WO2009060407A1

WO2009060407A1 - Differential filtering device

Info

Publication number: WO2009060407A1
Application number: PCT/IB2008/054643
Authority: WO
Inventors: Harish Kundur Subramaniyan
Original assignee: Nxp B.V.
Priority date: 2007-11-09
Filing date: 2008-11-06
Publication date: 2009-05-14

Abstract

The present applications relates to a filtering device (3) comprising a first amplifying unit (10) comprising a first connection (Z2) from an output of the first amplifying unit (10) to an input of the first amplifying unit (10). The filtering device (3) encompasses a second amplifying unit (12) comprising a second connection (Z6) from an output of the second amplifying unit (12) to an input of the second amplifying unit (12) wherein the output of the first amplifying unit (10) is coupled via a first impedance (Z4) to the input of the second amplifying unit (12). The filtering device (3) includes a third connection (Z3) from an output of the second amplifying unit (12) to an input of the first amplifying unit (10). The filtering device (3) comprises a second impedance (Z7) arranged at the output of an second amplifying unit (12). The filtering device (3) encompasses a fourth connection (Z5) arranged from an output of the second amplifying unit (12) to an output of the first amplifying unit (10) via the second impedance.

Description

Differential filtering device

FIELD OF THE INVENTION

The application relates to a filtering device, particularly to a notch and/or all pass filtering device, and to a transmitter comprising the filtering device.

BACKGROUND OF THE INVENTION

Analogue filtering units are widely used in signal conditioning devices. There are different kinds of filtering units, like low pass filter, high pass filter, notch filter, all pass filter and the like. The filtering units are generally denoted by their transfer function.

Filtering units can be divided into passive and active filtering units. Passive filtering units which usually consist of passive components, like inductors (L), resistors (R) and capacitors (C), are normally used at high frequencies. However, for lower frequencies, these so-called LRC filters have the drawback that the inductor value becomes very large and thus, the inductor itself gets quite bulky. An economical production of such filtering units becomes difficult. In these cases, active filtering units can be used comprising operational amplifiers or the like. Operational amplifiers are employed in combination with passive components such as resistors and capacitors. However, active filtering units are sensitive to active component parameter variations. Furthermore, the power consumption of such a filtering device is an additional drawback. Notch filtering units are used for canceling an unwanted frequency or frequency band. The quality of a notch filter can be exemplified by its quality factor Q. The quality factor Q depends on the mid frequency f_m and the bandwidth B. In prior art, several notch filters are known, like a passive or active Twin-T notch filter. These filters are accompanied by low Q poles. However, filtering units having low Q poles comprise the drawback of low frequency selectivity.

Another solution of prior art is a so-called Tow-Thomas biquad filter section. Such a filter section encompasses drawbacks, like the need of four amplifying units which results in a high power consumption. Additionally, all these filter units have a finite input impedance which raises the problem that a current signal is not suitable as an input signal. Thus, a current signal has to be converted to a voltage signal.

It is one object of the present application to provide reduced power consumption of a filtering device. A further object is to provide a better frequency selectivity of a filtering device. A further object is to allow the use of current signals as input signals. A further object is to provide a more generic filtering device.

SUMMARY OF THE INVENTION These and other objects are solved by a filtering device comprising a first amplifying unit comprising a first connection from an output of the first amplifying unit to an input of the first amplifying unit. The filtering device encompasses a second amplifying unit comprising a second connection from an output of the second amplifying unit to an input of the second amplifying unit wherein the output of the first amplifying unit is coupled via a first impedance to an input of the second amplifying unit. The filtering device includes a third connection from an output of the second amplifying unit to an input of the first amplifying unit. The filtering device comprises a second impedance arranged at an output of the second amplifying unit. The filtering device encompasses a fourth connection arranged from an output of the second amplifying unit to an output of the first amplifying unit via the second impedance.

The present filtering device can be realized in analogue circuit technology and can be implemented in a broad application field. It has been found according to the present application that the power consumption can be significantly reduced due to the use of two amplifying units. Such an amplifying unit may comprise at least two inputs and two outputs wherein the two inputs and two outputs respectively encompass opposite polarity.

Each amplifying unit comprises a connection or feedback loop from its outputs to its inputs having opposite polarity compared with the polarity of the outputs. Both connections may include at least one impedance according to an embodiment. The first amplifying unit is coupled to the second amplifying unit via a first impedance. Thereby, the positive output of the first amplifying unit can be connected to the negative input of the second amplifying unit, and the remaining terminals can be connected similarly.

Furthermore, the filtering device comprises a third connection. This can be a feedback loop from the outputs of the second amplifying unit to the input of the first amplifying unit. This feedback loop can be arranged between outputs and inputs having different polarity. According to an embodiment, the third connection comprises at least one impedance.

For achieving a generic filtering device, it is found, according to the present application, that a second impedance can be arranged at each output of the second amplifying unit. It is further found that an additional fourth connection from the output of the second amplifying unit to the output of the first amplifying unit via this second impedance causes a desired generic transfer function. The fourth connection can be arranged between outputs of the amplifying units having same polarity, different polarity or both. For the case, the used impedances are of the same kind, and two opposing polarity connections are used, the signals may tend to cancel. In some applications, like programming some coefficients of the numerator of the transfer function, this effect can be used. According to an embodiment, the fourth connection may include at least one impedance. By tuning this impedance, the desired transfer function and the desired filter respectively can be obtained. More particularly, the fourth connection may effect the numerator of the transfer function. According to embodiments of the present application, the filtering device can be used as a notch filter as well as an all pass filter, depending on the choice of the respective components, like the impedances. For instance, the numerator can be zero for a particular frequency due to the fact that the imaginary part of the numerator can be set to zero particularly depending on the choice of the impedance of the fourth connection. The filtering device according to the present application can be called a biquad filtering section although the transfer function differs completely from biquad filter sections known from prior art. Biquad filter sections known in the art comprise a low pass or band pass transfer function, whereas the filtering device according to the present application encompasses a more generic transfer function for realizing a notch filter, all pass filter or the like.

The present filtering device can be used for different applications, like a notch filter and/or all pass filter. Furthermore, the power consumption can be significantly reduced. What is more, the sensitivity to active components parameter variations of the present filtering device can also be reduced. In another embodiment, an impedance may comprise at least a capacitor and/or resistor and/or inductor. Since inductors are not cost-efficient for implementation on integrated circuits, the employed impedances may comprise only resistors and/or capacitors according to an embodiment. For example, the first and second impedances can be resistors. On the other hand, the impedance of the fourth connection may be a capacitor meanwhile the third connection may comprise a resistor as impedance. The first connection may comprise a parallel connection of a capacitor and resistor and the second connection may include a capacitor. However, according to other variants of the present application other components can be employed as respective impedances. According to a further embodiment, the filtering device may comprise a fifth connection arranged from an output of the second amplifying unit to an output of the first amplifying unit via the second impedance. The fifth connection may be connected in addition to the fourth connection, for example in parallel connection or cross connection. The fifth connection may comprise at least a third impedance. The fifth connection can be arranged between outputs of the amplifying units having same polarity, different polarity or both. For the case, the used impedances are of the same kind and two opposing polarity connections are used, the signals may tend to cancel. In some applications, like programming some coefficients of the numerator of the transfer function, this effect can be used. Furthermore, the fourth connection, as mentioned above, can also be arranged between outputs of the amplifying units having same polarity, different polarity or both. This may yield to at least four different transfer functions. The four transfer functions, in particular the numerator of these transfer functions, may differ by their different algebraic signs.

For instance, the impedance of the fourth connection may be a capacitor and the impedance of the fifth connection may be a resistor. Such a combination is particularly suitable to implement and tune a filtering device. For example, the resistor can be set to infinity resulting in a notch filter. For the case the value of the resistor being finite, an all pass can be realized. An easy tuning of the present filtering device is possible. However, according to other variants of the present application, other realizations of the impedances are also possible. In another embodiment of the present application, the impedances can be substantially arranged symmetrically. A symmetrical arrangement yields reduced disturbances and noise. The filtering device may include an input impedance which can be set to zero according to a further embodiment. It is found according to the application that the input impedance can be set to zero due to the above-mentioned configuration of the filtering device, in particular due to the fact that the impedances may be arranged symmetrically. For instance, at each input of the first amplifying unit an impedance can be attached. For the case, current signals are used as input signals, this input impedance can be set to zero. However, in practical applications the input impedance can be set merely to approximately zero, which may be sufficient. The filtering device may be attached directly to outputs from current steering digital analogue converters, passive mixers, high-speed Gilbert mixers or the like. Furthermore, noise can be significantly reduced. For the case, the input impedance is large or finite voltage signals can also be used as input signals.

At least one amplifying unit may be a differential amplifying unit according to an embodiment. Such an amplifying unit may comprise two transistors. The difference between the input signals of the differential amplifier can be amplified. These amplifying units are particular suitable for an employment within the present filtering device. According to another embodiment of the present application, at least one amplifying unit is at least an operational amplifier or operational transconductance amplifier. Operational amplifiers are widely used and can easily be implemented. An operational amplifier may encompass a differential amplifier. An operational transconductance amplifier can be employed in advantageous manner for wideband applications. Operational amplifiers as well as operational transconductance amplifiers may comprise at least two inputs of opposite polarity. Furthermore, they may provide at least two outputs of opposite polarity as well. In a further embodiment, a fourth impedance can be arranged at the output of the second amplifying unit via the second impedance. The fourth impedance can be arranged symmetrically at each output of the second amplifying unit. It is also possible to attach the impedance between both outputs of the second amplifying unit. In this case, the impedance may be a capacitor. The impedance can be attached additionally to implement a further pole location. A better stability can be achieved.

According to a further embodiment of the present application, the filtering device may encompass a third amplifying unit. An operational amplifier or operational transconductance amplifier can be employed as a third amplifying unit. The use of an additional third amplifying unit causes an improvement of filtering and/or amplifying and/or signal transformation. For instance, a current signal can be converted to a voltage signal.

In another embodiment, a sixth connection can be arranged from an output of the third amplifying unit to an input of the third amplifying unit. This connection may include at least a fifth impedance. A suitable realization of this impedance may be a parallel connection of a resistor and capacitor. The sixth connection can be arranged between inputs and outputs of the third amplifying unit comprising different polarity. However, according to other variants of the application, other forms of this impedance and of arranging the sixth connections are possible. The filtering device according to another embodiment can be implemented in CMOS technology and/or bipolar technology and/or BiCMOS technology and/or GaAs and/or as suitable discrete device. The essential power for driving the device can be reduced as well as required space can be small. Another aspect of the present application is a transmitter comprising at least the above mentioned filtering device. The present filtering device according to the application can be used widely in several signal- conditioning applications as an all-pass or notch-filtering device depending on the choice of the respective impedances. In particular, the present filtering device can be used in wireless and wired transceivers in both discrete and integrate forms.

These and other aspects of the present patent application become apparent from and will be elucidated with reference to the following Figures. The features of the present application and of its exemplary embodiments as presented above are understood to be disclosed also in all possible combinations with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures show:

Fig. 1 a first embodiment of the filtering device according to the application;

Fig. 2 a second embodiment of the filtering device according to the application;

Fig. 3 a third embodiment of the filtering device according to the application;

Fig. 4 a fourth embodiment of the filtering device according to the application.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of the drawings, exemplary embodiments of the present application will describe and point out the architecture of the filtering device comprising an increased quality factor Q and a less power consumption. Furthermore, it will be elucidated by the exemplary embodiments to allow the use of current signals as input signals and to reduce noise. Thereby, same units hold same reference signs.

Fig. 1 shows a first embodiment of a filtering device according to the present application. The shown filtering device 1 is symmetrically constructed. Thus, as shown in Fig. 1, each of the impedances exists two times. The filtering device 1 comprises a first amplifying unit 10 and a second amplifying unit 12. The amplifying units 10, 12 may be operational amplifier, operational transconductance amplifier or the like, wherein each amplifier 10, 12 comprises two inputs and two outputs. The outputs of the first amplifying unit 10 are coupled to the inputs of the second amplifying unit 12 via first impedances Z4. The positive polarized output of the first amplifying unit 10 may be coupled to the negative polarized input of the second amplifying unit 12 and the negative polarized output of the first amplifying unit 10 may be coupled to the positive polarized input of the second amplifying unit 12.

Furthermore, each amplifying unit 10, 12 comprises a feedback loop from its outputs to its opposite polarized inputs including an impedance Z2 and Z6 respectively. What is more, a third connection comprising impedance Z3 is arranged from the outputs of the second amplifying unit 12 to the inputs of the first amplifying unit 10. The terminals of the third connections encompass the same polarity.

At the outputs of the second amplifying unit 12, second impedances Z7 are attached. The outputs of the second amplifying unit 12 are connected to the outputs of the first amplifying unit 10 via these second impedances Z7. This fourth connection comprises impedance Z5. According to the shown embodiment, the two fourth connections are arranged such that the polarity of their endings is opposite.

As can be seen from Fig. 1, further impedances Z9 can be arranged at the outputs of the second amplifying unit 12 via impedances Z7. Impedances Z9 can be used for implementing an additional pole location. However, according to other embodiments of the present application, these impedances Z9 can also be omitted or merely one impedance Z9 can be attached. For instance, the impedance can be attached differentially between outputs of amplifying unit 12 via impedances Z7. Finally, the filtering device 1 comprises an input impedance Zl and an output impedance Z8. In the case the input signals are current signals, the input impedance Zl can be set to approximately zero. Thus current signals can be processed directly without the need of transferring the current signals into corresponding voltage signals. The filtering device 1 may be attached directly to outputs from current steering digital analogue converters, passive mixers, high speed Gilbert mixers or the like. The filtering device 1 may have low noise.

However, according to other variants of the present application, input impedances Zl may have higher values for processing voltage input signals. Fig. 2 shows a second embodiment of a filtering device according to the present application. The shown filtering device 2 differs merely in one detail from the filtering device 1 of Fig. 1. The fourth connections comprising impedances Z5 are attached between outputs of the amplifying units 10, 12 having the same polarity instead of different polarity. The effects of this different implementation are elucidated subsequently.

Fig. 3 shows a third embodiment of a filtering device according to the present application. The depicted filtering device 3 comprises filtering device 2 of Fig. 2. The embodiment of Fig. 3 differs from the embodiment of Fig. 2 by an additional fifth connection. This fifth connection comprises an impedance Zl 1. As can be seen from Fig. 3 both fifth connections are arranged between outputs of the amplifying units 10, 12 having same polarity. However, according to other variants of the application the fifth connections may be attached between outputs of the amplifying units 10, 12 having different polarity or may be omitted.

The starting point for designing the above shown embodiments is the transfer function given in the following equation

All filtering devices 1, 2, 3 depicted in Figs. 1-3 may realize this transfer function H(s). The shown generic transfer function H(s) can be used to realize an all pass filter and/or notch filter depending on the used impedances. The factors CO₂ ,(ύ_p , Q_z0 and Q_p0 are the angular frequency of zero locations, pole locations and their respective quality factors, parameter s is given by s=jω and factor A is the gain of the filtering device 1, 2, 3. The algebraic signs, especially the algebraic signs of the numerator of the transfer function H(s), depend on the implementation of the fourth and fifth connections.

For the case, impedances Z5 comprising a resistor are connected in parallel to a capacitor between outputs of the amplifying units 10, 12 having different polarity, the transfer function H(s) is given by

whereby the algebraic signs are positive. According to Fig. 3, if each of the impedances Z5 including a capacitor are arranged in the above manner and each of impedances ZI l including a resistor are arranged between outputs of the amplifying units 10, 12 having same polarity the transfer function H(s) is given by

whereby factor s/Cϋ_z Q_z0 comprises a negative algebraic sign. According to another embodiment, the fourth connections encompassing capacitors can be arranged between outputs comprising same polarity meanwhile the fifth connections comprising resistors can be arranged between outputs comprising different polarity resulting in the transfer function

whereby factor s/Cϋ_z Q_z0 comprises a positive algebraic sign and factor s²/Cϋ_z ² a negative algebraic sign. Finally, fourth and fifth connection comprising a capacitor and a resistor respectively may be arranged between outputs comprising same polarity resulting in the transfer function

whereby both factors s/Cϋ_z Q_z0 and s²/Cϋ_z ² comprises a negative algebraic sign. The alteration of the algebraic signs can be useful in systems for equalization, like in some kind of all pass- filtering devices. Furthermore, it can be advantageously for systems comprising operational amplifiers, which are non-ideal. Due to the fact that a non-ideal operational amplifier causes a finite Q_z0 a cancelling term using the opposite phase can be added to get infinite Q_z0 value, and thus to obtain a deeper notch. The specific values of the factors ω_z , (ύ_p , Q_z0 and Q_p0 depend on the used impedance components. In the following examples are given for possible realization of these factors, in particular of the factors of the numerator.

As already mentioned through varying the factors Cϋ_z and Q_z0 different filter functions can be generated. For the case a fifth connection is arranged within the filtering device using appropriate impedances, the following equations are given:

and

The subscripts correspond to the corresponding component values used to realize the impedances. Impedances Z5 and ZI l may comprise either a resistor or a capacitor or both in parallel. To achieve a notch filter the factor s/Cϋ_z Q_z0 can be set to zero or at least to approximately zero. Therefore, impedance ZI l can be made infinite resulting in an infinite value of Q_z0 and thus factor s/Cϋ_z Q_z0 is zero. For instance, impedances Z4, Z7 and ZI l can be implemented by a resistance and impedances Z5 and Z6 can be implemented by a capacitor. Thus the filtering device can easily designed by tuning the Q value through the resistance representing ZI l. For obtaining a notch filter this resistance can be set to infinite. Fig. 4 shows a fourth embodiment a filtering device according to the present application. The shown filtering device 4 encompasses the filtering device 2 known from Fig.

2. Additionally, a third amplifying unit 14 is arranged at the outputs of the filtering device 2. Furthermore, the outputs of the third amplifying unit 14 are coupled with its inputs via sixth connections comprising impedances ZlO. In this embodiment the fifth connection is omitted. The third amplifying unit 14 can be arranged for further improving filtering and amplification. Advantageously, an operational amplifier or an operational transconductance amplifier can be used. Operational transconductance amplifiers can be preferred for wideband applications. Further, as can be seen from Fig. 4, possible passive components, such as resistors and capacitors are arranged as impedances Zl to ZlO. For instance, impedance Z2 comprises a parallel connection of a resistor and capacitor and impedance Z5 encompasses a capacitor. The further forms of the remained impedances can be seen from Fig. 4. It shall be understood that according to other embodiments of the present application, other forms of the impedances Zl to ZlO are possible.

What is more, according to the shown embodiment, input impedance Zl is omitted. In other words impedance Zl is set to zero or at least to approximately zero. Thus the shown embodiment is in particular suitable for current signals used as input signals. The shown embodiment may be attached directly to outputs from current steering digital analogue converters, passive mixers, high speed Gilbert mixers or the like. Furthermore, noise can be significantly reduced due to the low value of impedance Zl.

Optional impedance Z9 is arranged according to the shown embodiment between the ends of impedances Z7 which are not directly coupled to the outputs of the second amplifying unit 12. According to the illustrated embodiment merely a capacitor is attached.

The transfer function H(s) of the filtering device 4 is given by following equation

It can be assumed that the transfer function Hi (s) does not include an additional pole caused by the stage including amplifying unit 14 although a third amplifying unit 14 is arranged. Thus, the output can be the current flowing into the input of the last stage.

The depicted transfer function Hi (s) is a generic transfer function and can be used to realize all-pass filtering devices, notch filtering devices or the like. The present filtering device 4 according to the application can be used widely in several signal conditioning applications as an all-pass or notch filter depending on the choice of the respective impedances Zl to ZlO. The additional pole location given by (Cϋ_r + s) can be tuned by optional impedance Z9. For example, impedance Z8 can be set to zero and the load can also be set to zero by using a virtual ground. Then, the pole locations may move to a very high frequency. By the use of suitable values of the impedances, factor CO₂ / Q_z0 can be set to zero resulting in a transfer function Hi (s) of a notch filtering device. Especially through the fourth connection and the arranged impedance Z5 respectively a desired transfer function Hi (s) of a filtering device can be obtained and is given for the case R7=R8 by equation

whereby

'

R3

A =

2 - Rl

"

and

CO₁, = r (C5 + 2 - C9)R7

An orthogonal tuning of CO^ and Qp is possible. The filtering device can be used as a generic biquad filtering section. Furthermore, it is readily clear for a person skilled in the art that the logical blocks in the schematic block diagrams as well as the flowchart and algorithm steps presented in the above description may at least partially be implemented in electronic hardware and/or computer software, wherein it depends on the functionality of the logical block, flowchart step and algorithm step and on design constraints imposed on the respective devices to which degree a logical block, a flowchart step or algorithm step is implemented in hardware or software. The presented logical blocks, flowchart steps and algorithm steps may for instance be implemented in one or more digital signal processors, application specific integrated circuits, using discrete hardware, field programmable gate arrays or other programmable devices. The computer software may be stored in a variety of storage media of electric, magnetic, electro -magnetic or optic type and may be read and executed by a processor, such as for instance a microprocessor. To this end, the processor and the storage medium may be coupled to interchange information, or the storage medium may be included in the processor.

Claims

CLAIMS:

1. A filtering device, comprising: a first amplifying unit (10) comprising a first connection from an output of the first amplifying unit (10) to an input of the first amplifying unit (10), a second amplifying unit (12) comprising a second connection from an output of the second amplifying unit (12) to an input of the second amplifying unit (12), wherein the output of the first amplifying unit (10) is coupled via a first impedance (Z4) to the input of the second amplifying unit (12), a third connection from an output of the second amplifying unit (12) to an input of the first amplifying unit (10), - a second impedance (Zl) arranged at an output of the second amplifying unit

(12), and a fourth connection arranged from an output of the second amplifying unit (12) to an output of the first amplifying unit (10) via the second impedance (Zl).

2. The filtering device according to claim 1, wherein each of the connections comprise at least one further impedance (Z2, Z3, Z6, Zl).

3. The filtering device according to claim 1, wherein impedance (Zl, Z2, Z3, Z4, Z5, Z6, Zl, Z8, Z9, ZlO, ZI l) comprises at least one of: A) resistance,

B) capacitor,

C) inductor.

4. The filtering device according to claim 1, further comprising a fifth connection arranged from an output of the second amplifying unit (12) to an output of the first amplifying unit (10) via the second impedance.

5. The filtering device according to claim 4, wherein the fifth connection comprising a third impedance (Zl 1) is connected in parallel to the fourth connection.

6. The filtering device according to claim 2, wherein impedances (Zl, Z2, Z3,

Z4, Z5, Z6, Z7, Z8, Z9, ZlO, Zl 1) are arranged substantially symmetrically.

7. The filtering device according to claim 1, further comprising an input impedance (Zl) which can be set to zero.

8. The filtering device according to claim 1, wherein at least one amplifying unit (10, 12, 14) is a differential amplifier.

9. The filtering device according to claim 1, wherein at least one amplifying unit (10, 12, 14) is at least one of:

A) operational amplifier,

B) operational transconductance amplifier.

10. The filtering device according to claim 1, further comprising a fourth impedance (Z9) arranged at the output of the second amplifying unit (12) via the second impedance (Z7).

11. The filtering device according to claim 1, further comprising a third amplifying unit (14).

12. The filtering device according to claim 11, further comprising a sixth connection comprising a fifth impedance (ZlO) arranged from an output of the third amplifying unit (14) to an input of the third amplifying unit (14).

13. The filtering device according to claim 1, wherein the filtering device is implemented in least one of:

A) CMOS technology, B) bipolar technology,

C) BiCMOS technology,

D) GaAs,

E) discrete device.

14. The filtering device according to claim 1, wherein the filtering device is a notch filtering device.

15. The filtering device according to claim 1, wherein the filtering device is an all pass filtering device.

16. Transmitter comprising a filtering device according to claim 1.