Preventing Limitations and Optimizing Devices
Measurement of phase noise is necessary to minimize its limitations and optimize narrow channel communication in RF transmission. Phase noise is caused by the instability of the oscillator and visible as variations, also referred to as ‘jitters’, in the time domain. This unintentional phase modulation (phase noise) negatively affects the signal quality and increases error rates in communication links. Phase noise can be a significant limiting factor in signal-generating devices, especially in mission-critical applications (e.g. aerospace and defense), but also in communications. Phase noise measurement plays an important role in preventing such limitations and optimizing signal-generating devices.
A variety of phase noise measurement techniques have been developed to meet different requirements, but three techniques are the most utilized: direct spectrum, phase detector, and two-channel cross-correlation.
Direct Spectrum Technique
The direct spectrum technique is the simplest and perhaps oldest technique and directly measures the power spectral density of the oscillator by using a spectrum/signal analyzer. This technique is useful for qualitative and quick evaluation on higher noise sources but is limited by both the dynamic range of the spectrum/signal analyzer and also by the amplitude noise mimicking phase noise. Because of these limitations, additional advanced techniques are required.
Advanced Techniques
Advanced solutions use a dedicated phase noise instrument that employs both a phase detector and two-channel cross-correlation techniques.
A phase detector is used to separate phase noise from amplitude noise by converting the phase difference of the two input signals into a voltage at the detector’s output. When the phase difference is set to 90° (quadrature), the voltage output will be zero volts. Any phase fluctuation from quadrature will result in a voltage fluctuation at the output.
The two-channel cross-correlation technique combines two duplicate single-channel reference sources/PLL systems and performs cross-correlation operations between the outputs of each channel. The two-channel cross-correlation technique achieves superior measurement sensitivity without requiring exceptional performance of the hardware components. However, the measurement speed suffers when increasing the number of correlations.
Both techniques operate on the baseband signal and require a downconverter.
Keysight offers a dedicated phase noise instrument that uses the advanced measurement techniques mentioned above to measure RF and microwave frequency phase noise. The Keysight RF phase noise measurement solution uses the following three products to provide accurate phase noise measurements with an extremely low noise floor.
- Keysight N5511A phase noise test system
- Keysight E5055A SSA-X signal source
- Keysight E5053A microwave downconverter
The following Application Note describes how Keysight’s Signal Source Analyzer is used for advanced phase noise measurement and transient measurements.
