Voltage Controlled Oscillator (VCO) in Phase-Locked Loop will always have some spurious signals present on its output. The amplitude and frequency of these spurious modulations may vary.

Rules for designing a low Phase Noise Voltage Controlled Oscillator (VCO) are as follows:

• Poor layout of the phase-locked loop oscillator circuitry (VCO) may increase the amplitude and number of the output spurious signals.

• Oscillator Phase Noise has two components: Phase Noise resulting from direct upconversion of white noise and flicker noise (1/f noise), and Phase Noise resulting from the changing phase of the noise sources modulating the oscillation frequency.

• In VCO design another source of Phase Noise increase are the non-linear capacitors (varactors) used in the LC resonator and its control lines.

• VCO have to maintain the Q of the resonator by avoiding forward bias on the varactor tuning diodes, limiting the signal swing across the tuning diodes to prevent heating and thermal effects. This can be achieved by placing the varactor circuit in the gate or base if possible.
• Two back-to-back varactors should be used to avoid self-rectification of the RF signal across the varactors, which always results in phase noise degradation.
• The noise from the varactor diode resistance can also become the dominant noise source. For good Phase Noise, the carrier signal effectively appearing across the varactor noise resistance should be maximized to maintain good Signal-to-Noise ratio at this point. By transforming the noise load resistance seen by the oscillating device to a lower value in the matching circuit, the Power-to-Noise ratio across the varactor can be maximized, although at the expense of tuning bandwidth since the matching circuit will restrict the obtainable capacitance variation.
• There is a compromise in order to avoid breakdown, saturation, or overheating effects in the varactor. These will all reduce the Loaded-Q.
• When frequency of the carrier increases, it is more difficult to achieve good Phase Noise
• It’s easy to achieve good Phase Noise when the frequency range covered by VCO is narrow; the tuning bandwidth must be small. Generated energy should be coupled from the resonator rather than from another portion of the active device so that the resonator limits the bandwidth.

• Increasing tuning sensitivity (measured in MHz / V) degrades Phase Noise.

• For a given frequency it’s easy to achieve good Phase Noise in VCO’s using a wide tuning voltage range.

• Temperature affects the Phase Noise. In a range of –55’C to +85’C the variation is +/- 3dB of the Phase Noise.

• Using of back-to-back varactor diodes in the tuning circuits has been found to eliminate effects of tuning circuit diode noise on oscillator signal spectral performance.