GB2326994A

GB2326994A - Polyphase filter with adjustable transfer function

Info

Publication number: GB2326994A
Application number: GB9813641A
Authority: GB
Inventors: Oliver Kromat
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1997-06-30
Filing date: 1998-06-24
Publication date: 1999-01-06
Anticipated expiration: 2018-06-24
Also published as: GB2326994B; FR2765418A1; DE19727869C1; FR2765418B1; GB9813641D0

Description

2326994 Polyphase filter with adjustable transfer function.

The invention relates to a polyphase filter with which, in addition to spectral channel filtering, for example damping of adjacent channels, in the case of negative frequencies the mirror sideband of the channel can likewise be damped.

A filter of this kind can be used, for example, in the receiver path of cordless telephones or mobile radio equipment. Basically the filter can be used everywhere where the signal to be filtered is split up into its imaginary and real parts.

Real filters, that is to say filters with an input and an output, have a real impulse response hr(t) The transfer function Hr(jw) is a rational real polynomial function in jw. The result is that for the transfer function Hr(jw) the following applies:

Hr(jw) = Hr(-jw) The amplification and damping of the filter is the same for positive and negative frequencies.

The impulse response of a polyphase filter, a filter with several inputs and several outputs (a multivariable filter), is a polyphase signal. For a two-phase filter this is a complex signal, h(t) = hr(t) + jhi(t). The transfer function H(jw) is as follows:

H(jw) = Hr(jw) + jHi(jw) and is a rational complex polynomial function, i. e. the frequency response can be different for positive and negative frequencies. With a polyphase filter the negative or positive frequency component of a complex signal can be damped. This can take place by means of a bandpass filter which results from the linear transformation of a low-pass filter. The conventional low-pass or bandpass transformation does not change the real properties of the low-pass filter in any way:

-2 jG) - j(,) c In this case the bandpass filter has the low-pass filter characteristic at w = + wc After the linear transformation iw - jwjW,: the bandpass filter now has a low-pass filter characteristic where w = +wc.

This transformation can only be realized by a polyphase filter because the transformation introduces complex coefficients into the rational polynomial transfer function of the filter. The following equation indicates this for a transformation of a lowpass filter of the first order.

HT H',p(jw) p 1 To manufacture a polyphase filter in the form of an integrated circuit, because of the manufacturing tolerances, a possibility for adjusting the polyphase filter may advantageously be provided. A displaceable filter curve makes it possible to adjust the polyphase filter.

A form of realization for adjusting the polyphase filter is described in M. Steyaert, J. Crols "Analog Integrated Polyphase Filters", Analog Circuit Design, Kluwer Academic Publishers, Netherlands, 1995, pages 149 to 166. The electronic adjustment of the polyphase filter is realized by means of switchable capacitor banks which require a very large amount of space because of their programming and the size of the additional capacitances. Half of the chip surface is covered by the switchable capacitor banks. The requirements with regard to noise demand that the capacitances must be large. The advantage of this operational amplifier solution is the large dynamic and linearity range.

A second possibility for adjusting the polyphase filter is in M. Koyama et al "A 2.5-Volt Active Low Pass Filter Using All-n-p-n Gilbert Cells with a 1-Vp_p Linear Input Range", IEEE Journal of Solid States Circuits, Vol. 28, No. 12, December 93, pages 1246 to 1253. The idea consists in using variable resistances (conductances). For this purpose, by varying the current with an emitter circuit the conductance (output current/input voltage) of the circuit is adjusted and in this way the corner frequency of a low-pass pole (conductance/load capacitance) is displaced. The advantage of such a variant lies in the space-saving realization. However, problems arise in the dynamic range, in the dependency of the smallsignal amplification on the control unit and the noise of the complex linear conductance structures.

According to the invention there is provided a filter with adjustable transfer function, having a polyphase filter having a plurality of coupling paths; a first set of adjustable amplifiers connected in series with the inputs of the polyphase filter, and a second set of adjustable amplifiers (WB1, WB2) arranged in the coupling paths of the polyphase filter.

The invention aims to provide a polyphase filter with an adjustable transfer function which, on the one hand, has a large dynamic and linearity range and, on the other hand, can be realized in a space-saving manner.

The damping in the passband of the polyphase filter remains constant in the event of a change of the amplification of the amplifiers if the amplification of the input-side amplifiers (the first set) corresponds to 1 + V, with V being the amplification of the outputside amplifiers (the second set).

In addition to the components of a conventional polyphase filter, the invention has a first set of amplifiers with adjustable amplification. The amplifiers of the first set are connected in series with the inputs of the polyphase filter. Furthermore, a second set of amplifiers with adjustable amplification is provided. The amplifiers of the second set are arranged in the coupling paths of the polyphase filter. Both sets of amplifiers are forward amplifiers, i.e. they multiply the input signal by an adjustable gain. They may, for example, be implemented by operational amplifiers, or by known transistor circuits.

The polyphase filter is preferably constructed in the usual way using operational amplifiers.

A specific embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows an embodiment of the polyphase filter in accordance with the invention.

Figure 2 shows various filter transfer functions with variation of the amplification.

The polyphase filter in accordance with the invention, according to Figure 1, has four input terminals Il, 12, 13, 14 which form the I and Q paths for a complex input signal. The input terminals I1 and 12 are connected to a first input-side forward amplifier WA1, while the input terminals 13 and 14 are connected to a second input-side forward amplifier WA2 (which make up a first set of amplifiers). The two inputside forward amplifiers WA1 and WA2 in turn are connected by way of two respective resistances R14, R15 is and R19, R20 to a first operational amplifier OP1 and a second operational amplifier OP2. At the output terminals 01, 02 of the polyphase filter which are connected to the outputs of the first operational amplifier OP1, and at the output terminals 03 and 04 of the polyphase filter which are connected to the outputs of the second operational amplifier OP2, real and imaginary partial components of the complex output signal are provided. The outputs of the two operational amplifiers OP1 and OP2 are furthermore each fed back by way of parallel- connected RC elements COS, R13 and C06, R16 and C07, R18 and C08, R21 to the inputs of the two operational amplifiers OP1 and OP2. In addition, the outputs of the two operational amplifiers OP1 and OP2 are connected to the inputs of two output-side forward amplifiers WBi and WB2 (which make up a second set of amplifiers). The first output of the first output- side forward amplifier WBi is connected by way of a resistance R23 to the inverting input of the second operational amplifier OP2. The second output of the first output-side forward amplifier WBi is connected by way of a resistance R24 to the non-inverting input of the second operational amplifier OP2. Correspondingly, the first output of the second outputside forward amplifier WB2 is connected by way of a resistance R17 to the inverting input of the first operational amplifier OP1 and the second output of the second output-side forward amplifier WB2 is connected by way of a resistance R22 to the non-inverting input of the first operational amplifier OP1.

The second set of amplifiers WBi and WB2 with adjustable amplification is therefore arranged in the coupling paths of the polyphase filter, whereas the first set of amplifiers with adjustable amplification WA1 and WA2 is connected in series with the inputs of the polyphase filter.

The input-side and output-side forward amplifiers WA1, WA2, WB1 and WB2 function as simple nonfeed back amplifiers with adjustable amplification. The amplification can be adjusted, for example, by a difference amplifier stage being provided, the amplification dependency of which is regulated by the current.

Combined with the two operational amplifiers OP1 and OP2, the capacitors CS, C6, C7 and C8 form integrators together with the corresponding resistances. At the input of the operational amplifier OP1 there is the voltage Uiffop, as the sum of the weighted voltages comprising input voltage OP1 (can be picked up between the terminals I1 and 12), output voltage OP1 (can be picked up between the terminals 01 and 02) and output voltage OP2 (can be picked up between the terminals 03 and 04). The voltages are weighted according to the value of the resistances.

At the input of the operational amplifier OP2 there is the voltage U.1iff, ,,,, as sum of the weighted voltages comprising input voltage OP2 (can be picked up between the terminals 13 and 14), output voltage OP2 and output voltage OP1. These voltages are likewise weighted according to the value of the resistances.

The polyphase filter according to Figure 1 is a twophase filter and represents an embodiment of the invention.

It is also possible to construct an n-pole filter, based on the embodiment of Figure 1, with further operational amplifiers. The n-pole filter has 2n operational amplifiers, 2n input-side and 2n outputside forward amplifiers with corresponding ohmic and capacitive protective circuits. The outputs of the first pole of the n-pole filter are applied to the inputs of the second pole of the n-pole filter etc.

The individual poles of the n-pole filter are linked to form a chain.

Figure 2 shows by way of example various filter transfer functions with variation of the amplification. On the abscissa the frequency f is plotted in hertz and it ranges from -3MHz to +3MHz. On the ordinate the damping of the polyphase filter in accordance with the invention is indicated in dB, beginning at -80dB and ending at 10dB. In all, ten filter transfer functions are shown in the diagram. In the case shown here they intersect at about -65OkHz and at this frequency have a damping of about -40dB. As can be seen, the damping of the polyphase filter is not mirror-symmetrical to the zero hertz axis. The filter transfer function of the polyphase filter can be adjusted in such a way that tolerances in the manufacture of the polyphase filter can be compensated. The filter transfer function of the polyphase filter can be adjusted in a large range by varying the amplification.

The damping in the passband of the polyphase filter remains constant in the event of a change of the amplification of the forward amplifiers WA1, WA2, WBi and WB2 if the amplification of the input-side forward amplifiers WA1 and WA2 corresponds to 1 + V, with V being the amplification of the output-side forward amplifiers WB1 and WB2. V can also be understood to be the coupling of the imaginary and real part paths of the polyphase filter.

The invention creates a simple space-saving possibility for influencing the transfer function of the filter. The polyphase filter in accordance with the invention combines the advantages of the large dynamic and linearity range of the operational amplifier solution with the space advantage of the polyphase filter with multiple resistors described above.

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Claims

Claims is 1. A filter with adjustable transfer function, having a

polyphase filter having a plurality of coupling paths; a first set of adjustable amplifiers connected in series with the inputs of the polyphase filter, and a second set of adjustable amplifiers (WB1, WB2) arranged in the coupling paths of the polyphase filter.
2. A filter according to claim 1 wherein the polyphase filter has a plurality of operational amplifiers and the said coupling paths couple the operational amplifiers.
3. A filter according to claim 1 or 2, where the amplification of the amplifiers of the first set corresponds to 1 + V where V is the amplification of the amplifiers of the second set.
4. An n-pole filter, where n is an integer greater than 1, having n filters according to any of claims 1 to 3 chained in series.
5. A polyphase filter substantially as herein described with reference to and as shown in Fig. 1 of the accompanying drawings.