Silicon-Bipolar IC Facilitates VCO Design - AN698

The frequency of a voltage-controlled oscillator (VCO) varies with the voltage applied to its tuning port. Operating in a phase-locked loop (PLL), the VCO provides a stable local oscillator (LO) for frequency conversion in superheterodyne receivers. VCOs are also used in transmit chains, where they upconvert the baseband signal to a radio frequency (RF) suitable for transmission over the airwaves. The frequency of a voltage-controlled oscillator (VCO) varies with the voltage applied to its tuning port. Operating in a phase-locked loop (PLL), the VCO provides a stable local oscillator (LO) for frequency conversion in superheterodyne receivers. VCOs are also used in transmit chains, where they upconvert the baseband signal to a radio frequency (RF) suitable for transmission over the airwaves (Figure 1). Figure 1. VCOs appear as part of the PLLs in this typical superheterodyne receiver. Design Considerations The VCO designer must consider several important performance parameters: ● Output level in dBm (dB relative to 1mW) ● Output harmonic level in dBc (dB relative to carrier power) ● Tuning sensitivity in Hz/V ● Load pulling of oscillation frequency in Hz p-p (for a given load voltage standing-wave ratio (VSWR) rotated through 360°) ● Frequency pushing, in Hz/V, of bias-supply change ● VCO phase noise, in dBc/Hz, at a given offset frequency The following paragraphs discuss each parameter in turn.