GB2189100A - A programmable electronic filter - Google Patents

Abstract

A filter including one or more variable resistance components ('R') controllable by variation of control current. Each resistance component comprises a multiplicity of series connected stages, each stage (Fig. 3) including a diode (D). The diode (16, 18,... 20) of successive stages are arranged in alternate sense. The resistance component may include a buffering transistor (14) at its input. The resistance may be made arbitrarily large merely by the design choice of including a sufficiently large number of stages. The filter may include a plurality of variable resistance components. For frequency scaling, these components would be connected in parallel to a common source of control current. Alternatively, for variation of shaping characteristics, they may be controlled independently.

Description

SPECIFICATION A Programmable Electronic Filter Technical Field The present invention concerns improvements in or relating to electronic filters more particularly filters of the type that are prog rammable-- that is to say, filters in which the frequency response or shape of the characteristic, or both, may be varied by the application of one or more electrical control signals.

Such filters have wide application, for example, in the field of radio and television tuning. In particular they have application to radio pagers. The latter would normally run on 1 .5v battery sources. Such battery sources typically have an end of life voltage of around 0.9v. For maximum battery economy, therefore filters for this application should ideally be able to operate satisfactorily down to low voltage (i.e. 0.9v orthereabouts) and, for maximum versatility at the same time a relatively wide potential response to frequency.

Background Art Transistor gyrator filters, for example, are in common use for this purpose. It is, however, a drawback of this type of filter, that ultimately the signal handling capacity is limited by the characteristics of the transistor.

Disclosure of the Inventor The present invention provides an alternative design of electronic filter. It is intended thus to provide a filter that is both versatile and capable of operation at low voltage.

In accordance with the invention thus there is provided a programmable electronic filter, a filter comprising: reactance and resistance components, characterised in that at least one said resistance component is variable and comprises a diode network having a multiplicity of series connected stages, each stage including at least, one diode which is interconnected between a pair of controllable current sources, the diodes of consecutive stages being arranged in alternate polarity sense.

The filter aforesaid may be either passive or active in the latter case, an amplifier being incorporated as part of the filter circuit.

In order to afford operation at the lowest voltages, it is preferable thus that each stage of the network incorporates but a single diode. The available supply voltage then is reduced by only one Vbe, the voltage drop across the one diode.

For impedance matching purposes, it is advantageous to incorporate a buffering transistor at the front end of the or each variable resistance component.

In such filters it would be usual to incorporate a plurality of variable resistance components. These may be controlled in parallel by a common current source. This then would serve to provide different filter characteristics that scale with frequency.

Alternatively, however, these components could be controlled independently to allow change in the shape of the filter characteristic.

Brief Introduction ofthe Drawings In the drawings accompanying this specification- Figures 1 and 2 are circuit diagrams showing a known filter, and a filter modified in accordance with this invention, respectively- Figures 3 and 4 are circuit diagrams of a single stage diode network, the latter figure being of a bipolar transistor implimentation; Figure 5 is a circuit diagram of a variable resistance component, part of the modified filter shown in figure 2 preceding; and Figures 6 and 7 are illustrative graphs showing a filter function as a function of frequency, for different values of the variable resistance.

Description of a Preferred Embodiment So that this invention may be better understood, an embodiment thereof will now be described and reference will be made to the accompanying drawings. The description that follows is given by way of example, only.

A known design of active filter is shown in Figure 1. This consists of a pair of like resistors R, an amplifier and a pair of capacitors C. Feedback is provided by one of the capacitors C which extends from the output of the amplifier to the junction of the two resistors R which are connected in series to the input of the amplifier. The other capacitor C extends between this input and system earth.

In the modification shown in Figure 2 variable resistance components 'R' have been substituted.

These therefore will permit control of the frequency response of the filter. As shown, the variable resistance, components 'R' are connected to a common control input. In this configuration the filter characteristics H(w) versus frequency (f) will thus be scaled as a function of control current/controlled resistance (See Fig. 6). Alternatively, the variable resistance components 'R' could be controlled independently to allow change of the shape of the filter characteristic. (See Fig. 7).

The basis of the variable resistance component 'R' is a diode network the a.c. resistance value of which is controlled by varying the level of biasing current.

A single stage of this network is shown in Figure 3.

This stage as shown incorporates but a single diode D. Others could be added in series but would have the effect of reducing available voltage further. The diode D is interconnected between high impedance sources of constant current, i.e. current sources S. In bipolar circuit design it is convenient to implement the diode D using a transistor Q (Fig. 4), the collector and base being shorted together.

With reference to Figure 5, the variable resistance component'R' comprises an input terminal 10, and an output terminal 12 the resistance between the two terminals being the resistance of the component with which we are concerned. This resistance is electronically variable as described hereinafter.

The input terminal 10 is connected to the base of a transistor 14, the collector of which is connected 'to a voltage supply rail Vcc and the emitter of which is connected to a first diode 16 in a diode chain series 16, 18...20. Each diode 16, 18,20, is connected between a pair of constant current sources 22, 24; 24, 26;...,28. Intermediate current sources 22, 24...26 supply a current of magnitude I and the terminal source 28 supplies a current of half this value 1/2, the magnitude of current flowing through each diode 16, 18,...20.

The resistance component therefore comprises N stages 1,2, 3...N. the first stage being a buffering transistor stage and the remaining stages, all diode stages. The resistance R of the component is equal to N.Re where 2kT Re= ql k being the Boltzman constant, Tthe Absolute Temperature, q the electron charge and 1/2 the current flowing through each diode.

Thus the resistance of this component is inversely proportional to the current I and this current I may be controlled by means of control signal applied to an input 30 which serves to control the current sources 22,24...26...28. Thus by applying a single external current control signal to terminal 30, the resistance of the component can be altered, and the frequency response of the filter changed.

The resistance thus is proportional to the number of stages N and to the inverse of the current magnitude I. Provided the number of stages is made reasonably large, then the current I can be relatively large. Thus for high value resistors by having a large number of stages the curernt I can still be allowed to be relatively large and, because it is relatively large, the current I can be more accurately controlled. This component is therefore very stable and can be used to produce a high value of resistance when made in an integrated circuit form. Filter performance thus can be much enhanced.

Although in the foregoing description and in the drawings an npn type transistor 14 and matched diodes 16,18 .,20 are described and shown, the pnp-type equivalent of this circuit is not procluded from the general scope of the claims that follow hereinafter.

Claims (9)

1. A programmable electronic filter comprising reactance and resistance components, characterised in that at least one said resistance component is variable and comprises a diode network having a multiplicity of series connected stages, each stage including at least one diode which is interconnected between a pair of controllable current sources, the diodes of consecutive stages being arranged in alternate polarity sense.

2. A filter is claimed in Claim 1, wherein each stage of the diode network comprises a single diode.

3. A filter, as claimed in either one of the preceidng claims, wherein the diode network of the resistance component is preceded by a buffer transistor.

4. A filter, as claimed in any one of the preceding claims, wherein the paired current sources are matched in value.

5. A filter, as claimed in any one of the preceding claims, when including an amplifier and configured thus as an active filter.

6. Afilter, as claimed in any one of the preceding claims, having a plurality of said variable resistance components.

7. A filter, as claimed in claim 6, wherein the variable resistance components are connected to receive control current from a common source.

8. An active filter constructed, adapted and arranged to operate substantially as described hereinbefore with reference to and as shown in Figures 2 and 5 of the accompanying drawings.

9. A filter as claimed in any one of the preceding claims when incorporated as part of a radio pager circuit.