Meaning
Undesired frequency modulations that appear as noise skirts on either side of a carrier signal degrade the spectral purity of a wireless transmitter. The presence of phase noise sidebands limits the ability of a receiver to resolve closely spaced signals and increases adjacent channel power leakage. This frequency-domain behavior is analyzed during synthesizer design and radio board validation, ending once the system meets the spectral mask requirements of the target standard.
Spectral Spreading
Phase noise originates from short-term frequency fluctuations in the local oscillator. These fluctuations spread the signal energy into the adjacent channels, which makes it difficult to comply with strict regulatory emissions limits.
Receiver Desensitization
Strong sidebands on the local oscillator of a receiver can mix with unwanted strong signals in adjacent channels and translate them into the intermediate frequency band. This process, known as reciprocal mixing, raises the noise floor of the receiver. As a result, the receiver loses the ability to detect weak incoming signals on its intended channel, which decreases the overall operating range of the device.
Mitigation Technique
Improving the quality of the oscillator and optimizing the phase-locked loop filter bandwidth can minimize the amplitude of these sidebands. Designers use low-noise voltage regulators to prevent power supply ripple from modulating the oscillator frequency.
