circuit |
| JP174545/1984 | 1984-08-20 | ||
| CA000482319A CA1241711A (en) | 1984-06-13 | 1985-05-24 | Low-pass filter circuit |
| CA000548540A CA1245309A (en) | 1984-06-13 | 1987-10-02 | Low pass filter circuit |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000482319A Division CA1241711A (en) | 1984-06-13 | 1985-05-24 | Low-pass filter circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1245309A true CA1245309A (en) | 1988-11-22 |
Family
ID=27167525
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000548540A Expired CA1245309A (en) | 1984-06-13 | 1987-10-02 | Low pass filter circuit |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1245309A (en) |
-
1987
- 1987-10-02 CA CA000548540A patent/CA1245309A/en not_active Expired
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| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |