VOLTAGE CONTROLLED OSCILLATOR WITH LOW-VOLTAGE GAIN STABILIZATION

Abstract: Voltage controlled oscillator (VCO) comprising a resonator (228), a plurality of tuning elements coupled to the resonator, a plurality of input ports, each being coupled to at least a selected one of the tuning elements, each of the input ports receiving a predetermined external control voltage (Vcourse, Vfine), wherein each of the tuning elements is characterized by a plurality of parameters and respective parameter values, wherein the parameter values vary non-linearly and inversely proportional to the respective control voltage for at least a selected one of the tuning elements (208), and wherein the parameter values vary non-linearly and proportional to the respective control voltage for at least a selected other of the tuning elements (214).

Description: Voltage Controlled Oscillator With Low-Voltage Gain Stabilization Field Of The Invention The present invention relates to tunable phase locked loop systems in general, and to systems which include a voltage controlled oscillator, in particular. Background Of The Invention Frequency synthesizer phase locked loops are widely used in communication transceivers. These loops typically include a voltage-controlled oscillator (VCO). In these conventional techniques, voltage controlled capacitors (varactors), are usually used to vary the resonant frequency of the VCO tuning network in accordance with a control voltage. The varactor has significantly non-linear response to applied voltage, especially at low voltages. Such a behavior leads to the use of high varactor control voltages, typically 12 Volts or more, to achieve the VCO tuning range that is needed for feedback transceiver applications. Reference is now made to Figure 1 , which is a schematic illustration of a conventional phase locked loop frequency synthesizer, generally referenced 100. Frequency synthesizer 100 includes a reference frequency source 102, a reference frequency divider 104, a digital-to-analog converter (DAC) 106, a controller 108, a phase detector 110, a loop filter 118, a VCO 120 and a feedback frequency divider 114. VCO 120 further includes a VCO tuning block 112 and a VCO active block 116. Reference frequency divider 104 is coupled to reference frequency source 102, controller 108 and phase detector 110. Feedback frequency divider 114 is coupled to phase detector 110, controller 108 and VCO active block 116. Loop filter 118 is coupled to phase detector 110 and to VCO tuning block 112. DAC 106 is coupled to VCO tuning block 112 and to controller 108. VCO active block 116 is coupled to VCO tuning block 112 and to feedback frequency divider 114. Reference frequency divider 104 receives a signal, having the frequency F R , from frequency source 102. Reference frequency divider 104 further divides this signal by N and provides the resultant signal, having the frequency F R /N, to phase detector 110. Feedback frequency divider 114 receives a feedback signal, having the output frequency F ouτ , from VCO active block 116. Feedback frequency divider 114 further divides this signal by M and provides the resultant signal, having the frequency F ouτ /M, to phase detector 110. Phase detector 110 compares these two signals, generates a respective output control signal and provides it to loop filter 118. Typically, loop filter 118 is a low-pass filter. It integrates the output control signal and provides the resultant output voltage V FlNE to VCO tuning block 112. Depending on the value of V RNE , VCO tuning block 112 adjusts the output frequency F ouτ so that it would be equal to the desired value — F R . This adjustment is performed within a comparatively narrow frequency range (fine-tuning). For a coarse tuning within a wide frequency range, DAC 106 provides a respective control signal V C0ARSE to VCO tuning block 112. Reference is now made to Figure 2, which is a schematic illustration in detail of a conventional VCO tuning block 112 of Figure 1. VCO tuning block 112 includes two inductors 150 and 152, a first varactor VR C 158, a second varactor VR F 160, three capacitors 154, 156 and 164 and a resonator 162. Varactor VR C 158 is coupled to inductor 150 and capacitor 154. Varactor VR F 160 is coupled to inductance 152 and capacitor 156. Resonator 162 is coupled to capacitors 154, 156 and 164. A varactor is a voltage-controlled diode whose capacitance changes with applied voltage. The character of this change is non-linear. Reference is now made to Figure 3, which is a graphical illustration of a typical dependence of varactor capacitance on applied voltage. At low voltages (for example, between 0.5 Volts and 1.5 Volts) the capacitance is maximal (for example, between 15 pF and 25 pF) and at high voltages (for example between 3.5 Volts and 4.5 Volts) the capacitance is minimal (for example, between 2 pF and 3 pF). Varactor voltage sensitivity, which is a derivative of varactor capacitance with respect to applied voltage, also changes in a non-linear manner. The voltage sensitivity of the varactor is much higher for the lower values of applied voltage than for the higher values of applied voltage. Referring back to Figure 2, the capacitance of varactor VR C 158 is controlled by the voltage provided by DAC 106 of Figure 1 via inductor 150. This steering voltage is responsible for a coarse tuning. The capacitance of varactor VR F 160 is controlled by the output voltage V FINE , provided by loop filter 118 of Figure 1. The output voltage V F , NE is provided to varactor 160 via inductor 152. The output control signal executes a fine tuning of the VCO. Since a VCO operating frequency is inversely proportional to the varactor capacitance, VCO sensitivity is much greater at low operating frequencies than at high operating frequencies. In order to increase the operating frequency range while keeping the VCO sensitivity at a low level, known devices use comparatively high voltages for VCO tuning (up to 13 Volts). New generations of IC for portable communication devices are based on low-voltage elements (3 - 5 Volts maximum). According to the foregoing explanation, variations of the control voltage within 5 Volts range will lead to a reduction of the operation frequency bandwidth and to large variations of the VCO gain. It is therefore desirable to have a low-voltage VCO circuit with varactors, which will provide the required operating frequency range and low VCO sensitivity. Prior art published United Kingdom patent application GB-A- 2,338,128 shows a voltage controlled oscillator. GB-A-2,338,128 describes a method for flattening the frequency modulation deviation over the frequency range of the VCO. This invention can be used in radio transmitters that use FM modulation and directly modulate the VCO. Prior art published United States patent US-A-4,003,004 is concerned with getting a linear response to the FM modulating signal from the VCO. Referring to figure 1 , the modulation signal goes through modulation FET amplifier 50 to the varactors 30/32, thus FM modulating the VCO. FET amplifier response is compensating for varactor nonlinearity and the response to modulating signal is linear. Summary Of The Present Invention It is an object of the present invention to provide a novel system, which includes voltage-controlled oscillator (VCO) with low-voltage gain stabilization, which mitigates the disadvantages of the prior art. It is another object of the present invention to provide a VCO with small gain variations in response to variations of the control voltage for coarse tuning. In accordance with the present invention, there is thus provided a VCO, which includes a resonator, a plurality of tuning elements, coupled to the resonator, and a plurality of input ports. Each of the input ports is coupled to at least a selected one of said tuning elements and receives a predetermined external control voltage. Each of the tuning elements is characterized by a plurality of parameters and respective parameter values. The parameter values vary non-linearly and inversely proportional to the respective control voltage for at least selected one of the tuning elements. The parameter values further vary non-linearly and proportional to the respective control voltage for at least selected other one of the tuning elements. In accordance with another aspect of the present invention, there is provided a VCO in a phase locked loop (PLL) of a frequency synthesizer. The PLL includes a phase detector for comparing a phase of a reference signal with a phase of an output VCO signal, and for generating an output signal for fine VCO tuning thereof. The output signal is responsive to the difference between the phases of the reference signal and the output VCO signal. The PLL further includes a controller and a control voltage source. The control voltage source generates a control voltage for the coarse VCO tuning at the output. The control voltage is responsive to the commands from the controller. The VCO includes three input ports, of which a first input port is coupled to the output of the phase detector, a second input port is coupled to the output of the control voltage source, and a third input port is coupled to a bias voltage source, which provides a predetermined bias voltage. The VCO further includes a resonator and a first, a second and a third tuning elements. The first tuning element is coupled to the resonator and to the first VCO input port. The second tuning element is coupled to the resonator and to the second VCO input port. The third tuning element includes a first port and a second port, wherein the first port is coupled to the second VCO input port, and the second port is coupled to the third VCO input port and to the resonator. Each of the first, second and third tuning elements possesses a capacitance, which is varying within a predetermined range in response to an applied voltage. The resonator possesses a resonance frequency, which is responsive to variations of the capacitance of the first, second and third tuning elements. Brief Description Of The Drawings The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: L _' Figure 1 is a schematic illustration of a prior art phase locked loop frequency synthesizer; Figure 2 is a schematic illustration in detail of VCO tuning block of Figure 1 ; Figure 3 is a graphical illustration of a prior art typical dependence of varactor capacitance on applied voltage; Figure 4 is a schematic illustration of a VCO tuning block, constructed and operative in accordance with a preferred embodiment of the present invention; and Figure 5 is a graphical illustration of a typical dependence of the capacitance of varactor VR 1 214 of Figure 4 on applied voltage. Detailed Description Of Preferred Embodiments The present invention mitigates the disadvantages of the prior art by providing a tunable VCO, which requires low control voltage values to provide a wide-band control range. Reference is now made to Figure 4, which is a schematic illustration of a VCO tuning block, generally referenced 200, constructed and operative in accordance with a preferred embodiment of the present invention. VCO tuning block 200 includes four inductors 202, 204, 206 and 224, seven capacitors 210, 212, 216, 218, 220, 226 and 230, a first varactor VR C 208, a second varactor VR^ 214, a third varactor VR F 222, and a resonator 228. Varactor VR C 208 is coupled to inductor 202 and capacitor 210. Varactor VR 1 214 is coupled to inductors 204 and 206 and to capacitors 212 and 216. Varactor VR F 222 is coupled to inductor 224 and to capacitors 220 and 226. Capacitor 218 is coupled to capacitors 220, 230, 212 and 210, and to resonator 228. Inductor 206 is coupled to inductor 202. DAC 106 of Figure 1 provides V C0ARSE voltage to varactor VR C 208, via inductor 202, and to varactor MR^ 214 via inductor 206. According a to preferred embodiment of the present invention, additional bias voltage (for example, +5V) is applied to varactor VR 1 214. In this case the capacitance of varactor VR 1 214 changes with the applied voltage V C0ARSE in a manner, which is opposite to that of varactor VR C 208. Reference is now made to Figure 5, which is a graphical illustration of a typical dependence of the capacitance of varactor VR 1 214 of Figure 4 on applied voltage. In contrast to the dependence, which was presented in Figure 3, the capacitance of varactor VR., 214, as well as its sensitivity, are minimal for low V COARSE values and maximal for high V C0ARSE values. Referring back to Figure 4, the capacitance of varactor VR C 208 is high and the capacitance of varactor VR 1 214 is low when V C0ARSE is low. In an opposite case where the value of V C0ARSE is high, then, the capacitance of varactor VR C 208 is low and the capacitance of varactor VR 1 214 is high. Thus, the resulting change of the capacitance, as well as that of the VCO sensitivity, over the entire range of V COARSE variations, is small. This allows applying comparatively low voltages (up to 5 Volts) for controlling VCO operating frequency over a wide frequency range, while keeping low VCO sensitivity. Returning now to a consideration of the prior art documents cited earlier, it is notable that the present invention provides the possibility of designing a wideband VCO, while keeping the VCO gain constant and small. The present invention is not intended to be used for flattening the FM deviation, as was the arrangement of GB-A-2338128. Here, reference is made to Figure 4 in GB-A-2338128 and to Figure 4 in the present invention. Vsteer in GB-A-2338128 corresponds to Vfine in the present invention. This is the line that is connected to the Loop Filter, and via this line, the phase locked loop is steering the VCO. The purpose of the present invention is to make the VCO sensitivity to this steering line as small as desired, and constant across the VCO frequency band. The purpose of GB-A-2338128 is to keep the VCO response to the audio modulation signal flat over the VCO frequency range. GB-A-2338128 does not keep VCO gain (dF/dVsteer) low and constant. Vcoarse in the present invention is controlling the VCO banding, and, at the same time, contributes to VCO gain flatness via the VR1 arrangement. Note that Vcoarse is a DC voltage. The VR1 (214) arrangement allows the present invention to get complementary varactor response as shown in figure 5 of the appended drawings. In summary, the present invention is completely different from GB-A-2338128, both in it's objectives and in how the invention works. The present invention differs from the arrangement of US 4,003,004. The present invention achieves small and constant VCO gain over the VCO frequency range. The objective of the present invention is therefore different from US 4,003,004. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. The scope of the present invention is defined only by the appended claims.

Claims: Claims 1. Voltage controlled oscillator (VCO) comprising: a resonator; a plurality of tuning elements coupled to said resonator; a plurality of input ports, each being coupled to at least a selected one of said tuning elements, each said input ports receiving a predetermined external control voltage; wherein each of said tuning elements is characterized by a plurality of parameters and respective parameter values; wherein said parameter values vary non-linearly and inversely proportional to said respective control voltage for at least selected one of said tuning elements; and wherein said parameter values vary non-linearly and proportional to said respective control voltage for at least selected other of said tuning elements. 2. The VCO according to claim 1 , wherein each of said tuning elements is characterized by a capacitance, said capacitance varying within a predetermined range in response to said control voltage. 3. The VCO according to claim 2, wherein said capacitance of each of said tuning elements depends non-linearly on said respective control voltage. 4. The VCO according to claim 3, wherein said capacitance of at least one of said tuning elements varies inversely proportional to said respective control voltage. 5. The VCO according to claim 3, wherein said capacitance of at least one of said tuning elements varies proportional to said respective control voltage. 6. The VCO according to claim 1 , wherein at least one of said tuning elements is a varactor. 7. Voltage controlled oscillator 1CO) in a phase locked loop (PLL) of a frequency synthesizer, the PLL including a phase detector for comparing a phase of a reference signal with a phase of an output VCO signal, the phase detector generating an output signal for a fine VCO tuning, the output signal being responsive to the difference between the phases of the reference signal and the output VCO signal, the PLL further including a controller, and a control voltage source, the controller coupled to the input of the control voltage source, the control voltage source generating a control voltage for the coarse VCO tuning at the output, the control voltage being responsive to the commands from the controller, the VCO having a first input port coupled to the output of the phase detector, a second input port coupled to the output of the control voltage source and a third input port, coupled to a bias voltage source, the bias voltage source providing a predetermined bias voltage, the VCO including: a resonator; a first tuning element, coupled to said resonator and to said first VCO input port; a second tuning element, coupled to said resonator and to said second VCO input port; a third tuning element, having a first port and a second port, wherein said first port is coupled to said second VCO input port and said second port is coupled to said third VCO input port and said resonator; wherein each of said first, second and third tuning elements possesses a capacitance, said capacitance varying within a predetermined range in response to an applied voltage, and, wherein said resonator possesses a resonance frequency, said frequency being responsive to variations of the capacitance of said first, second and third tuning elements. 8. The VCO according to claim 7, wherein said capacitance of said first, second and third tuning element depends non-linearly on applied voltage. 9. The VCO according to claim 8, wherein said capacitance of said first tuning element varies inversely proportional to said output signal, applied to said first VCO input port. 10. The VCO according to claim 8, wherein said capacitance of said second tuning element varies inversely proportional to said control voltage, applied to said second VCO input port. 11. The VCO according to claim 8, wherein said capacitance of said third tuning element varies proportional to said voltage, applied to said second VCO input port, and, wherein the dependence of said capacitance of said third tuning element on said voltage, applied to said second VCO input port is responsive to the value of said bias voltage applied to said third VCO input port. 12. The VCO according to claim 7, wherein said first tuning element is a varactor. 13. The VCO according to claim 7, wherein said second tuning element is a varactor. 14. The VCO according to claim 7, wherein said third tuning element is a varactor.

Inventors: BEN-AYUN MOSHE, ROZENTAL MARK, ZILBERMAN NIR, BEN-AYUN, MOSHE, ROZENTAL, MARK, ZILBERMAN, NIR
Applicants: BEN AYUN MOSHE, MOTOROLA ISRAEL LTD, ROZENTAL MARK, ZILBERMAN NIR, BEN-AYUN, MOSHE, MOTOROLA ISRAEL LIMITED, ROZENTAL, MARK, ZILBERMAN, NIR
Application year: 2001
Main classification: H03C3
IPC codes: H03B5, H03L7