The Post Office

British Telecommunications Public Limited Company

Alan J Greaves

John Mortimer Rollett

Phillip Ernest Greenaway

Charles Nightingale

Paul C Millar

Philip Ernest Greenaway

Phillip J Hunter

Speaker verification using memory address

A fourth (or higher) order filter has a series resonant section terminated by a parallel resonant section, both based on frequency dependent negative resistance (FDNR). The FDNR, resistance and capacitance (representing damping) of the series section are provided by a capacitive potential divider (C41,C42) connected between low impedance input (A) to the filter and an intermediate terminal forming the input of a unity-gain amplifier (A4) whose output is connected via a resistance (R4O) to the tapping point of the divider; the intermediate terminal is connected via a second resistance (R1) to the parallel section.

Active filter having series resonant active filter terminated by

A filter network that exhibits a bandstop response comprises a network of basically low pass or high pass structure. The network is developed by choosing a bandstop transfer function, a fourth-orderelliptic is disclosed, which is converted to a low pass function. This function describes a passive low pass LC ladder network including a negative impedance element. A Bruton transformation is applied to the passive LC network to give an active version including an ungrounded negative resistance network in a series arm and frequency-dependent negative resistances in shunt arms without requiring inductance coils for its implementation.

Bandstop filters

An equalizer network that exhibits stability and may be cascaded with a transmission channel is disclosed. The network includes only two operational amplifiers, resistors and capacitors and provides a bump shaped amplitude characteristic at resonance frequency. The equalizer may be modified to exhibit selectively a `bump` or `dip` at resonance, and may be so constructed that trimming operations and the independent selection of parameters is facilitated.

Equalizer networks providing a bump shaped response

A construction of an active filter having the same characteristics as a predetermined passive filter is described in which the capacitors of the passive filters are replaced by a topologically equivalent first network of resistors and the inductors of the passive network are replaced by a topologically equivalent second network of resistors, the two networks of the resistors being interconnected in a manner corresponding to the connections in the passive filter and connected to points corresponding to interconnection points of the passive filter by means of negative impedance converters, the converters connected to the first network having conversion ratios effective to convert resistance into negative capacitance and the converters connected to the second network having conversion ratios effective to convert resistance into negative inductance. The input and output to the active filter may be through similar negative impedance converters or may be directly to points in one or other resistive networks. An additional resistor connected between points of the active filter which correspond to terminals of a resonance circuit in the passive filter can provide Q-factor enhancement. A suitable design of negative impedance converter uses an operational amplifier the two inputs of which respectively form the input and output terminals of the converter with impedances connected from the output of the amplifier to its inputs.

Active filters utilizing networks of resistors and negative impedance

The invention relates to electrical oscillators arranged to oscillate so as to produce a substantially sinusoidal output signal. The said oscillator includes a general impedance transvertor circuit arrangement having two differential amplifiers connected only to a plurality of resistive and capacitive impedance elements and arranged to simulate a tuned circuit with sufficient feedback to maintain oscillation within the tuned circuit. The transvertor circuit is an active device in the form of an impedance invertor circuit or a convertor circuit.

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Ipswich, United Kingdom

Alan J Greaves