Voltage-controlled variable oscillator, in particular for phase-lock loops

Abstract: The voltage-controlled oscillator, comprises a differential stage formed by a pair of transistors ( T₃ and T₄) whose collectors define the output terminals between which an external resonating circuit (LC) is connected. A voltage generator (1) is connected between the two output terminals (A, B) and is suitable for generating a voltage which is proportional to the terminal voltage that exists across the resonating circuit (LC). The voltage generator output signal is applied to the resonation circuit (LC) through a pair of capacitors (Cs) to control the oscillating frequency. A bias current adjustment block (2) varies the bias current (Io) of the differential stage as the operating frequency varies in order to stabilize the peak-to-peak value of the output signal.

Description: The present invention relates to a voltage-controlled variable oscillator, in particular for phase-lock loops. As is known, a voltage-controlled oscillator (VCO) is an oscillator the frequency whereof varies according to the variation of an externally applied voltage. Said variable oscillators are extensively used in phase-lock loops (PLL), for example for conversion to video medium frequency. In particular, integratable variable oscillators, with a central frequency of 38.9 MHz and a variation interval of ± 500 KHz, are currently required for this application. The oscillator is furthermore required to provide a signal which must be as stable as possible in terms of frequency and peak-to-peak value as the temperature and the process parameters vary. In order to obtain the variability of the operating frequency, all current circuits use a capacitor in resonance with a fixed inductance. A known solution (shown in figure 1) has voltage-controlled varactors connected in parallel to the resonating circuit. These varactors can be internal or external to the integrated circuit; however, the integration of these varactors has problems in high speed processes since generally they cannot be integrated conveniently. On the other hand, the use of external components necessarily entails a more complicated application which in some cases is therefore undesirable. Another known solution provides the use of current controlled saturation capacitors obtained within an integrated circuit (see figure 2). According to this solution, by varying the current I, it is possible to directly bias the collector junctions of the transistors, thus modulating the equivalent capacity of the transistors themselves. However, this solution, despite being easily integratable, has numerous disadvantages related to the fact that the peak-to-peak output value depends on the operating frequency, the output frequency depends on the process parameters (e.g. on the inverse transit time) and the output characteristics depend on the temperature (through the saturation characteristics of the transistors). Document EP-A-0 080 280 discloses a solution of this kind used in a synchronous video detector circuit using phase-locked loops. Furthermore, document GB-A-2 043 378 discloses a voltage controlled oscillator which uses a parallel-resonant circuit to establish a frequency-sensitive phase characteristic in the loop by which the quiescent frequency is determined. A controllable gain circuit reinjects a variable-amplitude version of an oscillator output signal which phase shifted to establish the oscillation frequency. Given this situation, the aim of the present invention is to provide a voltage-controlled variable oscillator, in particular for phase-lock loops, which is capable of solving the disadvantages of the state of the art. Within the scope of this aim, a particular object of the present invention is to provide a variable oscillator which can provide a signal which is stable in terms of frequency and of peak-to-peak value as the temperature and process parameters vary. An important object of the present invention is to provide a variable oscillator which can be easily integrated. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by a voltage-controlled variable oscillator, in particular for phase-lock loops, as defined in the appended claims. The characteristics and advantages of the invention will become apparent from the description of a preferred embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is an electric diagram of a known solution; figure 2 is an electric diagram of another known solution; figure 3 is an electric diagram of a part of the circuit according to the invention; and figure 4 is a complete electric diagram of the oscillator according to the invention. With reference to figure 3, L and C respectively indicate the coil and the capacitor which constitute the external resonating circuit, which has a terminal voltage equal to V T sin(wt). The integrated circuit provided for driving the external resonating circuit comprises a differential stage which includes a pair of transistors T₃ and T₄, the collectors whereof are connected to the power supply V CC by means of respective resistors R. The collectors of the two transistors T₃ and T₄ are furthermore connected to the two output bump contacts for connection to the external resonating circuit. There are two feedback circuits, formed by the transistor T₁ and by the diode D₁ and respectively by the transistor T₂ and by the diode D₂, interposed between the collector of one of the transistors T₃ or T₄ and the base of the other transistor T₄ or T₃. The feedback circuits T₁,D₁ and T₂,D₂ ensure that the collector of the transistors T₃ and T₄ never drops below the voltage of the respective bases and that therefore these transistors never saturate. The circuit is completed by the current generator I O , connected to the joined emitters of the transistors T₃ and T₄, and the current generators I, which are connected to the bases of the transistors T₃ and T₄. Two capacitors C s are furthermore connected with one terminal to the collector of the transistors T₃ and T₄. According to the invention, a fraction K of the potential difference which occurs between the two output terminals (and therefore occurs across the terminals of the external resonating circuit) is applied to the other two terminals of the capacitors C s . At the points A and B therefore there is a voltage which is respectively equal to V T 2 sin (ωt) and - V T 2 sin (ωt) whereas at the points D and E the voltage is respectively equal to K V T 2 sin (ωt) and - K V T 2 sin (ωt) The current in the external resonating circuit i T is given by i T =G T ·V T cos(ωt) where G T is the load impedance given by: If V x is the voltage on the terminals of the capacitor C s connected to the collector of T₃, from the figure it can be immediately seen that V x = (K - 1) V T 2 sin (ωt) Since the current for a capacitor is given by i = C dv dt the current in C s is i x = C s ω (K - 1) V T 2 cos (ωt) When the differential of the oscillator is balanced, the following must be true: i x =i T and V T sin(ωt)=0 By replacing (1) and (4) in (5), the following is obtained: From (6), cos(wt)=1 and can be simplified. By solving with respect to w, the following is obtained: and consequently by varying K it is possible to vary the oscillator's operating frequency. If for example one chooses L=360.77 nanoHenry, C=45 picoFarad and C s =2.864 picoFarad, the oscillation frequency assumes the values comprised between 38.3 megahertz and 39.5 megahertz for K variable between -1 and 1. The factor K can be obtained by means of a linear multiplier, for example provided as a Gilbert multiplier. The complete solution with the Gilbert multiplier is thus shown in figure 4, wherein it is possible to identify, besides the diagram already shown in figure 3, the block 1, which indeed constitutes the linear multiplier (which is not described in detail, since it is well-known, and substantially comprises a Gilbert multiplier 3 and a voltage/current converter 4 interposed between the actual multiplier 3 and the points A and B) and a bias current adjustment block 2. In figure 4, V c indicates the external oscillation frequency control voltage. The block 2, which is substantially constituted by a differential stage formed by the transistors T₅ and T₆, which have emitter resistors and respectively receive, on their bases, a reference voltage V ref and the external control voltage V c , is advantageously provided in order to keep the peak-to-peak value of the oscillation constant. As can be seen from (2), the load impedance G T in fact depends on the frequency and more precisely the absolute value of the load impedance G T decreases as the operating frequency decreases. By increasing the bias current I o of the differential of the oscillator, its transconductance gain is thus increased, so that the total loop gain of the circuit is kept substantially constant. As can be seen from the preceding description, the invention achieves the proposed aim and objects. A variable oscillator (VCO) has in fact been provided which can be easily integrated (the capacity values required for C s are widely compatible with those which can be obtained by integrated capacitors) and is such as to be extensively independent of process parameter variations and of temperature variations. As can be seen from (8), the set frequency depends in fact only on the external components (L and C), on the integrated capacitors C s (which can be provided with good precision) and on the factor K, which can be controlled satisfactorily since it is set by means of a linear multiplier. The extreme symmetry of the circuit, which contributes to a reduction in sensitivity to manufacturing process spreads, is also stressed. Furthermore, by virtue of the feedback circuit T₁,D₁ and T₂, D₂ which prevents the transistors T₃ and T₄ of the differential stage from saturating, possible stray capacitances are reduced, thus providing greater insensitivity to process parameters. Computer simulations have shown that the output sinusoid has a distortion of less than 5 per mille, using the exemplifying values indicated above. Temperature sensitivity is of the following order of magnitude: 1 f o ∂f o ∂T ≃ 50 /ppm (parts per million) It has been observed that even a 10% spread between the capacitors C s gives rise to an extremely modest variation in the operating frequency. Relative sensitivity is of the following order of magnitude: 1 f o ∂f o ∂C s ≃ 30 ppm/spread percentage point The invention is furthermore conceptually simple and highly reliable. The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the appended claims. All the details may furthermore be replaced with other technically equivalent ones. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of the claims.

Claims: Voltage-controlled variable oscillator, in particular for phase-lock loops, comprising a differential stage formed by a pair of transistors (T₃ and T₄) which have collector terminals (A, B) which define output terminals suitable for connection to a resonating circuit (LC), characterized in that it comprises a voltage multiplier (1) which generates a voltage proportional to an external oscillation frequency control voltage (V c ) and to the terminal voltage which is present across said output terminals of the resonating circuit, one input (4) of said voltage multiplier being connected to at least one of said output terminals (A, B) and the output of said multiplier providing an output voltage being in phase with the respective input is connected to said at least one of said output terminals (A, B) by means of a capacitor (C s ) across which is present the voltage drop (V X ) between said terminal voltage and said voltage generated by said voltage multiplier. Oscillator according to claim 1, characterized in that said voltage multiplier (1) comprises a Gilbert multiplier (3). Oscillator according to the preceding claims, characterized in that it comprises a pair of capacitors (C s ), each of which is connected with one terminal to a respective output terminal and with the other terminal to the respective outputs of said Gilbert multiplier. Oscillator according to one or more of the preceding claims, characterized in that it comprises a pair of feedback circuits (T₁,D₁ and T₂,D₂)interposed between the collector terminal of one of said transistors (T₃ or T₄) of said differential stage and the base terminal of the other transistor (T₄ or T₃). Oscillator according to one or more of the preceding claims, characterized in that it comprises a variable current generator (2) which is connected to the emitter terminals of said transistors (T₃ and T₄) of said differential stage to vary the bias current of said differential stage as the operating frequency of the resonating circuit varies. Oscillator according to one or more of the preceding claims, characterized in that said variable current generator (2) comprises a differential stage which receives on its inputs a reference voltage (V ref ) and the oscillator's frequency control voltage (V c ).

Inventors: BETTI GIORGIO, ZUFFADA MAURIZIO, BETTI, GIORGIO, ZUFFADA, MAURIZIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1990
Main classification: H03B5
IPC codes: H03B5, H03L7