JP2003198326A - Complex band-pass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a distortion in a filter pass band which is caused by a frequency characteristic in OTA. <P>SOLUTION: A complex band-pass filter includes trans-conductance elements for respectively shifting the central frequency of the pass band. At least two or more trans-conductance elements are connected in parallel with little trans- conductance in each element. Besides, trans-conductance is made to be same in the whole trans-conductance elements. Thus, deterioration in the whole higher harmonic distortion of an output signal with respect to an input signal is improved. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a complex bandpass filter, and more particularly to a complex bandpass filter used in a Low-IF type receiver or the like for suppressing an image signal and performing channel selection.

[0002]

2. Description of the Related Art In recent years, with the spread of portable wireless terminals,
Instead of the conventional super-heterodyne receiving method, L which sets the intermediate frequency (IF) to several MHz or less
The ow-IF method is drawing attention. If this Low-IF system is adopted, an external IF filter having a large external dimension, which is required in the above-mentioned super-heterodyne system, can be removed, and the advantage that the receiving unit can be made into one chip and the cost can be reduced can be obtained. .

However, in this Low-IF system, suppression of an image signal is essential because the reception frequency (RF) and the frequency of the local oscillator are close to each other. As a kind of means for suppressing this image signal, there is one using an operational transconductance amplifier (hereinafter referred to as OTA), which is, for example, IEICE TRANS. FUNDAMENTALS, Vo.
l. E80-A, No. 9, September 1997, pages 1721-17
Xiaoxing ZHANG's paper, "Implement," published on page 24
ation of Active Complex Filter with Variable Param
eter Using OTAs "etc.

The OTA is usually represented by the symbol shown in FIG. 3, and both the input and output impedances are ideally infinite, and the output current I _out is given by the following equation.

I _out = gm · V _in (1) where V _in is the differential input voltage and gm is the output current I
It is the transconductance for _out , and ideally it is always constant with respect to the input voltage.

FIG. 4 shows the structure of the first-order complex bandpass filter described in the above paper. In this filter, the input signal X _R of the in-phase component of quadrature modulation is input, and g
a first OTA1 having a transconductance of m3a,
The output side of the first OTA1 is connected, and a third OTA3 having a transconductance of gm1a is provided. The third OTA3 outputs the output signal Y _{R of the in-} phase component. On the other hand, the input signal X _I of the quadrature component of quadrature modulation is input, the second OTA2 having the transconductance of gm3b and the output side of this second OTA2 are connected, and g
A fourth OTA4 having a transconductance of m1b is provided, and an output signal Y of an orthogonal component is output from the fourth OTA4.
_I is output.

Further, between the connection point A of the first OTA1 and the third OTA3 and the connection point B of the second OTA2 and the fourth OTA4, the fifth Oth is provided to perform a frequency shift function.
TA5 and sixth OTA6 are provided, and the fifth OTA5
Has a transconductance of gm2a, and the 6th OT
A6 has a transconductance of gm2b. Further, the first capacitor 7 having the capacitance Ca is connected between the connection point A and the ground, and the second capacitor 8 having the capacitance Cb is connected between the connection point B and the ground.

The transfer function H (s) [s: complex variable] of such a first-order complex bandpass filter is gm1a = gm1b = in the above transconductance.
gm1, gm2a = gm2b = gm2, gm3a = gm
When 3b = gm3 and Ca = Cb = C, it is given by the following equation.

[0009]

[Equation 1] In addition, j is an imaginary unit and j ² = −1.

According to such a complex bandpass filter, the input signal X _R of the in-phase component has a gain determined by the transconductance gm3a and a capacitance C.
a and a transconductance gm1a, the output signal Y _{R of the} passband width is obtained, and the input signal X _I of the quadrature component is the gain determined by the transconductance gm3b and the capacitance Cb and the transconductance gm1b. Output signal Y _{I of} determined pass bandwidth
Is obtained. Then, the capacitors Ca and Cb and the fifth OTA
Transconductance gm2a of the fifth and sixth OTA6
And the frequency is shifted in the positive direction by an amount determined by gm2b.

FIG. 5 shows a frequency characteristic in which the horizontal axis represents the angular frequency ω. According to the above complex bandpass filter, for example, the frequency characteristic 100 having a bandwidth of −ω ₀ to + ω ₀ is shown. From the center frequency ω
It can be shifted by _c , which results in a filter that passes positive frequencies but not negative frequencies.
Then, a high-order complex bandpass filter is formed by cascading such first-order complex bandpass filters.

FIG. 6 shows the structure of a fourth-order Butterworth type complex bandpass filter, which has a center frequency of 2 MHz and a pass band width of 1 MHz, for example. That is, the illustrated first filter 11 has gm1a = g
m1b = 29 μS (Siemens), gm2a = gm2b
= 113.6 μS, gm3a = gm3b = 31.4 μ
S, Ca = Cb = 10 pF, and the second
The filter 12 has gm1a = gm1b = 29 μS, gm1
2a = gm2b = 137.7 μS, gm3a = gm3b
= 31.4 μS, Ca = Cb = 10 pF, and the third filter 13 has gm1a = gm1b = 12μ.
S, gm2a = gm2b = 154.7 μS, gm3a =
The values of gm3b = 31.4 μS and Ca = Cb = 10 pF are set, and the fourth filter 14 at the final stage has gm1a = g.
m1b = 12 μS, gm2a = gm2b = 96.6 μ
S, gm3a = gm3b = 31.4 μS, Ca = Cb =
The value is set to 10 pF.

FIG. 7 shows the frequency characteristics of the fourth-order Butterworth type complex bandpass filter shown in FIG. 6. As shown in the drawing, the characteristic of the bandwidth for passing a positive frequency with a center frequency of 2 MHz is shown. can get.

Usually, the input voltage has a large value of several hundred mV, and within this range, the output current of the OTA and the input voltage must have a linear relationship. That is, as shown in FIG. 8, the transconductance of each OTA is required to be a straight line passing through the origin.

[0015]

However, in reality, as the transconductance of the first OTA1 to the sixth OTA6 becomes larger, the linear region of the input voltage-output current characteristic becomes narrower, and further, the amplitude of the input voltage becomes smaller. As becomes larger, the deviation from the ideal input voltage-output current characteristic becomes larger. In particular, the transconductances of the fifth OTA5 and the sixth OTA6 are
Therefore, as compared with the transconductance of the fourth OTA4, the value generally has a large value, so that the linear region becomes narrow and the total harmonic distortion characteristic of the output signal with respect to the input signal deteriorates.

FIG. 9 shows the actual input voltage of the fifth OTA5 having the transconductance of gm2a and the sixth OTA6 having the transconductance of gm2b included in the fourth-order Butterworth type complex bandpass filter of FIG.
A comparison between the output current characteristic and the ideal input voltage-output current characteristic is shown. The linear region becomes narrower as the transconductance becomes larger, and the transconductance becomes larger as the input voltage amplitude becomes larger. The deviation from the ideal input voltage-output current characteristics is large.

O having these input voltage-output current characteristics
FIG. 10 shows the total harmonic distortion ratio of the output signal when the frequency of the input signal of the fourth-order Butterworth type complex bandpass filter using TA is 2 MHz. From FIG. 10, the maximum value of the input signal voltage with a total harmonic distortion ratio of 1% or less is 440 mV _pp.
It can be seen that it is.

The present invention has been made in order to solve the above problems, and its purpose is to output an input signal due to the input voltage-output current characteristic of the transconductance of the fifth OTA5 and the sixth OTA6. It is an object of the present invention to provide a complex bandpass filter which can favorably improve the deterioration of the total harmonic distortion of a signal.

[0019]

In order to achieve the above object, the invention according to claim 1 provides a first transconductance element which receives an in-phase component signal of quadrature modulation and determines an output gain of the in-phase component signal. A first capacitor connected between the output terminal of the first transconductance element and ground, for inputting the output of the first transconductance element, and a first capacitor for determining the passband width of the in-phase component signal; An in-phase component output side element that determines the pass band width of the in-phase component signal by combination with a capacitor, a second transconductance element that receives the quadrature component signal of quadrature modulation and determines the output gain of this quadrature component signal, and this second transformer A second capacitor connected between the output terminal of the conductance element and the ground for determining the pass band width of the quadrature component signal. A quadrature component output side element for inputting the output of the second transconductance element and determining a pass band width of the quadrature component signal in combination with the second capacitor, the first transconductance element and the in-phase component output side element And a connection point between the second transconductance element and the connection point on the quadrature component output side, and includes a third transconductance element and a fourth transconductance element for shifting the center frequency of the pass band. In the complex bandpass filter provided with the frequency shifting element, each of the third transconductance element and the fourth transconductance element is configured by connecting at least two transconductance elements in parallel, and The third transconductance element It is characterized in that the total of the transconductances of at least two or more constituent transconductance elements is equal to the total of the transconductances of at least two or more transconductance elements forming the fourth transconductance element. To do.

The invention according to claim 2 is characterized in that at least two or more transconductance elements connected in parallel have the same transconductance.

According to the above arrangement, the individual transconductances of the transconductance elements forming the frequency shifting element can be reduced, so that the total harmonic distortion of the input signal due to the input voltage-output current characteristic can be reduced. It is possible to suppress deterioration. Then, by setting the total harmonic distortion for this input signal to the value set forth in claim 2, the deterioration of the total harmonic distortion can be significantly suppressed.

[0022]

1 shows the structure of a complex bandpass filter according to an embodiment of the present invention. This filter has a transconductance of gm3a and quadrature modulation as in the case of FIG. The third OTA 33, g having the transconductance of the first OTA 31, gm1a for inputting the input signal X _R of the in-phase component, and for outputting the output signal Y _R of the in-phase component, which is arranged after the first OTA 31.
A second OTA 32, gm, which has a transconductance of m3b and which inputs an input signal XI of a quadrature component of quadrature modulation
The second OTA with a transconductance of 1b
A fourth OTA 34, which is arranged in the subsequent stage of 32 and outputs the output signal YI of the orthogonal component, is provided. In addition, the first OT
Between the connection point A of A31 and the third OTA33 and the ground,
The first capacitor 39 having a capacitance Ca is connected to the second O
The second capacitor 40 having the capacitance Cb is connected between the connection point B of the TA 32 and the fourth OTA 34 and the ground.

Furthermore, between the connection point A and the connection point B, a seventh OTA 35 having a transconductance of gm2c and an eighth OTA 36 having a transconductance of gm2d are connected in parallel as a frequency shift element, And a ninth OTA 37 having a transconductance of gm2e and a tenth OTA 38 having a transconductance of gm2f are connected in parallel and provided, respectively.

FIG. 11 shows an example of a multi-input adder circuit in which two OTAs having different transconductance values are connected in parallel. V _i1 is input to the non-inverting input terminal of the OTA 21 having a transconductance of gm7, V _i2 is input to the inverting input terminal thereof, and V _i is input to the non-inverting input terminal of the OTA 22 having a transconductance of gm8.
_i3 and V _i4 are input to the inverting input terminal, respectively,
21 output and OTA22 are connected, output current I
₀ is represented by the following equation.

I ₀ = (V _i1 −V _i2 ) gm7 + (V _i3 −V _i4 ) gm8 (3) Here, the inverting input terminals of the OTA 21 and OTA 22 are connected to the ground, and the non-input terminals of the OTA 21 and OTA 22 are not connected. Input the signal of V _i1 = V _i2 = V _i from the inverting input terminal, and
21 and OTA22 have transconductance g
If m7 and gm8 are equal to gm7 = gm8 = gm7 ′, the above formula (3) becomes I ₀ = (2 · gm7 ′) · V _i (4).

According to the present invention, the fifth OTA 5 and the sixth OTA 6 for shifting the center frequency of the pass band of the complex band pass filter shown in FIG. 4 are configured by connecting two OTAs in parallel as shown in FIG. Is replaced by and connected.

A first-order complex bandpass filter as shown in FIG. 1 is formed by cascade connection as in FIG. 6, and a center frequency of the fourth-order Butterworth type complex bandpass filter is 2 MHz.
In order to set the pass band width to 1 MHz, gm1a = gm1b = 29 μS (Siemens) of the first filter, and from the equation (3), gm2c + gm2d = 113.6 μS, g
m2e + gm2f = 113.6 μS, gm3a = gm3
b = 31.4 μS, Ca = Cb = 10 pF, and the second filter in the next stage has gm1a = gm1b = 29.
μS, from the formula (3), gm2c + gm2d = 137.7
μS, gm2e + gm2f = 137.7 μS, gm3a
= Gm3b = 31.4 μS, Ca = Cb = 10 pF, and the third filter has gm1a = gm1b = 1.
2 μS, and from the formula (3), gm2c + gm2d = 154.
7 μS, gm2e + gm2f = 154.7 μS, gm3
a = gm3b = 31.4 μS, Ca = Cb = 10 pF,
The fourth filter at the final stage has gm1a = gm1b = 12μ
S, from equation (3), gm2c + gm2d = 96.6μ
S, gm2e + gm2f = 96.6 μS, gm3a = g
The values of m3b = 31.4 μS and Ca = Cb = 10 pF are set.

Here, the seventh OTA35 and the eighth OTA3
Since the sixth, ninth OTA 37, and tenth OTA 38 can be formed by elements having small transconductance, the linear region range of the input voltage-output current characteristic can be widened. As a result, it is possible to widen the linear region range as a whole. Also, if the number of OTAs connected in parallel is increased to three or four, the OTAs connected in parallel are
The transconductance of each can be further reduced, and the linear region can be further expanded.

Furthermore, the seventh OTA3 connected in parallel
5, 8th OTA36, 9th OTA37, 10th OTA3
By making the transconductances of 8 the same, the range of the linear region can be maximized. Therefore, gm2c = gm2d = gm2e = gm2f = 56.8 of the first filter.
μS, gm2c of the second filter = gm2d = gm2e =
gm2f = 68.9 μS, gm2c of the third filter = g
m2d = gm2e = gm2f = 77.4 μS, gm2c = gm2d = gm2e = gm2f = 4 of the fourth filter
The frequency of the input signal when set to a value of 8.3 μS is 2M
The total harmonic distortion of the output signal at Hz is shown in FIG. In FIG. 2, the dotted line is the conventional example and the solid line is the embodiment of the present invention. From Figure 2, confirm that the total harmonic distortion factor has increased from the maximum value of 1% or less of the input signal voltage is about 440 mV _pp to about 700 mV _pp, is a problem that a linear region is narrowed is resolved it can.

[0030]

As described above, according to the present invention,
In the complex bandpass filter, in order to shift the center frequency of the pass band, at least two or more transconductance elements are provided in parallel, so that the transconductance input of the frequency shift element is provided. It is possible to satisfactorily improve the deterioration of the total harmonic distortion of the output signal with respect to the input signal due to the voltage-output current characteristic, and it is possible to obtain a high-order complex bandpass filter with low distortion.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a complex bandpass filter according to an embodiment of the present invention.

FIG. 2 is a diagram showing a total harmonic distortion rate of a complex bandpass filter according to an example of the present invention.

[Fig. 3] Operational transconductance
It is a figure which shows the symbol example of an amplifier.

FIG. 4 is a circuit diagram showing a configuration of a conventional complex bandpass filter.

FIG. 5 is an explanatory diagram showing frequency shift in a complex bandpass filter.

FIG. 6 is a diagram showing a configuration of a fourth-order Butterworth type complex bandpass filter in which complex bandpass filters are cascade-connected.

FIG. 7 is a diagram showing frequency characteristics in the complex bandpass filter of FIG.

FIG. 8 is an ideal input voltage of the complex bandpass filter −
It is the figure which showed the output current characteristic.

FIG. 9 is a diagram showing a deviation from an ideal of an input voltage-output current characteristic generated in a complex bandpass filter.

FIG. 10 is a diagram showing a total harmonic distortion rate of a conventional complex bandpass filter.

FIG. 11 is a diagram showing a multi-input adder circuit including transconductance elements having different transconductances.

[Explanation of symbols]

1 31 ... 1st OTA 2, 32 ... Second OTA 3, 33 ... 3rd OTA 4, 34 ... 4th OTA 5 ... Fifth OTA 6 ... 6th OTA 7, 9 ... First capacitor 8, 40 ... Second capacitor 35 ... 7th OTA 36 ... Eighth OTA 37 ... 9th OTA 38 ... 10th OTA

Claims (2)

[Claims] 1. A first transconductance element for inputting an in-phase component signal of quadrature modulation and determining an output gain of the in-phase component signal, and an output terminal of the first transconductance element and a ground are connected, A first capacitor for determining a pass band width of the component signal;
Input the output of the transconductance element,
An in-phase component output side element that determines the pass band width of the in-phase component signal by combination with a capacitor, a second transconductance element that receives the quadrature component signal of quadrature modulation and determines the output gain of this quadrature component signal, and the second transconductance element A combination of a second capacitor which is connected between the output terminal of the transconductance element and the ground and which determines the pass band width of the quadrature component signal, and the output of the second transconductance element which is input to the second capacitor. An element on the output side of the quadrature component that determines the pass band width of the quadrature component signal by
A first connecting point between the first transconductance element and the in-phase component output side element and a second connecting point between the second transconductance element and the quadrature component output side for shifting the center frequency of the pass band; In a complex bandpass filter provided with a frequency shifting element composed of a third transconductance element and a fourth transconductance element, each of the third transconductance element and the fourth transconductance element has at least two transconductances. The elements are connected in parallel, and the total transconductance of at least two transconductance elements forming the third transconductance element is at least two forming the fourth transconductance element. Complex band-pass filter characterized by being configured to be equal to the sum of the transconductance of the transconductance element of the above. 2. The complex bandpass filter according to claim 1, wherein at least two or more transconductance elements connected in parallel have the same transconductance.