GB2160043A

GB2160043A - Audio frequency oscillator

Info

Publication number: GB2160043A
Application number: GB08415073A
Authority: GB
Inventors: Andrew Dunkley
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1984-06-13
Filing date: 1984-06-13
Publication date: 1985-12-11
Also published as: GB8415073D0

Abstract

An audio frequency oscillator comprising of a modified 2 port gyrator arranged to simulate an inductance with an associated parallel negative resistance. The gyrator comprises first and second operational amplifiers (10, 12) and an RC impedance chain (14, 16. 18, 20, 22 and 24) connected in a manner known per se. By the addition of a suitably sized resistor (32) to the gyrator circuit a parallel negative resistance can also be simulated. The oscillating circuit is completed by a capacitor (36) connected in parallel with the impedance chain. The frequency of oscillation is set by adjusting a potentiometer (16) in the impedance chain. By selecting 1% resistors and high Q capacitors a low distortion, low sensitivity to temperature changes audio oscillator is obtained which is suitable for application in portable and mobile transceivers. <IMAGE>

Description

SPECIFICATION Audio frequency oscillator This present invention relates to an audio frequency oscillator, more particularly to an oscillator formed by modifying a gyrator circuit simulating an inductance and a negative resistance.

Gyrators are known per se and are described in the prior art for example in 'The right gyrator trims the fat of active filters' by Thomas H. Lynch, Electronics, July 21, 1977, pages 115 to 119. This article points out that a gyrator is a lossless two-port circuit which inverts a load impedance and that a preferred realisation requires only two amplifiers and five impedances. The primary application of such gyrators is as active filters wherein a capacitance is used to simulate an inductance.

According to the present invention there is provided an audio frequency oscillator comprising a Riordan gyrator including first and second operational amplifiers and an impedance chain having a first capacitor so that the gyrator simulates an inductance; a resistance connected between interconnected inverting inputs of the operational amplifiers and a reference voltage line to simulate a negative resistance, the value of the negative resistance being such as to offset any associated parallel resistive damping, and a second capacitor connected in parallel with the impedance chain.

The audio frequency oscillator circuit made in accordance with the present invention is based on eliminating the damping which is associated with a parallel L.C. network. By eliminating the damping and coupling the second capacitance to the modified gyrator circuit then one has a lossless parallel L.C. circuit capable of oscillation. The frequency of the oscillator circuit may be set by providing a potentiometer in the impedance chain of the gyrator, the wiper of the potentiometer being coupled to the output of the second operational amplifier, the output of the first operational amplifier being connected to one side of the first capacitor.

Amplitude stability in the output of the oscillator, which output may be taken from the output of either one of the gyrator operational amplifiers, may be achieved by setting the clipping level of the amplifier and suitably selecting the value of the resistance.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic circuit diagram of an embodiment of the present invention, and Figure 2 is a symbolic representation of Figure 1.

The audio oscillator shown in Figure 1 comprises a Riordan gyrator constituted by first and second operational amplifiers 10, 12, respectively. The inverting inputs of the amplifiers 10, 12 are interconnected. As the gyrator is simulating an inductance, an RC impedance chain is provided. In the illustrated circuit, the impedance chain comprises a series arrangement of a first fixed value resistor 14, a potentiometer 16, second and third fixed value resistors 18, 20, a first capacitor 22 which is generally termed the gyrator capacitor, and a fourth fixed resistor 24. A junction 26 of the resistors 18, 20 is connected to the interconnection between the inverting inputs of the amplifiers 10,12.The non-inverting input of the amplifier 10 is connected to the end of the impedance chain adjacent the resistor 14, and the non-inverting input of the amplifier 12 is connected to the junction 28 of the first capacitor 22 and the fourth resistor 24. the low impedance output of the amplifier 10 is connected to the junction 30 of third resistor 20 and the first capacitor 22. The wiper of the the potentiometer 16 is connected to the output of the amplifier 12. The circuit described so far is effectively a gyrator which inverts a load impedance; the capacitance of the first capacitor 22, to simulate the impedance of an inductor. By connecting a second capacitor 36 between on the one hand, the reference voltage line 34, and on the other hand, the resistor 14 and the non-inverting input of the amplifier 10 a parallel L.C. circuit can be simulated.The circuit however will not oscillate as the circuit is not lossless and the associated parallel resistance Rcp will damp the circuit down. In order to produce sustained oscillation the damping effect of Rcp has to be eliminated by the introduction of a negative resistance.

this negative resistance can be simulated by the gyrator network by introducing a suitably sized resistor 32 between the inverting inputs of the amplifiers 10, 12 and a reference voltage line 34 to which the other end of the RC impedance chain is also connected. then the circuit frequency of the illustrated oscillator is set by adjusting the wiper of the potentiometer 16. An output of the oscillator can be derived from either one of the operational amplifiers 10, 12 and for the sake of completeness, an output terminal 36 is shown connected to the output of the amplifier 10.

In implementing the illustrated circuit the amplifiers 10, 12 may comprise a dual operational amplifier type MP 1485 or type 741 the resistors 20, 24 are of equal resistance R1, the resistors 14, 18 are of equal resistance R2 and the resistor 32 has a resistance R3. For operation at 3825HZ, R1 = 12K1, R2 = 8K25 and R3 = 1MQ although other values for R3 may be selected depending on the required negative resistance. The first and second capacitors 22 and 36 are multi-layer ceramic capacitors of 3n3F. The potentiometer 16 has a value of 1K# but if a larger value is chosen then a greater range of frequency adjustment is obtainable.By selecting 1% tolerance resistors 14,18,20, 24 and 32 and high Q capacitors 22, 36, then one obtains a high stability oscillator having a low distortion, for example less than 1.5% at 3825HZ and low sensitivity to temperature changes, for example 15HZ frequency drift over a temperature range of -30 C to +60 C.

Amplitude stability is achieved by setting the clipping level of the amplifiers 10, 12 and selecting the resistance value R3 of the resistor 32. As an example for a 10V supply, an 8V peak-to-peak oscillating waveform is obtainable.

Figure 2 is a symbolic representation of the circuit shown in Figure 1. The gyrator is shown as a two-port device 40 simulating an inductance L and a parallel connected negative resistance -Rcp. A resistor Rcp and a second capacitor 36 are connected in parallel with the device 40.

The resistor 32 may be arranged to be switched in and out of circuit as required. Thus with it out of circuit the gyrator can form a notch filter and when it is in circuit then say an alert tone can be generated.

The audio oscillator described may be fabricated as an integrated circuit or by thin or thick film techniques which makes it suitable for those applications, such as portable and mobile transceivers, in which space and power consumption are important considerations.

Claims

1. An audio frequency oscillator comprising a Riordan gyrator including first and second operational amplifiers and an impedance chain having a first capacitor so that the gyrator simulates an inductance; a resistance connected between interconnected inverting inputs of the operational amplifiers and a reference voltage line to simulate a a negative resistance, the value of the negative resistance being such as to offset any associated parallel resistive damping, and a second capacitor connected in parallel with the impedance chain.

2. An oscillator as claimed in Claim 1, wherein the impedance chain includes a potentiometer, the wiper of which is connected to an output of the second operational amplifier, the output of the first operational amplifier being connected to one side of the first capacitor.

3. An oscillator as claimed in Claim 1 or 2, wherein the gains of the operational amplifiers are set and the value of the negative resistance is selected to achieve a desired amplitude stability.

4. An oscillator as claimed in Claim 1, 2 or 3, wherein the resistances in the impedance chain and the negative resistance are 1% tolerance resistors and the first and second capacitors are high Q capacitors.

5. An audio frequency oscillator constructed and arranged to operate substantially as herein be fore described with reference to and as shown in the accompanying drawings.

6. An audio frequency oscillator comprising first and second operational amplifiers, each having an inverting input, a non-inverting Input and an output, said inverting inputs being interconnected; an impedance chain formed by series connected first fixed resistance, a potentiometer, second and third fixed resistances, a first capacitance and a fourth fixed resistance, the fourth resistance being coupled to a reference voltage line, a junction of said second and third resistances being coupled to said inverting inputs, the non-inverting input of the first amplifier being coupled to the first resistance, the non-inverting input of the second amplifier being connected to a junction of the first capacitance and the fourth resistance, the output of the first amplifier being coupled to a junction of the third resistance and the first capacitance, the output of the second amplifier being connected to a wiper of the potentiometer; a fifth resistance connected between said interconnected inverting inputs and the reference voltage line and a second capacitance coupled in parallel with the impedance chain.