Phase Noise

Oscillator Quality

The choice of the frequency reference is the primary factor determining the level of noise in a system. High quality crystal oscillators provide very low phase noise, which is necessary for complex modulation schemes that require precise timing. When an oscillator is used in a phase locked loop to generate a higher frequency, the noise from the reference and the loop filter components are multiplied.

Designers must balance the bandwidth of the loop filter to suppress the noise from the voltage controlled oscillator while not introducing too much noise from the reference. The thermal noise of the components and the flicker noise of the transistors are the physical sources of these fluctuations. Measuring this performance requires specialized equipment that can detect signals very close to the carrier.

Modulation Impact

In modern digital communications, information is encoded into the phase and amplitude of a signal. If the phase noise is too high, it can cause the symbols to rotate or smear in the constellation diagram, leading to an increased bit error rate. High order modulations like 256-qam are extremely sensitive to this effect because the distance between the symbol points is very small.

In a receiver, the noise from the local oscillator can mix with a strong adjacent signal and spread it over the desired weak signal, a process known as reciprocal mixing. This effectively raises the noise floor of the receiver and makes it impossible to detect the intended communication. Managing this noise budget is essential for the performance of high speed wireless links.

Jitter Relationship

There is a direct mathematical relationship between the noise in the frequency domain and the timing jitter in the time domain. Jitter represents the deviation of the clock edges from their ideal positions, which can cause timing errors in high speed digital to analog converters. By integrating the phase noise over a specific frequency range, engineers can calculate the root mean square jitter of a clock signal.

This allows for a comparison between the spectral performance of a radio and the timing requirements of a digital system. In wideband applications, the integrated noise over the entire channel bandwidth is often the most relevant figure. Reducing this noise involves choosing low noise power supplies and shielding the sensitive oscillator circuits from external interference.

The result is a more stable and reliable communication link.