JPH0834404B2 - Automatic filter adjustment circuit - Google Patents

Description

The present invention relates to an automatic filter adjusting circuit for correcting a change in characteristics of a filter circuit.

(B) Conventional technology It is known that when the filter circuit is integrated into an IC, the value of the capacitor varies and the frequency characteristics of the filter circuit deviate from what was expected. Therefore, an automatic filter adjusting circuit as shown in FIG. 2 has been proposed which automatically adjusts the variation of the frequency characteristic of the filter circuit. In FIG. 2, the filter (1) is for correcting the deviation, and a reference filter (2) of the same type as the filter (1) is provided. Oscillator (3)
Generates a first oscillation output signal φ ₁ having a phase of 0 degree and a second oscillation output signal φ ₂ having a phase of 90 degrees. The first oscillation output signal φ ₁ is applied to the phase detector (4) via the reference filter (2). The second oscillation output signal φ ₂ is directly applied to the phase detector (4). here,
If the reference filter (2) is a bandpass filter,
When the center frequency is f ₀ as shown in FIG.
It is assumed that a signal having a phase change of 0 degrees is passed.
The phase detector (4) assumes that the phase difference between both input signals (φ ₁ and φ ₂ ) is 90 degrees and the detection output becomes zero. The detection output of the phase detector (4) is simultaneously applied to the reference filter (2) and the filter (1) to change its filter characteristic. Therefore, if the characteristic of the reference filter (2) in FIG. 2 does not have a phase delay and is as expected, the output of the phase detector (4) becomes zero and no control is performed.
Further, if the characteristic of the reference filter (2) has a phase delay and the center frequency deviates from the frequency f ₀ , a phase detection output corresponding to the deviation amount is generated and the characteristic of the reference filter (2) is changed. As a result, the characteristics of the reference filter (2) are
The expected characteristic, that is, the phase delay becomes 0 degree, and is corrected to the center frequency f ₀ .

Therefore, the filter characteristic of the filter (1) is similarly corrected.

(C) Problems to be Solved by the Invention By the way, the reference filter (2) as a band-pass filter in FIG. 2 influences the response and range of the adjustment pull-in according to the value of the sharpness Q thereof. . Figure 4 shows
This relationship is shown, and when the sharpness Q is low, the change in phase with respect to the change in frequency is small and the response is poor. However, it has the advantage that the frequency pull-in range is wide. On the contrary, when 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-mentioned points, and a mutual operation of a differential amplifier and an oscillator that generates first and second oscillation output signals having a constant phase difference. A first filter whose frequency characteristic is determined by the conductance gm ₁ and the capacitance of the capacitor C ₁ connected to the differential amplifier, the transconductance gm ₂ of the differential amplifier, and the capacitor C connected to the differential amplifier. _{A second} filter whose frequency characteristic is determined by the capacitance of ₂ and a filter circuit to which the first oscillation output signal of the oscillator is applied; an output signal of the filter circuit; and a second oscillation output of the oscillator. A phase detection circuit that performs phase detection with a signal, a detection circuit that detects the absolute value of the shifted value when the output signal of the phase detection circuit deviates from a reference value, and the detection output signal of the phase detection circuit A detection output signal of the detection circuit is applied to the transconductance
In order to change the ratio of the transconductances gm ₁ and gm ₂ while keeping the product of gm ₁ and gm ₂ constant, the first current i ₁ and the transconductance gm that change the transconductance gm ₁ are changed. A control circuit for generating a second current i ₂ which changes ₂ and a sharpness Q of the filter circuit.
It is characterized by changing.

(E) Action According to the present invention, the amount of phase shift is detected by the detection circuit, and Q is increased near the center frequency by the detection output, and Q is decreased otherwise. The frequency pull-in range is wide, and once pulled, the response becomes high and the filter characteristic becomes stable.

(F) Example Figure 1 is a circuit diagram showing an embodiment of the present invention, (5)
The transconductance gm ₂ of the transconductance gm ₁ and first filter (8) and a second differential amplifier comprising a first capacitor of capacitance C ₁ (7) of the first differential amplifier (6) (9)
And a second filter ( 11 ) composed of a second capacitor (10) having a capacitance value C ₂ , and a band in which the first oscillation output signal φ ₁ from the oscillator (3) is applied to the input terminal (12). The pass filter, (13) is the first and second comparators (1
(4) and (15) and a constant current source (16), which detects the absolute value of the deviated value when the detection output of the phase detector (4) deviates from the reference value, and (17) Is 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)
And the product of the transconductance gm ₁ and gm ₂ (9) while a constant, the transconductance gm ₁ and to change the ratio of gm _2, first current i ₁ for changing the mutual conductance gm ₁ wherein a control circuit for generating a second current i ₂ for varying the mutual conductance gm ₂ and.

Incidentally, in FIG. 1, the same circuit elements as those in FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

In FIG. 1, the transconductances gm ₁ and gm ₂ of the first and second differential amplifiers (6) and (9) are determined according to the current values of the connected current sources (18) and (19). Therefore, the currents flowing through the current sources (18) and (19) are equal to the currents i ₁ and i ₂ applied from the control circuit (17). The bandpass filter ( 5 ) has a general configuration, and its center frequency f ₀ is And the sharpness Q is Becomes As is clear from the equation (2), the sharpness Q is changed according to the ratio of the mutual conductances gm ₁ and gm ₂ . Therefore, in the present invention, the sharpness Q is made variable by changing the ratio of the currents i ₁ and i ₂ 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 applied to the control circuit (17) as a control current i _0. . Then, the control circuit (17) changes the value of the output current i ₁ or i ₂ according to the control current i ₀ . Then
The transconductance gm ₁ or gm _{2 of} the first or second differential amplifier (6) or (9) changes, and the center frequency f ₀ changes as is apparent from the equation (1). The control current i ₀ continues to change until the phase difference between the input and output of the bandpass filter ( 5 ) becomes zero, and the center frequency f ₀ changes according to the change, so that the center frequency f ₀ is It is corrected to the original value (zero phase difference between input and output).

On the other hand, the value of the detection output voltage when the phase difference between both input signals (φ ₁ and φ ₂ ) of the phase detector (4) is exactly 90 degrees,
Set to V ₀ . Then, the reference voltage Vref ₁ of the reference power source (20) of the first comparator (14) in the detection circuit (13) is Vref ₁ = V ₀ −ΔV …………………… (1) Therefore, the reference voltage Vref ₂ of the reference power source (21) of the second comparator (15) becomes Vref ₂ = V ₀ + ΔV (2). Therefore, if the change in the voltage V ₀ is within ± ΔV, that is, if the frequency of the signal passing through the band pass filter ( 5 ) is near the center frequency f ₀ , the first and second comparators (14) and ( Both of the discrimination outputs of 15) become "H". Then, the first and second comparators (14) and (1
5) generates 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) controls to increase the current ratio i ₁ / i ₂ . Then, the transconductance ratio gm ₁ / gm ₂ increases,
As is clear from the equation (2), the bandpass filter ( 5 )
Sharpness Q becomes higher.

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

At this time, as is clear from the equation (1), when the mutual conductance ratio gm ₁ / gm ₂ is changed, if the mutual conductance product gm ₁ · gm ₂ is changed, the center frequency f ₀ becomes It changes and becomes a problem. Therefore, in the present invention,
If the mutual conductance gm ₁ is increased by n times, the mutual conductance gm ₂ is increased by 1 / n so that the product of the mutual conductances is always constant. It will be described later.

Next, when the change of the voltage V ₀ becomes large and exceeds the range of ± ΔV, the output currents of the first and second comparators (14) and (15) are stopped, and the constant current from the constant current source (16) is changed. Only is applied to the control circuit (17). Then, the control circuit (17) lowers the current ratio i ₁ / i ₂ , so that the sharpness Q of the bandpass filter ( 5 ) lowers contrary to the above case.

Therefore, the bandpass filter ( 5 ) in this case has an expanded frequency pull-in range. Thus, the sharpness Q of the bandpass filter ( 5 ) takes two values, which is shown in FIG.

FIG. 6 shows a specific circuit example of the control circuit (17) shown in FIG. 1, in which the current i ₀ shown in FIG. 1 is supplied to the first input terminal (22) and the second input terminal (23) is supplied. Is supplied with the output current of the detection circuit (13). The current i ₁ flows through the first output terminal (24) and the current i ₂ flows through the second output terminal (25). In the circuit of FIG. 6, since the currents flowing through the second input terminal (23) and the second output terminal (25) are equal,
The input current of the second input terminal (23) is also i ₂ . In the circuit of FIG. 6, when the current i ₀ of the first input terminal (22) is changed and the input current of the second input terminal (23) is constant, the base current of the transistor (26) changes. Current i ₁
You can change the value of. Further, in the circuit of FIG. 6, the following relation of current is established.

_{i 1 × i 2 = i 0} 2 ........................ (3) Therefore, a constant current i _0, the current i ₂ of the second input terminal (23) is changed, the current i ₁ is the It changes while maintaining the relationship of equation (3). Therefore, the current ratio i ₁ / i ₂ can be changed while keeping the product of the currents i ₁ and i ₂ constant.

Next, another embodiment of the detection circuit (13) of FIG. 1 will be described. In the example of FIG. 1, by providing the upper and lower reference voltages Vref ₁ and Vref ₂ with respect to the voltage V ₀ , the sharpness Q of the bandpass 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 the detection circuit in that case is shown in FIG. In FIG. 7, the input terminal (27) has a first
The output voltage of the phase detector (4) in the figure is applied, and the voltage of the reference power source (28) is the standard time voltage of the phase detector (4).
Set to V ₀ . Now, assuming that the voltage V ₀ is applied to the input terminal (27), the respective currents flowing through the transistors (29) and (30) become maximum and the voltage at the output terminal (31) becomes minimum. When the voltage of the input terminal (27) changes in the positive direction or the negative direction, the transistor (2
The current flowing in 9) or (30) decreases and the output terminal (31)
Voltage increases.

Therefore, if the detection circuit of FIG.
Can be changed linearly.

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

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a characteristic diagram for explaining the conventional automatic filter adjusting circuit, FIGS. 3 and 4 are characteristic diagrams for explaining FIG. 2, and FIG. 5 is a characteristic diagram for explaining FIG. FIG. 6 is a circuit diagram showing a concrete circuit example of the control circuit (17) in FIG. 1, and FIG.
FIG. 9 is a circuit diagram showing another embodiment of the detection circuit (13) shown in FIG. (3) …… Oscillator, (4) …… Phase detector, ( 5 ) ……
Band pass filter, (13) …… Detection circuit, (17) ……
Control circuit.

Claims (2)

[Claims] 1. A frequency according to an oscillator for generating first and second oscillation output signals having a constant phase difference, a transconductance gm ₁ of a differential amplifier and a capacitance of a capacitor C ₁ connected to the differential amplifier. A first filter whose characteristics are determined, a transconductance gm ₂ of the differential amplifier and a capacitor C ₂ connected to the differential amplifier
And a second filter whose frequency characteristic is determined by the capacitance of the filter, to which the first oscillation output signal of the oscillator is applied, and the output signal of the filter circuit and the second oscillation output signal of the oscillator. And a detection circuit for detecting the absolute value of the shifted value when the output signal of the phase detection circuit deviates from a reference value, and the detection output signal of the phase detection circuit and the detection The detection output signal of the circuit is applied and the transconductances gm ₁ and gm
the product of the m ₂ while maintaining constant the to vary the ratio of the mutual conductance gm ₁ and gm _2, change the transconductance gm ₁ first current i ₁ for changing the said transconductance gm ₂ And a control circuit for generating a second current i ₂ to be generated, wherein the sharpness Q of the filter circuit is changed. 2. The automatic filter adjusting circuit according to claim 1, wherein the detection circuit includes a current output type window comparator.