JP2805171B2 - Active filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention <relates> is, g _m amplifier used in the sound multiplex demodulator, etc. (g _m is the transconductance) an active filter circuit used.

<Related Art> In a television receiver compatible with audio multiplex broadcasting, an audio multiplex demodulator is provided in an audio receiving circuit in order to extract left and right audio signals during stereo broadcasting and main / sub audio signals during two audio broadcasting. I have. This audio multiplex demodulator converts the audio multiplex signal
It is provided with a variety of filter circuit including a bandpass filter for separating into a transport multiplex control signal of the transfer sub-audio signals and 3.5 F _H of the 2f _H.

Typically these filter circuit consists active filter, the amplifier of the active filter, IC of easiness, g _m amplifier etc. that gain can be adjusted is frequently used. Shows an active filter using the g _m amplifier in Figure 3.

Active filter 20, two g _m amplifier 21a, 21b and two capacitors 22a, and a 22b, g _m amplifier 21a, 21b of the
g _m value and the capacitor 22a, the time constant determined by the capacitance of 22b, passing signals of a specific frequency. g _m amplifier
21a and 21b are buffer amplifiers 23a and 23b that convert the output to a voltage in order to convert the input voltage to a current and output the current.
Is combined with

The <Problems that the Invention is to Solve> active filter using such g _m amplifier, because there are variations in the capacitance of the capacitor, unavoidable that the frequency characteristic is shifted in accordance with the variations thereof.
Conventionally, the deviation of the frequency characteristic is modified by changing the g _m value of g _m amplifier, there is a problem that complicates the manufacture of such sound multiplex demodulator for that.

In addition, the signal input to the filter circuit is not limited to the audio multiplex signal, and its frequency fluctuates as a general phenomenon.
In particular, in the case of a filter circuit having a band-pass filter, there is a risk that a signal to be separated is extracted with distortion.

The present invention has been made in view of such circumstances,
An active filter circuit in which the deviation of the frequency characteristic is automatically corrected even if the capacitance of the capacitor varies,
It is an object of the present invention to provide an active filter circuit with a built-in band pass filter that can automatically follow the center frequency of the filter even when the frequency of the input signal fluctuates.

Active filter circuit according to the present invention <Means for Solving the Problems> uses a filter circuit body of active filter using the g _m amplifier, the same active filter as the active filter used in the filter circuit main A band-pass filter that passes only a predetermined frequency band with respect to the frequency of the input signal, and a signal of the predetermined frequency that has passed through the band-pass filter using the same active filter as the active filter used in the filter circuit body. A phase shifter for transferring, a phase difference detector for detecting a phase difference with respect to a reference phase shift angle of a signal having passed through the phase shifter, and a filter so that the phase difference detected by the phase difference detector becomes equal. change g _m value of g _m amplifier in active filter in the circuit body, the band-pass filter and the phase shifter It includes a g _m control circuit for, the.

<Operation> If the capacitance of the capacitor of the active filter used in the phase shifter deviates from the normal value, the phase of the signal passing through the phase shifter is shifted from the reference phase shift angle by an angle corresponding to the deviation. Make a difference. Active filter used in the phase shifter is the same as the active filters used in the filter circuit body, so that the phase difference is eliminated, g _m value of g _m amplifier of the active filter in the filter circuit body By being changed, the deviation of the frequency characteristic is eliminated. Since g _m value of g _m amplifier active filter is also changed in the phase shifter, it does not shift the frequency characteristic of the filter circuit body is modified more than necessary.

When the frequency of a signal (for example, a carrier multiplex control signal) to be separated from an input signal (for example, a voice multiplex signal) fluctuates by using the output of a controlled bandpass filter as a control reference, Te, the signal passing through the phase shifter is not the reference phase angle, the control for changing the g _m value of g _m amplifier in active filter of the band-pass the filter as the retardation is eliminated is.
That is, the band-pass filter automatically follows the above-mentioned frequency fluctuation, and the frequency characteristic of the band-pass filter is always kept optimal.

<Example> Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of an active filter circuit according to the present invention.

The active filter circuit 10 shown here is provided in an audio multiplex demodulator of an audio receiving circuit in a television receiver compatible with audio multiplex broadcasting, and g _m in the audio multiplex demodulator is provided.
Various active filters using an amplifier are used as a filter circuit main body, and the deviation of the frequency characteristic due to the variation in the capacitance of the capacitor is automatically corrected.

The active filter circuit 10 converts the voice multiplexed signal to 3.5 f
Bandpass filter (B retrieving the _H transport multiplex control signals
PF) 11. The band-pass filter 11 uses the same active filter as the active filter used in the filter circuit body.

Transport multiplex control signal 3.5 F _H retrieved by the band-pass filter 11, it passes through the phase shifter 12. The phase shifter 12 uses the same active filter as the active filter used in the filter circuit body, and shifts the phase of a passing signal by a reference phase shift angle of 90 deg.

The signal whose phase has been shifted by the phase shifter 12 is input to a phase difference detector 13 composed of a balanced modulator. Phase difference detector 13
Compares the phase of the signal after passing through the phase shifter 12 with the phase of the signal before passing through the phase shifter 12 based on 90 deg, and generates an error voltage if the phase difference deviates from 90 deg. .

Error voltage generated in the phase difference detector 13, g _m control circuit 14
Given to. g _m control circuit 14, as described above an error voltage becomes 0, the filter circuit body, the band-pass filter 11
And it has a configuration to control in accordance with the error voltage g _m value of g _m amplifier of the phase shifter 12 in the active filter used respectively.

In the active filter circuit, if the capacitance of the active filter used in the phase shifter 12 deviates from a normal value, the signal after passing through the phase shifter 12 and the signal before passing through the phase shifter 12 Does not become 90 deg. Therefore, generate a phase difference detector 13 is an error voltage, by g _m value of g _m amplifier in active filter phase shifter 12 by the g _m control circuit 14 is controlled, the phase difference is 90deg. As a result, the active filter of the phase shifter 12, in order to absorb the deviation from a normal value of capacitance, is changed g _m value of g _m amplifier, a state in which the frequency characteristics are corrected.

Active filters used in the filter circuit body and the band-pass filter 11 is the same as the active filter of the phase shifter 12, and g _m value of g _m amplifier like active filter of the phase shifter 12 is since controlled by g _m control circuit 14, the deviation of the variation due to the frequency characteristics of these capacitor capacity, is eliminated as with active filter of the phase shifter 12.

When the shift in the frequency characteristics of the active filter in the filter circuit body and in the band-pass filter 11 is eliminated, the shift in the frequency characteristics of the active filter in the phase shifter 12 is also eliminated. No error voltage is generated, and the frequency characteristics of the active filters in the filter circuit main body and the band-pass filter 11 are not corrected more than necessary.

Incidentally, by the output control reference of the band-pass filter 11 which is controlled, the frequency of the carrier multiplex control signal 3.5 F _H to be separated from the sound multiplex signal input varies with respect to its variation, The signal passing through the phase shifter 12
90deg and should not, occur error voltage corresponding to the phase difference by the phase difference detector 13, the g _m control circuit 14, the frequency characteristic of the band-pass filter 11 is controlled to the optimum position. That is, the band-pass filter 11 automatically follows the above-mentioned frequency fluctuation, and the frequency characteristic of the band-pass filter 11 is always kept optimal.

FIG. 2 is a block diagram showing another example of the active filter circuit according to the present invention.

The active filter circuit 10 shown here is a phase shifter
12 is divided into a first phase shifter 12a and a second phase shifter 12b. The first phase shifter 12a sets the phase of the passing signal to 4
The phase is advanced at a reference phase shift angle of 5 degrees, and the second phase shifter 12b delays the phase of the passing signal by the reference phase shift angle of 45 degrees.

Also in the active filter circuit, the first phase shifter 12
By the phase difference between the signal passing through the signal and the second phase shifter 12b that has passed through the a is compared with 90deg reference, the g _m value control of the active filter so that the phase difference is eliminated is performed, the filter circuit body The deviation of the frequency characteristics of the active filters in the above is automatically eliminated.

Incidentally, for reference, the active filter circuit 10 and the filter circuit main body as described above are generally incorporated in an IC. Among such active filter circuits 10, for example, a band-pass filter 11 and a phase shifter
As compared with the case of No. 12, when ICs manufactured from different wafers are compared, there is variation (absolute variation) in the capacitor capacitance formed in each IC. But,
The capacitance of the capacitor formed in one IC is such that the band-pass filter 11 and the phase shifter 12 use the same active filter as the active filter used in the filter circuit body (that is, the capacitance of the capacitor). Are assumed to have the same design target value) and the following related reasons. In other words, the capacitances of the capacitors formed on one wafer can be easily formed in the same manner with high accuracy. However, when the capacitors are formed on different wafers, the capacitance between the different wafers can be increased. It is because of the IC manufacturing feature that it is difficult to generate a variation in capacitance due to the circuit. Accordingly, capacitance formed in one IC includes a band-pass filter 11, relative variations in the phase shifter 12 can be regarded as no other hand, an active filter using the g _m amplifier The frequency response is g _m
Because it is determined by the capacitance of the capacitor formed in the IC,
The center frequency of the band-pass filter 11 and the frequency at which the input / output phase shift difference of the phase shifter 12 becomes 90 degrees are the same frequency (for example, f1
And [Ie, the absolute value of each frequency may vary between different ICs, but within one IC due to variations that have relatively the same tendency,
It is possible to design at the same frequency (f1)]. Therefore, the center frequency of the input signal frequency (for example, f2).
And the center frequency of the band-pass filter 11 [the frequency at which the input / output phase shift difference of the phase shifter 12 is 90 degrees is the same. ] (F1) if the is shifted, by a loop consisting of the phase shifter 12 and the phase difference detector 13 and g _m control circuit 14., output shift retardation of the phase shifter 12, the center frequency of the input signal frequency 90deg at (f2)
It is controlled so that Accordingly, the center frequency (f1) of the band-pass filter 11 also automatically follows so as to match the center frequency (f2) of the input signal frequency.

When observing different ICs, the center frequency of the band-pass filter 11 has an absolute variation due to the absolute variation of the capacitor capacitance. the center frequency of the 11, for example, when offset from f1 (out center frequency of the design.) up to a frequency of f3 is the phase shifter 12 and the phase difference detector 13 and g _m control circuit 14. loop comprising The phase shift difference between the input and output of the phase shifter 12 is controlled to be 90 degrees at the designed center frequency (f1) of the bandpass filter 11. Accordingly, the center frequency of the bandpass filter 11 is automatically corrected from f3 to the designed center frequency f1.
Therefore, if the center frequency of the input signal frequency is f1, this state continues, but if the center frequency of the input signal frequency shifts to f2 as described above, the bandpass filter
As described above, the center frequency (f1) of the eleventh automatically follows the center frequency (f2) of the input signal frequency.

<Effects of the Invention> As described above, in the case of the active filter circuit according to the present invention, in order to absorb the variation in the capacitance of the active filter of the filter circuit within the body, the g _m amplifier
g _m value is automatically changed, since the deviation of the frequency characteristic is automatically eliminated, even if there are variations in the capacitance in the active filter in the filter circuit body, the g _m amplifier for correcting a frequency characteristic g There is an advantage that adjustment of the _m value is not required.

Further, a band-pass filter that passes only a predetermined frequency band with respect to the frequency of the input signal using the same active filter as the active filter used in the filter circuit body, and an active filter used in the filter circuit body A phase shifter that transfers the signal of the predetermined frequency that has passed through the band-pass filter using the same active filter, and a phase difference detector that detects a phase difference with respect to a reference phase shift angle of the signal that has passed through the phase shifter. phase difference as phase difference detected by the detector is eliminated, the g _m amplifier in active filter in the band pass filter g _m
Since a g _m control circuit for changing the value, by a controlled are band-pass filter outputs a control reference signal to be separated from the signal input (e.g. sound multiplex signal) (for example, conveying multiple Even if the frequency of the control signal changes, the change automatically follows the change and the frequency characteristic of the band-pass filter can always be kept optimal. In other words, since the active filter circuit is controlled to the optimum center frequency even if the frequency of the input signal to be filtered fluctuates, the active filter circuit can extract the signal to be separated from the input signal without distortion. There is also an advantage that can be provided.

[Brief description of the drawings]

Block diagram illustrating an example of an active filter circuit according to Figure 1 the present invention, FIG. 2 is a block diagram showing another example of the active filter circuit according to the present invention, the active filter Figure 3 is with g _m amplifier FIG. 10 ...... active filter circuit 12 ...... phase shifter 13 ...... phase difference detector 14 ...... g _m control circuit 20 ...... active filter 21a, 21b ...... g _m amplifier

Claims (1)

(57) [Claims] A filter circuit body of active filter as claimed in claim 1] with g _m amplifier, a predetermined frequency band with respect to the frequency of the input signal using the same active filter as the active filter used in the filter circuit body only A band-pass filter that allows the signal to pass therethrough, a phase shifter that transfers the signal of the predetermined frequency that has passed through the band-pass filter using the same active filter as the active filter used in the filter circuit body, and a signal that passes through the phase shifter A phase difference detector for detecting a phase difference of the obtained signal with respect to a reference phase shift angle, and a filter circuit body, a bandpass filter, and an active filter in the phase shifter so that the phase difference detected by the phase difference detector is eliminated. active filter times, characterized by comprising a g _m control circuit for changing the g _m value of g _m amplifier .