JPH0846479A - Capable of automatic calibration low-pass filter - Google Patents

Abstract

PURPOSE:To automatically make a culibration of an LPF to obtain an accurate cut-off frequency. CONSTITUTION:In a variable LPF 10, variable capacitance diodes 22 and 24 are adjusted by the output control voltage of a digital/analog converter 40, and it has the cut-off frequency decided by the capacitance of variable capacitance diodes 22 and 24 and the inductor of the 1st inductor 16. A 2nd inductor 30 is connected in parallel with the variable capacitance diode 24 and becomes a tank circuit. It comprises a VCO 34 with an amplifier 32. The variable LPF 10 obtains the specific capacitance of the variable capacitance diodes 22 and 24 so as to give the required cut-off frequency to the variable LPF 10. Further, the specific output frequency of the VCO 34 by the capacitance is obtained. The control means controls the output voltage of the digital/analog converter 40 so as to make the output frequency of the VCO 34 to the specific output frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic calibratable low pass filter device for automatically calibrating a cutoff frequency.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram of a conventional third-order variable low-pass filter (LPF) 10 capable of voltage-controlling a cutoff frequency. Both ends of an inductor 16 having an inductance L0 are connected to the input terminal 12 and the output terminal 14, respectively, and one ends of capacitors 18 and 20 each having a capacitance of C0 are connected. The cathode of a variable capacitance diode 22 whose anode is grounded is connected to the other end of the capacitor 18, and the variable capacitance diode 24 whose anode is grounded is connected to the other end of the capacitor 20.
The cathode of is connected. Resistors 26 and 28 are connected in series between the other ends of the capacitors 18 and 20, and a control voltage Vcont is supplied to the connection point of both resistors. The variable-capacitance diodes 22 and 24 have the same capacitance C1 that changes with the control voltage Vcont. The capacitance C0 is much larger than the capacitance C1 and blocks the direct current component. The cutoff frequency fcut of the LPF 10 is determined by the capacitance C1 of the variable capacitance diodes 22 and 24 and the inductance Lo of the inductor 16.

One method of calibrating the cutoff frequency fcut of the LPF 10 of FIG. 3 is to investigate beforehand the relationship between the control voltage Vcont and the capacitance C1 of the variable capacitance diodes 22 and 24, and to find the desired cutoff frequency in relation to the inductor 16. Control voltage V so that
Adjust cont. According to another method, when a sine wave having a frequency equal to a desired cutoff frequency and a predetermined amplitude A is applied to the input terminal of the LPF 10, the frequency is sufficiently lower than the cutoff frequency and the amplitude is equal to the predetermined amplitude A. The control voltage Vcont is adjusted so that the ratio of the respective output amplitudes when the sine wave is applied becomes 0.71: 1.

[0004]

However, in the former method, the relationship between the control voltage and the variable capacitance diode may change due to the influence of temperature and the like, and it is difficult to accurately set the capacitance C1. In the latter case, a variable frequency oscillator or a plurality of oscillators are required, and the work of properly adjusting the control voltage Vcont is troublesome.
Furthermore, even if it is set once, the cutoff frequency may change due to changes in the element characteristics due to temperature changes and changes in the control voltage, and it is not possible to maintain an accurate cutoff frequency.

Accordingly, it is an object of the present invention to provide an auto-calibratable low pass filter which can be automatically calibrated to have the desired exact cutoff frequency.

[0006]

In the first embodiment of the automatically calibratable low-pass filter device of the present invention, the variable LPF is
The capacitance of the variable capacitance diode is adjusted by the output control voltage of the digital-analog converter, and the cutoff frequency is determined by the capacitance of the variable capacitance diode and the inductor of the first inductor. The second inductor is
It is connected in parallel with the variable capacitance diode to form a tank circuit. This tank circuit is connected to the two inputs of the amplifier and constitutes a voltage controlled oscillator (VCO). The control means consisting of the frequency counter and the microprocessor is V
The output frequency of CO is checked and the output control voltage of the digital-analog converter is controlled according to the output frequency. Also, in the second embodiment, the tank circuit including the first inductor and the variable capacitance diode is connected to the amplifier by connecting the two input terminals of the amplifier in parallel to the first inductor without using the second inductor. This is what constitutes a VCO.

[0007]

The cutoff frequency of the variable LPF is determined by the values of the first inductor and the two variable capacitance diodes. Also,
The oscillation frequency of the VCO is determined by the values of the variable capacitance diode and the second inductor in the first embodiment, and is determined by the values of the first inductor and the two variable capacitance diodes in the second embodiment. The variable LPF obtains a specific capacitance of the variable capacitance diode for having a desired cutoff frequency, and further obtains a specific output frequency of the VCO by this capacitance. Controls the output voltage of the digital-analog converter. This will automatically calibrate the LPF to have the exact cutoff frequency desired.

[0008]

1 is a circuit block diagram showing an embodiment of an automatic calibratable low-pass filter according to the present invention. In this figure, the LPF 10 is the same as the LPF 10 shown in FIG. 3, and therefore, the respective constituent elements are denoted by the same reference numerals as those in FIG. 3, and the description of the configuration thereof is omitted here to avoid duplication.

One variable capacitance diode 2 of the LPF 10
4 has a known inductance Lca at one end that is not the ground side.
One end of an inductor 30 having l is connected via a capacitor 31. The other end of the inductor 30 is grounded. The capacitor 31 is connected for the purpose of preventing the impedance of the inductor 30 from affecting the LPF 10, and has a capacitance that is about 1/10 of the capacitance of the variable capacitance diodes 22 and 24. The parallel circuit composed of the variable capacitance diode 24 and the inductor 30 is a tank circuit, and is connected between the two input terminals of the amplifier 32. The amplifier 32 is, for example, a Motorola MC
It is an amplifier such as the 1648 type and constitutes a voltage controlled oscillator (VCO) 34 together with the tank circuit. A switch 35 is connected between a common connection point of the variable capacitance diode 24 and the inductor 30 that is not on the ground side and between the ground. The VCO 34 oscillates when the switch 35 is in the ON state, and is in the non-oscillation state when the switch is in the OFF state. The oscillation frequency fcal of the VCO 34 is determined by the capacitance C1 of the variable capacitance diode 24 which changes according to the control voltage Vcont supplied to the connection point of the resistors 26 and 28 and the inductance Lcal of the inductor 30, and is represented by the following equation. It fcal≈1 / 2π · (Lcal · C1) * (1/2) (1) (where, N * n represents Nth power of n)

The output signal of the VCO 34 is supplied to the frequency counter 36, and the number of its cycles within a predetermined period is counted. The MPU 38 outputs VC from the output of the frequency counter 36.
Obtain the frequency of the output signal of O34 and adjust the DAC accordingly.
The output digital value that determines the output voltage of 40 is changed.

Next, referring to FIG. 1, a calibration operation for setting the cutoff frequency fcut of the LPF 10 to a desired value fcut 'will be described. When performing this calibration operation, switch 35
Is turned off. As described above, the cutoff frequency fcut of the LPF 10 is determined by the values of the inductor 16 and the variable capacitance diode 24. The inductance L0 of the inductor 16 is known, and the capacitance C1 'of the variable capacitance diode 24 for setting the cutoff frequency fcut of the LPF 10 to a desired value fcut' is calculated. Also,
As represented by the equation (1), the output frequency fcal of the VCO 34 is determined by the capacitance C1 of the variable capacitance diode 24 and the inductor Lcal of the inductor 30 which change according to the control voltage Vcont.
The output frequency fca when the capacitance of is C1 '
l'is obtained from this equation. The MPU 38 monitors the output of the frequency counter 36, and controls the output control voltage Vcont of the DAC 40 so that the frequency obtained from this output becomes fcut '. In this way, the variable capacitance diode 2 is adjusted so that the cutoff frequency of the LPF 10 becomes the desired value fcut ′.
The capacitance of 4 is set to a predetermined value C1 '. To keep the cutoff frequency of the LPF 10 at a desired value at all times, the switch 35 is kept in the ON state. In addition, when performing the above calibration operation intermittently as necessary, MPU3
After 8 stores the output digital value corresponding to the desired cutoff frequency, the switch 35 is once turned off.

FIG. 2 is a circuit block diagram showing another embodiment of the automatic calibratable low-pass filter device of the present invention.
In this figure, the same components as those in FIG. 1 are designated by the same reference numerals, and here, one input end of an amplifier 42 corresponding to the amplifier 32 in FIG. 1 is directly connected to one end of the inductor 16, The other input end is connected to the other end of the inductor 16 via the switch 43. In this case, the inductor 16, the capacitors 18, 20, 22, and 24 form a tank circuit and together with the amplifier 42 form a VCO 44. The VCO 44 oscillates when the switch 43 is on. Capacitance Co of capacitors 18 and 20
Is much larger than the capacitance C1 of the varactor diodes 22 and 24, the oscillation frequency fc of the VCO 44 is
al is substantially determined by the values of the inductance L0 of the inductor 16 and the capacitance C1 of the variable capacitance diodes 22 and 24. Since the diodes 22 and 24 are connected in series, fcal is expressed by the following equation. fcal ≒ 1 / 2π ・ (L0 ・ C1 / 2) * (1/2) ・ ・ ・ (2)

The output signal of the VCO 44 is supplied to the frequency counter 36, and the number of its cycles within a predetermined period is counted. The MPU 38 outputs VC from the output of the frequency counter 36.
Obtain the frequency of the output signal of O34 and adjust the DAC accordingly.
The output digital value that determines the output voltage of 40 is changed. .

In FIG. 2, the cutoff frequency f of the LPF 10 is
To perform the calibration operation for setting the cut to the desired value fcut ', the switch 43 is turned on. As above,
First, the cutoff frequency fcut of the LPF 10 is set to a desired value fcut ′.
The capacitance C1 'of the varactor diode 24 is calculated, and the oscillation frequency fcal' when the capacitance of the varactor diode 24 is C1 'is calculated from the equation (2). The MPU 38 sets the DAC4 so that the output frequency of the frequency counter 36 becomes fcut '.
The output control voltage Vcont of 0 is controlled, and the capacitance of the variable capacitance diode 24 is set to the predetermined value C1 'so that the cutoff frequency of the LPF 10 becomes the desired value fcut'.

Although one embodiment of the present invention has been described above, it will be apparent to those skilled in the art that various modifications can be made without departing from the gist of the present invention. For example, third-order variable L
Although the case where the present invention is applied to the PF has been described above, the present invention can also be applied to a higher order variable LPF.

[0016]

According to the present invention, the cutoff frequency of the variable LPF is determined by the values of the first inductor and the two variable capacitance diodes, and the output frequency of the VCO is the value of the variable capacitance diode and the second inductor in the first embodiment. In the second embodiment, it is determined by the values of the first inductor and the two variable capacitance diodes. The specific capacitance of the varactor diode is determined so that the variable LPF has a desired cutoff frequency, and the VC
The specific output frequency of O is obtained, and the control means digitally adjusts the output frequency of the VCO to the specific output frequency.
Controls the output voltage of the analog converter. This gives L
The PF is automatically calibrated to have the exact cutoff frequency desired.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing an embodiment of an automatic calibratable low-pass filter device of the present invention.

FIG. 2 is a circuit block diagram showing another embodiment of the automatically calibratable low-pass filter device of the present invention.

FIG. 3 is a circuit diagram showing a conventional Butterworth type third-order low-pass filter.

[Explanation of symbols]

 16 First Inductor 22, 24 Variable Capacitance Diode 30 Second Inductor 32, 42 Amplifier 34, 44 Voltage Controlled Oscillator 36, 38 Control Means 40 Digital-Analog Converter

Claims (2)

[Claims] 1. A digital-analog converter, and a capacitance of a variable capacitance diode is adjusted by an output control voltage of the digital-analog converter, and a cutoff frequency determined by the capacitance of the variable capacitance diode and the inductance of the first inductor is adjusted. A controllable variable low-pass filter, a second inductor connected in parallel to the variable capacitance diode, an amplifier connected to the variable capacitance diode and the second inductor to form a voltage controlled oscillator, and the voltage controlled oscillator And a control means for controlling the output control voltage of the digital-to-analog converter according to the output frequency of the low-pass filter device. 2. A digital-analog converter and a capacitance of a variable-capacitance diode can be adjusted by an output control voltage of the digital-analog converter to control a cutoff frequency determined by the capacitance of the variable-capacitance diode and the inductance of an inductor. Variable low-pass filter, an amplifier having two input terminals connected in parallel to the inductor to form a voltage-controlled oscillator, an output frequency of the voltage-controlled oscillator, and the digital-analog conversion according to the output frequency. And a control means for controlling the output control voltage of the instrument.