JPH0983294A - Gm-c filter circuit and gm-c filter adjusting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the Gm-C filter circuit in which a required circuit scale is reduced and the accuracy of the filter is improved and to provide the adjustment method of the Gm-C filter. SOLUTION: In order to adjust a Gm-C filter (BPF) 10, a signal with a center frequency of an ideal band pass filter is given to a reference signal input terminal 21. A phase comparator 11 provides an output of a phase lag (lead) signal to an output terminal. Then an up-down counter 12 increments (decrements) its count based on an output of the phase lag (lead) signal by one. A fine- adjustment bias current generating circuit 13 decides an output current by a count output 27 of the up-down counter 12. Since a Gm of each Gm amplifier is controlled based on a bias current supplied to each bias terminal to control a center frequency of the Gm-C filter 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Gm-C filter circuit for automatically adjusting frequency characteristics and a method for adjusting a Gm-C filter.

[0002]

2. Description of the Related Art A Gm-C filter is a time-continuous filter, unlike a sampling filter such as a switched-capacitor filter.

An example of a conventionally known Gm-C filter is shown in FIG. In FIG. 5, 50 is a Gm-C filter composed of a Gm amplifier and a capacitor, 51 is a Gm-C type low-pass filter composed of a Gm amplifier and a capacitor, 5
Reference numeral 2 is a phase comparator, 53 is a low-pass filter, and 54 and 55 are comparators. Then, the PLL circuit 56 is configured by these respective elements 51 to 55.

FIG. 6 shows a specific circuit configuration of the PLL circuit 56 shown in FIG. In FIG. 6, 61 to 64 are Gm
Amplifiers 65 and 66 are capacitors. When the input terminal is 67 and the output terminal is 68, the Gm-C filter 51 having these elements 61 to 66 has a low-pass filter characteristic, and at the same time, has a phase shift in the low range as shown in FIG. Is 0 °, the phase shift is 180 ° in the high range, and the phase shift is 90 ° at the cutoff frequency f _c . That is, when the frequency of the input signal matches the cut-off frequency f _c , the filter input signal and the filter output signal pass through the comparators 55 and 54, respectively, and further function as the phase comparator 52. By passing through (EXOR), an output signal having a so-called duty ratio of 50% in which the frequency is double that of the input signal and the periods of the high level logic and the low level logic are equal to each other is obtained. At this time, even if the signal output from the phase comparator 52 passes through the integrator 53 (LPF) which also functions as a low-pass filter, the DC output level of the integrator 53 does not change, and the phase locked state can be realized.

Assuming that the cutoff frequency f _c ′ of the filter composed of the elements 61 to 66 shown in FIG.
When it is smaller than _c , the phase delay becomes larger than the design value (= 90 °), as can be seen from FIG. As a result, the high-level logic period of the output signal of the phase comparator 52 is shorter than that of the low-level logic period, so that the output level of the integrator 53 is lowered. And integrator 5
The bias voltage generated when the output level of No. 3 is lowered increases the Gm values of all the Gm amplifiers 61 to 64. Especially Gm of Gm amplifiers 62 and 63
Since the m value determines the cutoff frequency of the Gm-C filter 51 itself, the cutoff frequency also increases as the Gm value increases. Thus, the output level of the integrator 53 shifts in the direction where the cutoff frequency of the filter 51 becomes equal to the design value, and finally the phase comparator 52
When the duty ratio of the output signal becomes 50%, that is, when the cutoff frequency of the filter 51 becomes equal to the design value _{(f c '= f c)} , the integrator output settles to a constant level. Also, when the cutoff frequency of the filter 51 is higher than the design value, the same operation is performed, and finally, the cutoff frequency of the filter 51 becomes equal to the design value, and the output of the integrator falls to a certain level.

On the other hand, the Gm-C filter 50 of FIG. 5 has exactly the same configuration as the low-pass Gm-C filter 51 used in the PLL circuit 56, and the Gm of the Gm amplifier used therein. If the value and the capacitance value are also the same, the characteristics of the filter 50 and the filter 51 are the same. However, since the Gm value of the actually configured Gm amplifier cannot be realized as the design value due to the variation between the elements of the MOSFET, an error occurs between the filters.

Here, the circuit configuration of the filter 50 is shown in FIG. The circuit configuration shown in FIG. 9 is exactly the same as that of the Gm-C filter 51 of FIG. The cutoff frequency of the filter 50 is proportional to the geometric mean of the Gm values of the Gm amplifiers 92 and 93. Similarly, the cutoff frequency of the filter 51 (see FIG. 6) is proportional to the geometric mean of the Gm values of the Gm amplifiers 62 and 63. Assuming that the Gm of the filter 51 (see FIG. 6) is
The geometric mean value of the Gm values of the amplifiers 62 and 63 is calculated by the filter 5
When the Gm value of the Gm amplifiers 92 and 93 of 0 (see FIG. 9) is larger by 1% than the geometric average, the cutoff frequency of the filter 51 is higher by 1% than that of the filter 50.

Even if the Gm amplifiers are designed in exactly the same manner as described above, an error occurs between the Gm values due to a process problem, so that the characteristic of the filter 50 (see FIG. 9) is the same as that of the filter 5.
1 (see FIG. 6) does not completely match. Moreover, this error is due to the SCF frequently used in LSI.
Since it is larger than (switched capacitor filter), it was difficult to put it to practical use.

[0009]

As a method of solving such a problem, there is known a method of adjusting the characteristics required for a filter by using a correction method using an adaptive filter. As a correction method using this adaptive filter, for example, KARAN A. IEEE, CI by KOZMA et al.
RCUITS AND SYSTEMS 1991/1
A method of publishing the January issue on page 1241 is known.

FIG. 10 shows the configuration of a Gm-C filter circuit using the above adaptive filter method. In the figure, 121 is an ideal input signal source, 122 is a Gm-C filter, 123 to 126 are gradient filters, 127 is an ideal output signal source, 128 to 131 are multipliers, and 132 to 135 function as cumulative adders. The integrator 136 is a subtractor.

Here, the gradient filters 123 to 126 are used for generating gradient coefficients (described in each block) necessary for correcting each coefficient of the filter. Is a predetermined output of the Gm-C filter 122. The circuit configuration of each gradient filter is basically the same as that of the Gm-C filter.

In order to update the variables of the Gm-C filter 122, a signal is input to the Gm-C filter 122,
The output is compared with an ideal output signal to operate so that the error signal ε becomes zero.

However, when a circuit for performing such an operation is configured, there is a problem that the circuit scale becomes extremely large.

Accordingly, an object of the present invention is to provide
It is an object of the present invention to provide a Gm-C filter circuit and a method for adjusting a Gm-C filter in which the required circuit scale is reduced and the filter accuracy is improved.

[0015]

In order to achieve the above object, a Gm-C filter circuit according to the present invention is a band pass type having a Gm amplifier having a control terminal for controlling a Gm value and a capacitor. A Gm-C filter, a phase comparison means for comparing a phase relationship between a reference signal input to the Gm-C filter and a signal output from the Gm-C filter, and output from the phase comparison means. And a control signal generator for supplying a control signal for controlling the Gm value to the control terminal based on a count output of the up / down counter in response to a comparison signal And means. In addition, the reference signal input to the Gm-C filter is switched to a processing input signal, and an output signal from the Gm-C filter is input to a predetermined output end without being input to the phase comparison means. It is also possible to adopt a configuration provided with a switching means for guiding. here,
The control signal generating means includes a current source that generates a reference bias current for controlling the Gm value, a current source that generates a fine adjustment bias current corresponding to the count output of the up / down counter, and the reference bias. It is preferable to employ a configuration including an adder that adds a current and the fine adjustment bias current.

Further, the Gm-C filter adjusting method according to the present invention is provided with a control terminal for controlling the Gm value.
In adjusting the frequency characteristic of a bandpass Gm-C filter having an m amplifier and a capacitance, a phase between a reference signal input to the Gm-C filter and a signal output from the Gm-C filter. The relationships are compared, the up / down counter is counted up or down in response to a comparison signal obtained as a result of the comparison, and the G
A control signal for controlling the m value is supplied to the control terminal. After the adjustment of the frequency characteristic of the Gm-C filter is completed, the reference signal input to the Gm-C filter is switched to the processing input signal, and the output signal from the Gm-C filter is immediately output. It shall be led to a predetermined output terminal. Here, when a control signal for controlling the Gm value is supplied to the control terminal, a current source for generating a reference bias current for controlling the Gm value and a count output of the up / down counter are supported. A current source that generates a bias current for fine adjustment,
It is preferable to use an adder that adds the reference bias current and the fine adjustment bias current, and supply the addition bias current obtained from the adder to the control terminal.

According to the present invention described above, the center frequency is adjusted and controlled so that the frequency characteristic (specifically, phase shift) of the Gm-C filter becomes equal to that of the ideal filter at the ideal center frequency. The accuracy of the frequency characteristics of the filter can be improved.

[0018]

DETAILED DESCRIPTION OF THE INVENTION An example of an embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 shows a block diagram of a Gm-C filter circuit to which the present invention is applied. In FIG. 1, 10 is Gm
Gm-C filter consisting of amplifier and capacitor, 11 is G
A phase comparator for comparing the phase of the input signal 23 of the m-C filter 10 and the phase of the output signal 24 of the Gm-C filter 10, and 12 is a count value that increases or decreases according to the polarity of the output of the phase comparator 11. Up / down counter. That is, when the output of the phase comparator 11 is positive, it counts up, and when it is negative, it counts down.

Reference numeral 13 is a fine adjustment bias current generating circuit in which an output current value (bias current value) 30 is determined according to the output signal 27 of the up / down counter 12, and 14 is a self-adjustment (feedback) of the frequency characteristic of the Gm-C filter 10. Bias current generating circuit (PLL circuit) for generating a reference bias current I _PLL required for control), and 15 is a bias current generating circuit 14 and a fine adjustment bias current generating circuit 13.
_Is an adder for adding each of the output currents 29 (I _PLL ) and 30 (I _TUNE ). Then, the output current 31 (I _FIL ) of the adder 15 is supplied to a bias terminal for adjusting the frequency characteristics of the Gm-C filter 10.

Reference numeral 20 is an input terminal for introducing the input signal of the filter 10, 21 is a reference signal input terminal for introducing a reference signal for self-adjusting the frequency characteristic of the filter 10, 16
And 17 are switches for switching a signal input to the Gm-C filter 10, 22 is an output terminal for outputting an output signal of the filter 10, and 18 and 19 are filters 10
Is a switch for switching between outputting the output signal to the output terminal and outputting the output signal to the phase comparator 11, and a terminal 28 for applying a reference clock signal to be input to the bias current generating circuit 14.

Next, the operation of the Gm-C filter shown in FIG. 1 will be described.

Bandpass filter (Gm-C filter) 1
FIG. 2 shows the frequency characteristics of 0. Here, FIG. 2A shows frequency-gain characteristics. FIG. 2 (B) is a frequency-phase characteristic, as shown in FIG., The phase has a property that becomes zero at the center frequency f _o. That is, the input signal frequency band pass filters equal to the center frequency f _o (hereinafter referred to as BPF), the output signal of the phase is consistent with the input signal.

First, in order to adjust the Gm-C filter (BPF) 10, a signal of the center frequency of the ideal bandpass filter is given to the reference signal input terminal 21 as a training signal. At this time, the switches 17 and 19 are turned on, and the switches 16 and 18 are turned off. If the center frequency of the BPF 10 is smaller than the ideal value, the output phase will lag behind the ideal value. Therefore, the phase comparator 11 outputs the signal as the phase delay to the phase delay output terminal. Then
The up / down counter 12 increments the count value of the counter 12 by 1 based on the phase delay signal output. For example, when the initial value of the counter 12 is “0”, the output of the counter 12 is “1” based on the output of the phase comparator 11.

The output current of the fine adjustment bias current generating circuit 13 is determined by the count output 27 of the counter 12.
For example, when the count value of the counter 12 is "N", the output current I _TUNE is given by the following equation.

[0026]

I _TUNE = N × I _ref (1) where I _ref is a minute reference current proportional to the current generated by the bias current generating circuit 14, and N is a count value of the counter 12. That is, N ranges from the minimum value N _min to the maximum value N
the N _min = -N _max as an integer value in the range of _max. Since the output current 31 (I _FIL ) of the adder 15 adds the output current I _PLL of the bias current generation circuit 14 and the output current I _TUNE of the fine adjustment bias current generation circuit 13, the output current I _PLL
FIL is given by the following equation.

[0027]

## EQU2 ## I _FIL = I _PLL + I _TUNE (2) In this case, since I _TUNE increases due to the phase delay, the output current I _FIL also increases. Thus, the frequency characteristics of the Gm-C filter 10 are controlled by the output bias current I _FIL .

Now, it is assumed that the Gm-C filter 10 is designed so that the center frequency increases as the bias current _IFIL increases. Then, the phase lag of the Gm-C filter 10 is reduced by such an operation.
By repeating such a counter operation, the input / output phase difference of the Gm-C filter 10 finally becomes zero at the reference input signal frequency, and as a result, the center frequency finally matches that of the ideal filter.

Further, temporarily, the Gm-C filter (BPF) 1
When the center frequency of 0 is larger than the ideal value, the output phase leads the ideal value. Therefore, the phase comparator 11 outputs the signal as the phase lead to the phase lead output terminal. The up / down counter 12 decrements the count value of the counter 12 by 1 based on the phase lead signal output.
For example, if the initial value of the counter 12 is "0", the count value of the counter 12 becomes "-1" based on the output of the phase comparator 11.

As a result, the output of the counter 12 becomes small, the output current I _{TUNE of the} fine adjustment bias current generating circuit determined based on the output of the counter 12 becomes small, and the output current I _FIL from the adder 15 also becomes small. . Due to the decrease in the output current I _FIL , the center frequency of the Gm-C filter 10 decreases.

By repeating the same operation of the up / down counter 12, the input / output phase difference of the Gm-C filter 10 becomes zero at the reference input signal frequency,
As a result, the center frequency finally matches that of the ideal filter.

FIG. 3 shows the Gm-C filter 1 shown in FIG.
A circuit configuration of 0 is shown. Here, 37 to 40 are Gm amplifiers,
Reference numerals 41 and 42 denote capacitances, and the Gm-C filter 10 is constituted by these elements 37 to 42. And
The output bias current I _FIL from the adder 15 is supplied to each Gm value control bias terminal of the Gm amplifier.
Each G is controlled by a bias current supplied to each bias terminal.
By controlling the Gm value of the m amplifier, the center frequency of the Gm-C filter 10 is controlled. In addition, M
The OSFET 43 is an element of the current mirror circuit and converts the current value I ₁ into a voltage value (a current I ₂ proportional to I ₁ flows through the MOSFET 44: see FIG. 4).

FIG. 4 shows a detailed circuit configuration of each Gm amplifier shown in FIG. Here, 44 is the MOSFET of FIG.
MOS forming a current mirror circuit in pair with 43
A FET, acts as a current source formed by the bias current control circuit, Gm value of each Gm amplifier is determined by the current value I _2. Specifically, the Gm value is given by the following equation.

[0034]

Gm = 2 (I ₂ K) ^0.5 (3) Accordingly, when the current I _1, that is, the bias current I _FIL from the adder 15 increases, the Gm value increases, and as a result, the center frequency increases.

Further, 45 and 46 are input MOSFETs for receiving positive and negative signals at their gates, 47 and 48 are load MOSFETs, and a signal for in-phase signal adjustment is applied to each gate terminal.

Note that the present invention is not limited to the configuration of the Gm amplifier shown in FIG. 4, but can of course be applied to other general Gm amplifiers.

[0037]

As described above, in the present invention, the frequency characteristic of the Gm-C filter is self-adjusted by controlling the Gm value of the Gm amplifier, so that a filter having excellent frequency characteristic accuracy can be realized. Can be.

On the other hand, in the conventional circuit, a MOSFET having a large channel length and a large channel width was used in order to improve the relative accuracy, but this increased the chip size. By implementing the present invention with respect to such a conventional circuit, it becomes possible to obtain a filter having a small chip size as a whole and high accuracy.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a Gm-C filter to which the present invention is applied.

FIG. 2 is a diagram showing frequency characteristics of a Gm-C filter to which the present invention is applied.

3 is a diagram showing a specific circuit example of the Gm-C filter shown in FIG.

FIG. 4 is a diagram showing a circuit diagram of each Gm amplifier shown in FIG.

FIG. 5 is a block diagram showing an example of a conventionally known Gm-C filter.

6 is a diagram more specifically showing the PLL circuit 56 shown in FIG.

7 is a diagram showing a phase characteristic of the Gm-C filter 51 shown in FIG.

8 is a diagram showing a phase characteristic of the Gm-C filter 51 shown in FIG.

9 is a detailed circuit diagram of the Gm-C filter 50 shown in FIG.

FIG. 10 is a diagram showing an example of a Gm-C filter using a conventional adaptive filter.

[Explanation of symbols]

 10 Gm-C filter 11 Phase comparator 12 Up / down counter 13 Fine adjustment bias current generation circuit 14 Bias current generation circuit 15 Adder

Claims (6)

[Claims] 1. A band-pass Gm-C filter having a Gm amplifier having a control terminal for controlling a Gm value and a capacitor; a reference signal input to the Gm-C filter;
Phase comparison means for comparing the phase relationship with the signal output from the -C filter, and in response to the comparison signal output from the phase comparison means,
An up / down counter for counting up or down and a control signal generating means for supplying a control signal for controlling the Gm value to the control terminal based on the count output of the up / down counter. And G
m-C filter circuit. 2. The method according to claim 1, wherein the reference signal input to the Gm-C filter is switched to a processing input signal, and an output signal from the Gm-C filter is added to the phase comparison means. A Gm-C filter circuit comprising switching means for leading to a predetermined output end without inputting to the Gm-C filter circuit. 3. The control signal generating means according to claim 1, wherein the current source generates a reference bias current for controlling the Gm value, and a fine adjustment bias current corresponding to the count output of the up / down counter. A Gm-C filter circuit, comprising: a current source for generating the above; and an adder for adding the reference bias current and the fine adjustment bias current. 4. A reference signal input to the Gm-C filter when adjusting frequency characteristics of a bandpass Gm-C filter having a Gm amplifier having a control terminal for controlling a Gm value and a capacitor. And the Gm
-Compare the phase relation with the signal output from the C filter, and count up or down the up-down counter in response to the comparison signal obtained as a result of the comparison, and output the count output of the up-down counter. Based on the above, a control signal for controlling the Gm value is supplied to the control terminal. 5. The method according to claim 4, wherein after the adjustment of the frequency characteristic of the Gm-C filter is completed, the reference signal input to the Gm-C filter is switched to the processing input signal and the Gm-C filter is input. Gm-C characterized in that an output signal from the C filter is immediately led to a predetermined output terminal.
How to adjust the filter. 6. The current source for generating a reference bias current for controlling the Gm value when supplying a control signal for controlling the Gm value to the control terminal, according to claim 4, A current source that generates a fine adjustment bias current corresponding to the count output of the counter and an adder that adds the reference bias current and the fine adjustment bias current are used, and the added bias current obtained from the adder is G which is supplied to the control terminal
Adjustment method of m-C filter.