The present invention generally relates to voltage controlled oscillators (VCOs). More particularly, this invention relates to programmable varactors for VCOs.
An electronic oscillator may be an electronic circuit that produces an electric signal. A voltage controlled oscillator (VCO) may be an electronic oscillator. A VCO may be used, for example, in a radio transceiver such as a mobile telephone.
The frequency of a VCO output signal may be controlled by an input tuning voltage of the VCO. The frequency may be tuned by a variable capacitor having a particular capacitance. As shown, the particular capacitance may be a function of the input tuning voltage
            f      ⁡              (        C        )              =          1              2        ⁢        π        ⁢                  LC                      ,      C    =                  C        0            +              C        CT            +                        C          VAR                ⁡                  (          V          )                      ,          ⁢            K      V        =                                                ⅆ            f                                ⅆ            V                                      =                                                                            ⅆ                f                                            ⅆ                C                                      ⁢                                          ⅆ                C                                            ⅆ                V                                                              =                              2            ⁢                          π              2                        ⁢                          Lf                                                                                ⁢                3                                      ⁢                                                                          ⅆ                  C                                                  ⅆ                  V                                                                            ∝                      2            ⁢                          π              2                        ⁢            L            ⁢                                                  ⁢                          N              3                        ⁢                                                                          ⅆ                  C                                                  ⅆ                  V                                                                                      where L is the VCO tank circuit inductance, C is the total tank circuit capacitance, C0 is the parasitic tank capacitance, CCT is the coarse tune bank capacitance, CVAR is the varactor capacitance, N is the phase locked loop (PLL) counter value (divide ratio), V is the varactor control voltage, and KV is the magnitude of VCO gain. When the PLL is clocked, the VCO oscillation frequency equals the target frequency, i.e., f=N*fref, where fref is the PLL reference frequency.
The VCO gain KV is proportional to f3 as shown above. Due to the width of the frequency tuning range (i.e., the large range of f and N), VCO gain variation is very large over the VCO frequency range, with a ratio of 2:1 to 8:1 from the high end to the low end of the frequency range depending on the actual frequency plan.
There are disadvantages associated with large VCO gain variation. PLL bandwidth varies with VCO gain, causing spur and integrated phase noise problems. PLL dynamics also vary, causing a settling time problem. Phase noise is worse at higher frequency according to Leeson's equation and worse with higher VCO gain due to amplitude modulation to phase modulation (AM-PM) conversion. As shown above, VCO gain is larger at higher frequency, which makes phase noise even worse.
Traditionally, VCO gain compensation is accomplished by adjusting a charge pump current, i.e., decreasing charge pump current ICP when VCO gain is high, and vice versa. Although the loop transfer function H(S) remains the same if the product of VCO gain and ICP is constant, the noise from the loop filter resistor R has a different transfer function for a different VCO gain:
            L      R        ⁡          (      s      )        =      4    ⁢    KTR    ⁢                  C        Z                              C          Z                +                  C          P                      ⁢          1              1        +                              sRC            Z                    ⁢                                    C              P                        /                          (                                                C                  Z                                +                                  C                  P                                            )                                            ⁢                                                              K              V                        /            s                                1            +                          H              ⁡                              (                s                )                                                                2      where CZ and CP are the zero and pole loop capacitors, respectively, R is the PLL loop filter resistor, K is the Boltzmann constant 1.38e-23 Joule/Kelvin, and T is temperature in Kelvin. A higher KV results in more noise contribution from the loop filter resistor.
As can be seen, there is a need for improved VCO gain compensation. In particular, it is desirable to maintain a relatively constant VCO gain other than by adjusting a charge pump current.