JPH0461508A - Automatic adjustment circuit for filter - Google Patents

Abstract

PURPOSE:To widen a frequency lock range and to improve the response by detecting a phase shift, increasing the Q near a center frequency with its detection output and decreasing the Q in other frequencies. CONSTITUTION:When a phase difference of both input signals phi1 and phi2 of a phase detector 4 is accurately 90 deg. and a change in a detection output voltage V0 is within + or - V, that is, a frequency of a signal passing through a band pass filter 5 is nearly equal to a center frequency f0, a discrimination output of 1st and 2nd comparators 14, 15 is both at logical H. Then a constant current from the 1st and 2nd comparators 14,15 is added to a constant current from a constant current source 16, the result is fed to a control circuit 17 to increase the sharpness Q of the band pass filter 5. On the other hand, when a change in a voltage V0 exceeds a range of + or - V, the current outputted from the 1st and 2nd comparators 14, 15 is stopped and the sharpness Q is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an automatic filter adjustment circuit for correcting changes in characteristics of a filter circuit.

(B) Conventional Technology It is known that when a filter circuit is integrated into an IC, the capacitor values vary, and the frequency characteristics of the filter circuit deviate from the expected ones. Therefore, an automatic filter adjustment circuit as shown in FIG. 2 has been proposed, which automatically adjusts the fluctuations in the frequency characteristics of the filter circuit. In FIG. 2, a filter (1) is used to correct deviations, and a reference filter (2) of the same type as the filter (1) is provided. The oscillator (3) generates a first oscillation output value signal φ2 with a phase of 0 degrees and a second oscillation output value signal φ2 with a phase of 90 degrees. The first oscillation output value number -3 is applied to the phase detector (4) via the reference filter (2). Further, the second oscillation output value number φ□ is directly applied to the phase detector (4). here,
Assuming that the reference filter (2) is a bandpass filter,
Assume that the frequency characteristic is such that when the center frequency is f, a signal with a phase change of 0 degrees is passed, as shown in FIG. In addition, the phase detector (4) receives the inner input signal (≠, and φ,)
Assume that the detected output becomes zero when the phase difference between the two is 90 degrees. The detection output of the phase detector (4) is simultaneously applied to the reference filter (2) and the filter (1) to change their filter characteristics. Therefore, if the characteristic of the reference filter (2) in FIG. 2 is as expected, with a phase lag, the output of the phase detector (4) will be zero and no control will be performed. Further, if the characteristic of the reference filter (2) has a phase lag and the center frequency deviates from the frequency r, a phase detection output corresponding to the amount of deviation is generated and changes the characteristic of the reference filter (2). As a result, the characteristics of the reference filter (2) are the expected characteristics, that is, the phase delay is 0 degrees, and the center frequency can be corrected to r0.

Therefore, the filter characteristics of filter (1) are similarly corrected.

(c) Problems to be solved by the invention By the way, the reference filter (2) as a bandpass filter in FIG. 2 affects the responsiveness and range of adjustment pull-in depending on the value of its sharpness Q. . FIG. 4 shows this relationship. When the sharpness Q is low, the change in phase with respect to a change in frequency is small and the responsiveness is poor. However, it has the advantage of a wide frequency pull-in range. On the other hand, if the sharpness Q is high, the situation is completely opposite to the above case.

In this way, the sharpness Q could only be set to a value that satisfies the two conditions as much as possible.

(d) Means for Solving the Problems The present invention has been made in view of the above points, and includes an oscillator that generates first and second oscillation output values having a constant phase difference, and a differential amplifier. A first filter whose frequency characteristics are determined by the mutual conductance gm and the capacitance of the capacitor CI connected to the differential amplifier, and the mutual conductance IL of the differential amplifier and the capacitor C1 connected to the differential amplifier. The second frequency characteristic is determined by the capacitance.
a filter circuit that prevents the application of a first oscillation output value signal of the oscillator; and a phase detection circuit that performs phase detection between the output signal of the filter circuit and the second oscillation output value signal of the oscillator. , a detection circuit that detects the absolute value of a shifted value when the output signal of the phase detection circuit deviates from a reference value; a detection output signal of the phase detection circuit and a detection output signal of the detection circuit are applied; A first method for changing the mutual conductance gm1 in order to change the ratio between the mutual conductances gmI and gm while keeping the product of the mutual conductances gm1 and gmt constant.
and a control circuit that generates a second current i2 that changes the current 11 and the mutual fundance gm, and is characterized in that the sharpness Q of the filter circuit can be changed.

(According to the present invention), the amount of phase shift is detected by the detection circuit, and the detected output allows the Q to increase near the center frequency, and otherwise decreases the Q. The frequency pull-in range is wide, and once the frequency is pulled in, the response becomes high and the filter characteristics become stable.

(5) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (5) shows the mutual conductance gm of the first differential amplifier (6) and the first capacitor (7 ), the mutual conductance gm of the second differential amplifier (9), and the second filter (11) consisting of the second capacitor (10) with a capacitance value C3. Terminal (1
2) is a bandpass filter that can apply the first oscillation output value <1!1 from the oscillator (3), and (13) is a constant current with the first and second comparators (14) and (15)! (16
), which detects the absolute value of the detected output of the phase detector (4) when it deviates from the reference value;
and (17) are applied with the detection output signal of the phase detector (4) and the detection output signal of the detection circuit (13), and the first and second differential amplifiers (6), (9), and (7) Keeping the product of mutual conductance gm1 and gmt constant,
The control circuit generates a first current i that changes the mutual conductance gm, and a second current i2 that changes the mutual conductance gm, in order to change the ratio of the mutual conductance gm+ and gm.

In FIG. 1, circuit elements that are the same as those in FIG. 2 are denoted by the same reference numerals, and explanations thereof will be omitted.

In FIG. 1, the first and second differential amplifiers (6) and (
9) mutual conductance gm, and gm.

is determined according to the t current values of the connected power sources (18) and (19). Therefore, the currents flowing in currents 1 (18) and (19) are controlled by the electric current Kf,i, applied from the control circuit (17).
and i.

shall be equal to The bandpass filter (5) has a general configuration, and its center frequency f0 is as follows, and its sharpness Q is as follows. As is clear from equation (2), the sharpness Q is changed depending on the ratio of mutual conductance gmI to gmt. Therefore, in the uninvention, the sharpness Q is made variable by changing the ratio of the t-flow 11 and i generated from the control circuit (17).

In FIG. 1, a detection output corresponding to the phase change caused by the bandpass filter <5) is generated at the output end of the phase detector (4), and is applied as a control current 10 to the control circuit (17). Then, the control circuit <17) changes the value of the output current i1 or i according to the control current 10. Then, the first or second differential amplifier (6) or (9)
transconductance gm.

Or, gm changes, and as is clear from equation (1), the center frequency r changes. The control tii continues to change until the phase difference between the input and output of the bandpass filter (5) becomes zero, and the center frequency f changes in accordance with the change, so the center frequency f6 changes from the original (the phase difference between input and output is zero).

On the other hand, the two-power signals Cd and φ,) of the phase detector (4)
The value of the detection output voltage when the phase difference is exactly 90 degrees is ■,
shall be. Then, the reference voltage V ref of the reference power supply (20) of the first comparator (14) in the detection circuit (13)
, is Vref,=V. −ΔV ・・・・・・・・・・・・・・・
......(1), and the second comparator (1
The reference voltage V ref of the reference power supply (21) in 5) is: Vref, -+V, +ΔV ・・・・・・・・・・・・
......(2). Therefore, the voltage V
If the change in 0 is within ΔV, that is, if the frequency of the signal passing through the bandpass filter (5) is around the center frequency f0, the first and second comparators (14) and (
The discrimination outputs of 15) are both rH. Then, the first
The second comparators <14) and <15) generate a constant current, which is added to the constant current of the constant current source (16) and applied to the control circuit (17). Then, the control circuit <17
) is controlled to increase the current ratio 1 l/11. Then, the mutual conductance ratio g m + / g
m t increases, and as is clear from equation (2), the sharpness Q of the bandpass filter (5) increases.

Therefore, the bandpass filter (5) in this case has good response and stable filter characteristics.

At this time, as is clear from equation (1), when the mutual conductance ratio gm, /gm is varied, and the mutual conductance product gm, ·gm, is varied, the center frequency f, It changes and becomes a problem. Therefore, in the present invention, if the mutual conductance gm+ is multiplied by n, the mutual conductance gm1 is multiplied by 1/n so that the product of the mutual conductances is always constant. More on that later.

Next, when the change in the voltage V becomes large and exceeds the range of ΔV, the output currents of the first and second comparators (14) and (15) stop, and the constant current from the constant TFT source (16) only is applied to the control circuit (17). Then, the control circuit (17) lowers the current ratio 1 r/1 t, so the band pass filter (
5) The sharpness Q decreases.

Therefore, the bandpass filter (5) in this case has an expanded frequency pull-in range.In this way, the bandpass filter (5) has two values, sharp & (Hi), as shown in Fig. 5. It is shown as follows.

FIG. 6 shows a specific circuit example of the control circuit (17) in FIG. 1, and the first input terminal (22) has the current i in FIG.
is supplied to the second input terminal (23), and a detection circuit (13) is supplied to the second input terminal (23).
) output current is supplied. Then, the first output terminal (2
4) has a current i, and the second output terminal (25) has a current i.
, has a flowing structure. In the circuit of Fig. 6, the second
Since the currents flowing through the input terminal (23) and the second output terminal (25) are equal, the input current at the second input terminal (23) is also assumed to be i. In the circuit of FIG. 6, the first input terminal (22
), the second input terminal (23)
If the input current of is constant, then the transistor (26)
The base current of is changed, and the value of current i can be changed. Further, in the circuit shown in FIG. 6, the following current relationship holds true.

1 HX 1z=1 o ・・・・・・・・・・・・
・・・・・・・・・・・・・・・(3) Therefore, the current i,
When the current i at the second input terminal (23) changes while keeping the current i constant, the current iI changes while maintaining the relationship expressed by equation (3). Therefore, it is possible to change the current ratio i, /i, while keeping the product of the electric current WE s + and i constant.

Next, another embodiment of the detection circuit (13) shown in FIG. 1 will be described. In the example shown in FIG. 1, by providing upper and lower reference pressures V ref and V ref with respect to the voltage V, the sharpness Q of the band pass filter (5) is switched to a binary value as a result. However, the sharpness Q may be changed linearly according to the frequency shift. An example of a detection circuit in that case is shown in FIG. In FIG. 7, the output voltage of the phase detector (4) in FIG. 1 is applied to the input terminal (27), and the voltage of the reference power supply (28) is the standard voltage V of the phase detector (4). , is set to . Now, if the heavy pressure V is applied to the input terminal (27), the currents flowing through the transistors (29) and (30) will be the maximum, and the heavy pressure at the output terminal (31) will be the lowest. When the voltage at the terminal (27) changes in the positive or negative direction, the transistor (29) or (3) changes accordingly.
0) decreases, and the voltage at the output terminal (31) increases.

Therefore, by using the detection circuit shown in FIG. 7, the sharpness Q can be changed linearly.

(G) Effects of the Invention As described above, according to the present invention, it is possible to provide an automatic adjustment circuit for a filter that can automatically correct variations such as phase fluctuations of a filter. Further, according to the present invention, since the sharpness Q of the filter to be corrected is varied according to the amount of frequency (phase) shift, the advantage is that the frequency pull-in range can be widened and the response can be improved. have

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention.
A circuit diagram showing a conventional automatic adjustment circuit of a filter, FIGS. 3 and 4 are characteristic diagrams for explaining FIG. 2, FIG. 5 is a characteristic diagram for explaining FIG. 6 is a circuit diagram showing a specific circuit example of the control circuit 17) in FIG.
This figure is a circuit diagram showing another embodiment of the detection circuit (13) in FIG. 1. (3)...Oscillator, (4)...Phase detector, (
5)...bandpass filter, (13)...detection circuit, (17)...control circuit.

Claims (2)

[Claims] (1) The frequency characteristics are determined by an oscillator that generates first and second oscillation output signals having a certain phase difference, the mutual conductance gm_1 of the differential amplifier, and the capacitance of the capacitor C_1 connected to the differential amplifier. and a second filter whose frequency characteristics are determined by the transconductance gm_2 of the differential amplifier and the capacitance of the capacitor C_2 connected to the differential amplifier.
a filter circuit to which an oscillation output signal is applied; a phase detection circuit that performs phase detection between the output signal of the filter circuit and a second oscillation output signal of the oscillator; a detection circuit that detects the absolute value of the shifted value in the case of up to, the transconductance g
a control circuit that generates a first current i_1 that changes the mutual conductance gm_1 and a second current i_2 that changes the mutual conductance gm_2 in order to change the ratio between m_1 and gm_2; An automatic filter adjustment circuit characterized in that the sharpness Q is varied. (2) The automatic filter adjustment circuit according to claim 1, wherein the detection circuit includes a current output type window comparator.