GB2069788A

GB2069788A - Improvements in or relating to tuneable quartz overtone oscillators

Info

Publication number: GB2069788A
Application number: GB8104635A
Authority: GB
Original assignee: INST TELE I RADIOTECH
Current assignee: INST TELE I RADIOTECH
Priority date: 1980-02-13
Filing date: 1981-02-13
Publication date: 1981-08-26
Also published as: FR2475821A1; PL222005A1; DE3105134A1; PL127641B1

Abstract

A tunable quartz overtone oscillator is provided having, between the base of the transistor 1 and a common point 2 a two-terminal network connected in series and consisting of an inductance element 3 and a capacitance element 4 forming, together with a capacitance 5 connected between the base and the emitter of the transistor, an in-series circuit of resonance frequency higher than the frequency of the fundamental resonance of the quartz resonator 6 but lower than the frequency of the utilized overtone oscillation of the quartz resonator. Between the common point 2 and the emitter of the transistor 1 a two-terminal network is connected in-series comprising a tuning capacitor and a quartz resonator. <IMAGE>

Description

SPECIFICATION Improvements in or relating to tuneable quartz overtone oscillators The invention relates to tuneable quartz overtone oscillators.

A known design of a quartz overtone oscillator with tuneable frequency (U.S. patent No. 4001724) is a Colpitt's three-point generating circuit with a cascade output. In this circuit, for the purpose of obtaining a required range of tuning and selection of the required overtone oscillation, two inductive elements are connected in series and parallel to a quartz resonator. For tuning of frequency, a varactor diode is connected in series with a third inductive element. A fourth inductive element between the varactor and the emitter of a transistor serves for elimination of undesirable oscillations in the circuit.

Another known design of an oscillator with tuneable frequency (U.S. patent No. 3916344) operates in a three-transistor Butler's circuit.

A two-terminal network consisting of a quartz resonator and an inductance element connected in parallel is connected between the emitters of transistors which constitute a tuned amplifier. The load of the first transistor in the amplifier is a rejector circuit consisting of a second inductance element and a variable capacitance element in a form of a capacitance diode.

The above described circuits are characterized by a high degree of complexity and require the use of at least two inductance elements which aggravate in a substantial way the thermal and long-term frequency stability of the signal of the oscillator. Also, inductance elements are of large size and are inconvenient for assembly since they require screening.

According to the invention, there is provided a quartz overtone oscillator with tuneable frequency operating in a one-transistorthree-point Colpitt's circuit, characterized in that between the base of a transistor and a common point, a two-terminal network is connected which consists of an inductance element and a capacitance element and forms together with a capacitance connected between the base and the emitter of the transistor, an in-series circuit of a resonance frequency higher than the frequency of the fundamental resonance of a quartz resonator, but lower than the frequency of the utilized overtone oscillator of the quartz resonator, and between the common point and the emitter of the transistor, an in-series two-terminal network is connected which consits of a tuning capacitance and the quartz resonator.

It is thus possible to provide a circuit having a minimum number of inductance elements and improved frequency stability of tuneable overtone oscillation.

Such an oscillator can be provided having a wide range of tuning and a good frequency stability of the signal, because of the reduction in the number of inductance elements to one induction element which serves for selection of the required overtone oscillation and for widening of the range of frequency tuning. Such an oscillator can maintain these properties within the whole frequency range of presently produced quartz resonators and in the metric waves range (upto about 300 MHz).

The invention will be further described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows the basic circuit of an oscillator constituting a preferred embodiment of the invention; and Figure 2 shows a practical circuit of the oscillator operating at the frequency of 132.7 M Hz, particularly useful in the range of metric waves.

In Figure 1 there is connected between the emitter of a transistor 1 and a common point 2 a series resonant circuit comprising an inductance element 3 and capacitance elements 4 and 5. This circuit determines the selection of the chosen overtone oscillation of a resonator 6.

The overtone is selected by selecting the values of the elements 3 and 5 so that the resonant frequency of the circuit is hig her than the frequency of funda mental oscillation of the quartz resonator 5 but is lower than the frequency of the selected overtone of the resonator. In practice, the reactance of the resonant circuit is selected to have the same modulus as the reactance of the static capacitance of the quartz resonator at the operating frequency of the oscillator.

The branch consisting of the quartz resonator 6 and a tuning capacitance element 7 connected in series is connected between the emitter of the transistor 1 and the common point 2. At the operating frequency of the oscillator, this branch has a capacitive nature so that the resonator operates below the frequency of series resonance of the selected overtone oscillation. The resonator when operating in this frequency range is very susceptible to frequency tuning by means of external reactance owing to a relatively small rate of change of reactance of the resonator as a function of frequency. The element tuning the frequency of the oscillator is the variable capacitor7 which is usually in the form of a variable capacitance diode.

The principle of operation of the circuit shown in Figure 2 is identical to that of the basic circuit. The main differences between these circuits are limited to earthing of a different electrode of the transistor 8, the use of a resistor 9 for attenuating undesirable oscillations because of the interelectrode capacitance ofthe quartz resonator 10, and the use of a diode 11 together with an isolating resistor 12 as the variable capacitance. Earthing of the base of the transistor 8 in this circuit reduces harmful interelectrode capacitance of the resonator 10, whose essential components are the capacitances between electrodes and the usual metal casing of the quartz resonator 10.These capacitances in the circuit of Figure 2, are not added to the interelectrode capacitance of the resonator 10, but increase by a negiigible amount the capacitances 13 and 14 without any unfavourable consequences.

The use of the resistor 9 for attenuating the u ndesirable oscillations becomes necessary at operating frequencies above 60 MHz owing to the reactance of the interelectrode capacitance of the resonator 10 which decreases with increasing frequency and, in consequence, increases the susceptibility of the circuit to the excitation of undesirable oscillations.

Capacitor 14 permits precise selection of reactance of an inductive character, which is provided by the parallel connection of the capacitor and an inductance element 15. This is advantageous from the practical point of view, since the inductance element 15 should be of stable value, i.e. a noncontrnlled or fixed element. Capacitors 16, 17, and 18 serve for separation of the constant component from the variable component of voltages and currents in the circuit In the circuit the full tuning range of about 50 kHz is obtained, which in the measure of relative frequency changes is 3.8 x 10-4.

In one practical example of the circuit of Figure 2, the various components were of the following types and values.

Resonator 10 type T 807/8 made by C.E.P.E.

(oscillation of the seventh overtone) Transistor 8BFR90 Capacitors 1343 pF Varicap 11 BBl05B 14.2.5 pF Inductance 150.2 #H 163.3 nF Resistors 9 2.2kQ 173.3 no 12510kfl 183.3nF 20 Okfl 21 10kQ 22 12kQ The parameters of the oscillator of Figure 2, and especially its high operating frequency and wide range of tuning,make it suitable for applications e.g.

in UHF FM radio-telephones with direct modulation of frequency.

Claims

1. A quartz overtone oscillator with tuneable frequency operating in a one-transistor three-point Colpitt's circuit, characterized in that between the base of a transistor and a common point, atwo- terminal network is connected which consists of an inductance element and forms together with a capacitance connected between the base and the emitter of the transistor an in-series circuit of a resonance frequency higherthan the frequency of the fundamental resonance of a quartz resonator but lowerthan the frequency of the utilized overtone oscillator of the quartz resonator, and between the common point and the emitter of the transistor an in-series two-terminal network is connected which consists of a tuning capacitance and the quartz resonator.

2. A tuneable quartz overtone oscillatorcomprising a non-inverting amplifying element having an input connected via a circuit comprising an inductor and a firstcapacitorto a common point, a second capacitor connected between the input and output of the amplifying element, and a series connected tuning capacitor and quartz resonator connected between the common point and the output of the amplifying element, the first and second capacitors and the inductor forming a series-resonantcircuitwhose resonance frequency is higher than the fundamental resonance frequency of quartz resonator but lower than the overtone frequency.

3. An osciliatorasclamed in claim 1 or2, in which the variable capacitance is a variable capacitance diode.

4. Atuneable quartz overtone oscillator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.