Meaning
Shift in the output frequency of an oscillator caused by changes in the load impedance at its output port can disrupt wireless communication links. This effect, known as frequency pulling, occurs when power amplifiers turn on or antennas experience shifts in their physical environment. The changing impedance alters the phase of the signal reflected back to the oscillator, shifting its resonance frequency.
Isolators or buffer amplifiers are placed between the oscillator and the antenna to minimize this feedback.
Impedance Mismatch
Antennas have an impedance that varies based on their physical surroundings, such as the proximity of a human hand or a metal sheet. When these variations occur, the reflection coefficient at the oscillator interface changes. This mismatch pulls the oscillator away from its target frequency, potentially leading to packet loss or regulatory violations.
Buffer Stage
Inserting a high-isolation amplifier stage after the local oscillator protects it from load variations. This buffer stage presents a stable, high-impedance load to the oscillator regardless of what happens downstream at the antenna. The amplifier stage blocks reflected power from traveling back into the oscillator.
VCO Design
Voltage-controlled oscillators are particularly sensitive to these load fluctuations because of their high tuning sensitivity. Designers select oscillators with low pulling figures, typically specified in megahertz of frequency shift for a given voltage standing wave ratio.
