CN1983808A - Bandwith-adjusting low-channel filter - Google Patents

Abstract

The invention is concerned with the low pass filter that the bandwidth of the pass band is changeable, the characteristic is: the filter is consists of the low pass filter with fixed bandwidth, the decoding circuit and the accessional capacitance; the decoding circuit creates controlling signal by inputting signal decoding, the controlling signal connects and sends to the controlling port of the accessional capacitance to control the accessional capacitance whether or not parallel connects with the capacitance of the low pass filter with fixed bandwidth, the accessional capacitance weather or not is parallel connection and the quantity of the parallel connection capacitance group can make capacitance value of the filter to change, in order to change the system function parameter of the filter, achieve to adjust the bandwidth of the low pass filter, and solve the shortage of the low pass filter with fixed pass band in the act application.

Description

The low pass filter that a kind of pass band width is adjustable

Technical field

The invention relates to the integrated low pass filter of a kind of simulation, specifically is the adjustable low pass filter of a kind of pass band width.

Background technology

Filter can be divided into two kinds of analog filter and digital filters by implementation method, can be divided into three kinds of low pass filters, high pass filter, band pass filter by function.Simulation low-pass filter since device use few, but the elimination high-frequency noise has comparatively mature C MOS switched capacitor technique realize etc. being widely used in advantage in all kinds of analog circuitry systems.

It is zero frequency range (stopband) that ideal low-pass filter has the frequency range (passband) of a finite gain and a gain.But according to Circuit theory, this filter is impossible realize.Actual low pass filter has passband and stopband that gain is very little of a finite gain, also has a transition band that passband and stopband are separated.Fig. 1 is the spectrum diagram of low pass filter, and wherein T (jw) is the transfer function of low pass filter, W _pBe cut-off frequecy of passband, W _sBe stopband cut-off frequency, W _pAnd W _sBetween be exactly transition region, transitional region is more little, the performance of filter is just good more.

But at present, the low pass filter in the application system has only one by frequency, promptly has only a fixing pass band width, and this is extremely inconvenient in many systems.For example, when the frequency of the input clock signal of a system has improved, if the pass band width of filter can not improve accordingly, whole system will go wrong.

Summary of the invention

Has only a kind of fixed passband frequency deficiency in actual applications in order to overcome existing low pass filter, the invention provides the adjustable low pass filter of a kind of pass band width, it is characterized in that: this filter is by fixed bandwidth low pass filter, and decoding circuit and additional capacitor are formed; Input signal produces control signal through decoding circuit decoding, control signal connects the control port that sends to additional capacitor, whether the control additional capacitor is connected in parallel on the electric capacity of fixed bandwidth low pass filter, the additional capacitor parallel connection whether and the size of shunt capacitance appearance value the capacitance of this filter is changed, thereby change the system function parameter of filter, realize regulating the bandwidth of low pass filter.

Additional capacitor of the present invention by a plurality of electric capacity by and the capacitance group formed of the mode that connects, its capacitance group quantity equals the progression of fixed bandwidth low pass filter.

Each electric capacity of forming additional capacitor all has a control port, and this port is connected the output of decoding circuit, is controlled on its electric capacity that whether is connected in parallel on fixed bandwidth low pass filter by the control signal that decoding circuit produces.

The resistance of filter of the present invention is simulated by switch and electric capacity, and switch is the cmos analog switch that a pair of NMOS, PMOS pipe are formed in parallel.

Description of drawings

Fig. 1 is the spectrogram of low pass filter

Fig. 2 is the circuit theory diagrams of preferred embodiment of the present invention

Fig. 3 is the circuit diagram of switch of the present invention and capacitance simulation resistance

Fig. 4 is the structure chart of single additional capacitor

Fig. 5 is the structure chart of decoding circuit

Embodiment

Accompanying drawing 2 is the circuit theory diagrams of preferred embodiment of the present invention, and this circuit has three major parts: a decoding circuit, realize 2 supplied with digital signal are decoded into 3 control signals; One is complete, the low pass filter with three rank fixed passband bandwidth, and an additional capacitor includes three groups of electric capacity, and every group of electric capacity is formed in parallel by three electric capacity, and 2 the port A and the B of additional capacitor are connected in parallel on the electric capacity of fixed-bandwidth filter.The operation principle of circuit is such: as shown in Figure 5, FH and FL are two supplied with digital signal, they are input in the decoding circuit, produce 3 control signal F0, F1, F2, these three control signals are connected in turn on the control port Ct1 of every group of additional capacitor, and whether the control additional capacitor is parallel on the filter.For example, if F0=1, F1=0, F2=0.C1a then, c2a, c3a is connected in parallel on C1, and C2 is on the C3.The appearance value of electric capacity is because c1a, c2a, and incorporating into of c3a and increasing, therefore the system function of filter also just changes, so the pass band width of filter has just changed.

For the pass band width of more detailed description filter is how to change with electric capacity, we carry out quantitative analysis.Filter in the preferred embodiment circuit theory diagrams of the present invention is the low pass filter of one three rank bandwidth, and we list nodal voltage equation according to Fig. 2:

$\left\{\begin{array}{c}\frac{\mathrm{Vi}}{\mathrm{Ri}}=-\frac{V1}{R}-\frac{V1}{X1}-\frac{V2}{R}\\ -\frac{V1}{R}=-\frac{V2}{X2}-\frac{V0}{-R}\\ \frac{V2}{R}=-\frac{V0}{R}-\frac{V0}{X3}\end{array}\right.---\left(1\right)$

Solve an equation, obtain

$\frac{R}{\mathrm{Ri}}\mathrm{Vi}=-(1+{\mathrm{RC}}_{1}S)({\mathrm{RC}}_2{\mathrm{<figure-callout\; id="2"\; label="SV"\; filenames="A20051010227500111.png"\; state="\{\{state\}\}">SV</figure-callout>}}_2+V_0)-{\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="A20051010227500111.png"\; state="\{\{state\}\}">V</figure-callout>}}_2$

$=(1+{\mathrm{RC}}_{1}S)\&lsqb;{\mathrm{RC}}_{2}S(1+{\mathrm{RC}}_{3}S)V_0+V_0\&rsqb;+(1+{\mathrm{RC}}_{3}S)V_0---\left(2\right)$

Make C1=C3, then

$\frac{R}{R_i}{V}_i=(1+{\mathrm{RC}}_{3}S)\&lsqb;{\mathrm{RC}}_{2}S(1+{\mathrm{RC}}_{3}S)+2\&rsqb;V_0$

$={\mathrm{RC}}_3(S+\frac{1}{{\mathrm{RC}}_3})\&lsqb;S(S+\frac{1}{{\mathrm{RC}}_3})+\frac{2}{R^2{C}_2{C}_3}\&rsqb;V_0{R}^2{C}_2{C}_3---\left(3\right)$

Arrangement can obtain the system transfer function of filter:

$\frac{V_0}{V_i}=\frac{1/R_i{R}^2{C}_2{C}_3^2}{(S+\frac{1}{{\mathrm{RC}}_3})\&lsqb;{\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="A20051010227500111.png"\; state="\{\{state\}\}">S</figure-callout>}}^2+\frac{1}{{\mathrm{RC}}_3}S+\frac{2}{R^2{C}_2{C}_3}\&rsqb;}---\left(4\right)$

We know that the canonical form of three rank Butterworth system functions is

Comparison expression (4) and formula (5), and consider frequency normalization, we can obtain

Solve C1, C2, C3, draw

Wherein, W _cThe passband that promptly is filter is by frequency,

Because resistance is the switching capacity simulation in the circuit, when calculating, can be represented R by clock frequency and electric capacity:

$R=\frac{1}{{4f}_s{C}_R}---\left(9\right)$

F wherein _sIt is input clock frequency.

Formula (8) and formula (9) are brought in the formula (7), put in order

$\left\{\begin{array}{c}C3=\frac{C_R}{\mathrm{\&pi;}\frac{f_c}{2f_x}}\\ \mathrm{<figure-callout\; id="2"\; label="C"\; filenames="A20051010227500111.png"\; state="\{\{state\}\}">C</figure-callout>}2=\frac{2C_R}{\mathrm{\&pi;}\frac{f_c}{2f_s}}---\left(10\right)\\ C1=C3=\frac{C_R}{\mathrm{\&pi;}\frac{f_c}{2f_s}}\end{array}\right.$

From formula (10), we can be clear that pass band width f _cWith the direct relation between capacitor C 1, C2, the C3.When changing capacitance, pass band width is just corresponding have been changed.Formula (10) is a theoretical foundation of the present invention.

The concrete course of work of the low pass filter that pass band width of the present invention is adjustable is as follows: as Fig. 2, when FH, FL are low level, F0, the F1, the F2 that produce through decoding circuit are low level, C1a, C1b, C1c, C2a, C2b, C2c, C3a, C3b, these electric capacity of C3c are all not and link on the filter, C1tot=C1, C2tot=C2, C3tot=C3, filter have a pass band width f _C1When FH low level, FL high level, F0 is a high level, and F1, F2 are low level, and C1a also goes up C1, C2a and also goes up C2, C3a and go up C3, and at this moment, it is f that C1tot=C1+C1a, C2tot=C2+C2a, C3tot=C3+C3a, filter have a pass band width _C2In like manner also can analyze when FH be high level, when FL is low level, C1tot=C1+C1b, C2tot=C2+C2b, C3tot=C3+C3b, filter have a pass band width f _C3FH is high level, when FL is low level, C1tot=C1+C1c, C2tot=C2+C2c, C3tot=C3+C3c, filter have the 4th pass band width f _C4So just realized four pass band width fc1, fc2, fc3, the fc4 that filter is adjustable.

Fig. 3 has represented how to use electric capacity and switch artifical resistance.In integrated circuit, the resistance that realize high value is very difficult, often needs to account for very big chip area, and in switched-capacitor circuit, we often use switch and electric capacity to come artifical resistance.Fig. 3 has schematically represented the implementation method of electric capacity and switch artifical resistance.In this kind simulation, $R=\frac{1}{{4f}_s{C}_R}\&CenterDot;$

Fig. 4 has represented additional capacitor and has been connected into the implementation method of circuit.Two cmos analog switch closures when en=1, additional capacitor just and be connected in the filter, and when en=0, switch disconnects, the conducting of two NMOS pipes, the additional capacitor two ends all have been connected on the common-mode voltage, electric capacity can not be recharged.

Fig. 5 has represented the realization of decoding circuit, $F0=\stackrel{\&OverBar;}{\mathrm{FH}}\&CenterDot;\mathrm{FL},$ $F1=\mathrm{FH}\&CenterDot;\stackrel{\&OverBar;}{\mathrm{FL}},$ ,F2=FH·FL。

Described most preferred embodiment of the present invention so in detail.Yet, the people that common skill is arranged in this area be it is evident that, can not break away from the spirit or scope of the present invention ground the embodiment that discloses is here made many changes, so present embodiment should be as limitation of the present invention.

Claims (6)

1, the adjustable low pass filter of a kind of pass band width is characterized in that: this filter is by fixed bandwidth low pass filter, and decoding circuit and additional capacitor are formed; Input signal produces control signal through decoding circuit decoding, control signal connects the control port that sends to additional capacitor, whether the control additional capacitor is connected in parallel on the electric capacity of fixed bandwidth low pass filter, whether and the size of shunt capacitance appearance value the additional capacitor parallel connection, the capacitance of this filter is changed, thereby change the system function parameter of filter, realize regulating the bandwidth of low pass filter.

2, additional capacitor according to claim 1 is characterized in that: this additional capacitor by a plurality of electric capacity by and the capacitance group formed of the mode that connects.

3, additional capacitor according to claim 1 and 2 is characterized in that: the capacitance group quantity of additional capacitor equals the progression of fixed bandwidth low pass filter.

4, according to claim 1, the described additional capacitor of 2 or 3 arbitrary claims, it is characterized in that: each electric capacity of forming additional capacitor all has a control port, this port is connected the output of decoding circuit, is controlled on its electric capacity that whether is connected in parallel on fixed bandwidth low pass filter by the control signal that decoding circuit produces.

5, the adjustable low pass filter of pass band width according to claim 1, it is characterized in that: the resistance of this filter is simulated by switch and electric capacity.

6, switch according to claim 5.It is characterized in that: this switch is the cmos analog switch that is formed in parallel by a pair of NMOS, PMOS pipe.

Images