Method and apparatus for extending a VCO tuning range

[0002] The present invention relates to extending the tuning range of a voltage controlled oscillator (VCO).

[0003] In developing products which are compatible with industry standards, such as GSM/DCS (Global System for Mobile Communications/Digital Communication Systems), it is necessary to tailor the design within the constraints imposed by those standards. For example, an oscillator may be required to have a wide tuning range, while limiting phase noise and tuning sensitivity variation. At times, it can be challenging to achieve all of the requirements given commercially available components and their inherent tolerances. It is particularly difficult to do so with a conventional tank circuit—an inductor and a capacitor in parallel.

[0004] Phase noise of the VCO is determined largely by the Q (quality factor) of the tank circuit. A VCO with less phase noise can be achieved if there is a larger Q. This will result in lower total RMS phase error in the system and an output signal with a cleaner spectrum. RMS phase error and spectrum purity are two of the most important specifications for any communications system.

[0005] A varactor may be used in designing a tank circuit. The voltage dependent capacitance of the varactor can be varied to tune the VCO frequency. The tank circuit Q is dependent on the Q factors of its components, and the Q of a varactor is typically much lower than that of other capacitances or inductances. When a varactor is present in a tank circuit to tune the VCO frequency, the Q of the varactor is usually the limiting factor of a tank circuit Q.

[0006] The degradation of the tank circuit Q caused by a varactor can be reduced by using a coupling capacitance to reduce the impact of the varactor capacitance on performance of the tank circuit. However, this also reduces the tuning range of the VCO. Around a given operating frequency, the tuning range can depend on the capacitance ratio of the tank circuit, which results from the impact on the tank circuit of the variable capacitance of the varactor. Therefore, modifying the impact of the varactor capacitance on the tank circuit by adjusting the coupling capacitance is a trade-off. Extending tuning range degrades Q and phase noise, and vice versa.

[0007] In addition, capacitance of a varactor generally varies nonlinearly with respect to the control voltage. Consequently, increasing the impact of the varactor capacitance on the tank circuit also degrades the tuning sensitivity variation of the VCO.

[0008] Besides adjusting a coupling capacitance, the tuning range also can be extended by using a varactor with a greater range of capacitance. However, any design is limited by what is commercially available to implement the design. Varactors with a greater range of capacitance generally have a lower Q and more non-linearity. The tuning range can be extended using a varactor with a greater range of capacitance, but the tuning sensitivity and phase noise of the VCO will suffer.

[0009] An improved and novel way to extend the tuning range of a VCO while also satisfying the other constraints is to connect a shunt inductance in parallel with the varactor. Prior art did not recognize this way to extend tuning range of a VCO tank circuit without affecting tuning sensitivity variation and phase noise.

[0010] The features of the present invention which are believed to be novel are set forth below with particularity in the appended claims. The invention, together with further advantages thereof, may be understood by reference to the following description in conjunction with the accompanying figures. FIG. 3 illustrates an embodiment of the invention.

[0011] FIG. 1 shows an example of a variable capacitance in series with a coupling capacitance.

[0012] FIG. 2 shows an example of a variable capacitance in parallel with a shunt inductance.

[0013] FIG. 3 shows the circuit diagram of one embodiment of part of a VCO according to the invention.

[0014] FIG. 1 is an illustration, for discussion purposes, of a variable capacitance C_vin series with a coupling capacitance C_c. The two in series may be viewed together as an effective capacitance C_swith an impedance $\begin{array}{cc}\frac{1}{\mathrm{j\omega }{C}_s}=\frac{1}{\mathrm{j\omega }{C}_v}+\frac{1}{\mathrm{j\omega }{C}_c}.C_v{C}_c=C_s{C}_c+C_s{C}_v,\mathrm{and}{C}_s=C_v\left(\frac{C_c}{C_c+C_v}\right).& \left(1\right)\end{array}$

[0015] As the coupling capacitance C_cis made larger relative to the variable capacitance C_v, the variable capacitance C_vhas a greater impact on the effective capacitance C_sof the two capacitances in series.

[0016] A varactor is not only a variable capacitance but also has resistance associated with its impedance, and is generally a relatively low Q component. The Q factor is inversely related to capacitance and resistance. Since C_sis always less than C_v, the coupling capacitance C_calways results in the combined components in series having a higher Q than a varactor alone. However, the impact of the varactor and its low Q on the combined components in series increases as C_cis made larger relative to C_v.

[0017] Similarly, as C_cis made larger relative to C_v, the ratio of a change in C_v(or _ΔC_v) to C_vhas a greater impact on the ratio of a change in C_s(or _ΔC_s) to C_s. As C_vchanges to C_v−_ΔC_v, the change in C_smay be represented as $\begin{array}{c}\Delta C_s=\frac{C_c{C}_v}{C_c+C_v}-\frac{C_c\left(C_v-\Delta C_v\right)}{C_c+C_v-\Delta C_v}\\ =\frac{C_c{C}_v\left(C_c+C_v-\Delta C_V\right)-C_c\left(C_v-\Delta C_v\right)\left(C_c+C_v\right)}{\left(C_c+C_v\right)\left(C_c+C_v-\Delta C_v\right)}\\ =\frac{C_c{C}_v\left(C_c+C_v-\Delta C_V\right)-C_c\left(C_v-\Delta C_v\right)\left(C_c+C_v\right)}{\left(C_c+C_v\right)\left(C_c+C_v-\Delta C_v\right)}\\ =\frac{C_c^2\Delta C_v}{\left(C_c+C_v\right)\left(C_c+C_v-\Delta C_v\right)}.\end{array}$

[0018] The ratio of _ΔC_sto C_smay be represented as $\begin{array}{cc}\begin{array}{c}\frac{\Delta C_s}{C_s}=\frac{C_c^2\Delta C_v\left(C_c+C_v\right)}{\left(C_c+C_v\right)\left(C_c+C_v-\Delta C_v\right)C_c{C}_v}=\frac{C_c\Delta C_v}{C_v\left(C_c+C_v-\Delta C_v\right)}\\ =\left(\frac{\Delta C_v}{C_v}\right)\left(\frac{C_c}{C_c+C_v-\Delta C_v}\right).\end{array}& \left(2\right)\end{array}$

[0019] The ratio of _ΔC_sto C_sis always less than the ratio of _ΔC_vto C_v. However, as C_cis made larger relative to C_v, not only does C_vhave a greater impact on C_s, but the ratio of _ΔC_vto C_vhas a greater impact on the ratio of _ΔC_sto C_s.

[0020] Adding a coupling capacitance in series with a varactor lowers the effective capacitance of the combination, raises the Q, and reduces the capacitance ratio relative to the varactor alone. As the coupling capacitance is made larger relative to the capacitance of the varactor, the impact of the low Q and the capacitance ratio of the varactor increases. If this series combination of FIG. 1 is part of a tank circuit of a VCO, a larger coupling capacitor can extend tuning range, but it will simultaneously increase phase noise and tuning sensitivity variation.

[0021] FIG. 2 is an illustration, for discussion purposes, of a variable capacitance C_vin parallel with a shunt inductance L. The parallel combination may be viewed together as an effective capacitance C_eff, with an impedance $Z=\frac{1}{\mathrm{j\omega }{C}_{\mathrm{eff}}}=\frac{1}{\mathrm{j\omega }{C}_v+\frac{1}{\mathrm{j\omega }L}}=\frac{\mathrm{j\omega }L}{1-{\omega }^2{\mathrm{LC}}_v}.1-{\omega }^2{\mathrm{LC}}_v=-{\omega }^2{\mathrm{LC}}_{\mathrm{eff}}.C_{\mathrm{eff}}=\frac{1-{\omega }^2{\mathrm{LC}}_v}{-{\omega }^{2}L}=C_v-\left(\frac{1}{{\omega }^{2}L}\right).$

[0022] Therefore, C_effis less than C_v. However, the absolute value of the change of C_effis the same as the absolute value of the change of C_v. If C_vchanges from C₁to C₂, the change in C_effmay be represented as $\Delta C_{\mathrm{eff}}=\left(C_1-\frac{1}{{\omega }^{2}L}\right)-\left(C_2-\frac{1}{{\omega }^{2}L}\right)=C_1-C_2.$

[0023] Compared to a variable capacitance C_vwithout a shunt inductance L, the two in parallel have a smaller effective capacitance C_effwithout altering the absolute value of the change in capacitance, C₁−C₂, or _ΔC_v.

[0024] When a varactor with a shunt inductance is substituted for a varactor alone in a series combination with a coupling capacitor, the smaller C_effof the two in parallel can be substituted for the C_v(of the varactor alone) in equations (1) and (2) discussed above. However, the _ΔC_vremains the same. Therefore, the capacitance ratio can be higher, without increasing the impact of the low Q and nonlinearity of the varactor.

[0025] The operating frequency of a tank circuit is a function of a circuit capacitance, such as C_sof FIG. 1, and a separate circuit inductance not illustrated in FIGS. 1 or 2. If CS is reduced by inclusion of the shunt inductance L in parallel with the variable capacitance C_v, the desired operating frequency can be maintained by increasing the separate circuit inductance.

[0026] Compared to the impact of a varactor alone on a tank circuit, the impact of an effective capacitance C_eff, of a shunt inductance in parallel with the varactor, is to extend the tuning range about a given operating frequency without degrading phase noise or tuning sensitivity variation.

[0027] FIG. 3 shows an example of circuitry for part of a VCO which is a differential multivibrator. A tank circuit 10 is shown above the dashed line. Terminals 11 and 12 of the tank circuit 10 are connected to two transistors 21 and 22 which are part of an integrated circuit below the dashed line.

[0028] The example tank circuit 10, shown in FIG. 3, has a common cathode dual diode varactor 30 with a control voltage Vcon connected to the cathode junction 31. A shunt inductance 13 is connected across the anodes 32 and 33 of the varactor 30. The anodes 32 and 33 are each connected to ground via a resistor 14 and 15, respectively. They are each connected to one of the tank circuit terminals 11 and 12 via a capacitor 16 and 17, respectively, as shown. A supply voltage Vsup is connected to terminals 11 and 12 via inductors 18 and 19, respectively.

[0029] An embodiment of the tank circuit 10 shown in FIG. 3 was built using an Alpha 1234 hyper-abrupt junction varactor 30. The capacitance of each of the diodes ranged from about 10 pF to about 4 pF over a control voltage Vcon of about 0 to about 3 volts. For this illustrative example, a shunt inductance 13 of about 18 nH was chosen, capacitors 16 and 17 were each chosen to be about 18 pF, and inductances 18 and 19 were each chosen to be about 5.6 nH. The capacitance and inductance values were chosen to enhance design robustness. By including shunt inductance 13, the tuning range was increased 50% without changing the phase noise or the tuning sensitivity variation.

[0030] The embodiments discussed and/or illustrated are examples. They are not exclusive ways to practice the present invention, and it should be understood that there is no intent to limit the invention by such disclosure. Rather, it is intended to cover all modifications and alternative constructions and embodiments that fall within the scope of the invention as defined in the following claims.