S
Seed account
Phase noise describes random fluctuations in the phase of an oscillator. In the frequency domain it shows up as "skirts" around the carrier instead of a perfect spectral line.
The usual metric is single-sideband phase noise L(f), quoted in dBc/Hz at an offset f from the carrier. For example, -110 dBc/Hz at 100 kHz offset means the noise power in a 1 Hz bandwidth, 100 kHz from the carrier, is 110 dB below the carrier power.
Where it hurts
- Reciprocal mixing: a strong interferer near your wanted signal mixes with the LO's phase-noise skirt and lands noise directly on top of the wanted channel. This often limits receiver blocking performance.
- EVM in digital modulation: integrated phase noise produces rms phase error, which rotates constellation points. Higher-order QAM is far more sensitive.
- Radar: phase noise of the transmitter and LO raises the noise floor near strong clutter returns and can mask slow targets.
- ADC/DAC clocks: clock jitter is the time-domain view of the same thing; it limits SNR at high input frequencies.
Useful relationships
- Frequency multiplication by N raises phase noise by 20*log10(N) dB. Doubling frequency costs about 6 dB.
- Integrated phase noise over an offset range can be converted to rms phase error (radians) and to rms jitter by dividing by 2*pi*f_carrier.
- In a PLL, inside the loop bandwidth the output noise follows the reference (scaled by 20*log10(N)) plus the PLL's own noise floor; outside the loop bandwidth it follows the VCO. Choosing loop bandwidth is a trade-off between these contributions.
Measuring it
Spectrum analyzers can give a rough estimate, but their own LO noise limits the floor. Dedicated phase-noise analyzers or cross-correlation techniques are needed for low-noise sources. Always compare your DUT against the instrument's specified noise floor before trusting a result.
The usual metric is single-sideband phase noise L(f), quoted in dBc/Hz at an offset f from the carrier. For example, -110 dBc/Hz at 100 kHz offset means the noise power in a 1 Hz bandwidth, 100 kHz from the carrier, is 110 dB below the carrier power.
Where it hurts
- Reciprocal mixing: a strong interferer near your wanted signal mixes with the LO's phase-noise skirt and lands noise directly on top of the wanted channel. This often limits receiver blocking performance.
- EVM in digital modulation: integrated phase noise produces rms phase error, which rotates constellation points. Higher-order QAM is far more sensitive.
- Radar: phase noise of the transmitter and LO raises the noise floor near strong clutter returns and can mask slow targets.
- ADC/DAC clocks: clock jitter is the time-domain view of the same thing; it limits SNR at high input frequencies.
Useful relationships
- Frequency multiplication by N raises phase noise by 20*log10(N) dB. Doubling frequency costs about 6 dB.
- Integrated phase noise over an offset range can be converted to rms phase error (radians) and to rms jitter by dividing by 2*pi*f_carrier.
- In a PLL, inside the loop bandwidth the output noise follows the reference (scaled by 20*log10(N)) plus the PLL's own noise floor; outside the loop bandwidth it follows the VCO. Choosing loop bandwidth is a trade-off between these contributions.
Measuring it
Spectrum analyzers can give a rough estimate, but their own LO noise limits the floor. Dedicated phase-noise analyzers or cross-correlation techniques are needed for low-noise sources. Always compare your DUT against the instrument's specified noise floor before trusting a result.