How Do Amplifiers Affect Signal Phase Noise?

Phase noise is critical to systems, like Electronic Warfare and 5G Communications, requiring precise frequency stability. Oscillators are typically the determining factor in the signal chain’s phase noise performance. However, in extremely low phase noise systems, the amplifier phase noise contribution is considered. This tech note explores amplifier’s phase noise contribution, demonstrating:

• Amplifiers only contribute 1/f and White PM noise to a signal chain.
• 1/f noise is apparent at approximately 1‑100 kHz offset frequencies.
• Amplifier 1/f noise adds linearly in series and subtracts linearly in parallel.
• Oscillator higher order phase noise dominates at <1 kHz offset frequency.

Phase Noise Regions

Typical phase noise measurements are divided into regions, each attributed to a different physical phenomenon. Figure 1 identifies the phase noise regions and their corresponding mechanisms, and displays typical oscillator and amplifier phase noise trends. High order (1/f4, 1/f3, and 1/f2) phase noise is caused by the cavity effects from oscillators, and are not contributed by amplifiers.