Understanding Signal Generator Phase Noise

Phase noise refers to short-term, rapid fluctuations in the phase of a frequency signal. In the frequency domain, an ideal signal appears as a single delta function at the carrier frequency. Real-world RF signal generators, however, exhibit phase fluctuations that spread the carrier's energy into adjacent frequencies, forming noise sidebands on both sides of the carrier.

It is important to distinguish phase noise from long-term frequency stability. Long-term stability, often specified as a reference oscillator aging rate (such as ±1 ppm/year or ±0.01 ppm/year with Option 002 on the Keysight N5182B), describes frequency drift over days or years. Phase noise describes rapid, microsecond-scale instabilities that affect the instantaneous spectral purity of the signal.

Phase noise is specified in decibels relative to the carrier per Hertz (dBc/Hz) at a given frequency offset from the carrier. Because phase noise levels decrease as the offset from the carrier increases, a standalone dBc/Hz value is meaningless without specifying both the carrier frequency and the offset.

When evaluating high-performance generators, compare these figures at identical offsets. For example, the Keysight N5182B specifies a phase noise of -146 dBc/Hz at a 20 kHz offset from a 1 GHz carrier. In comparison, the Rohde & Schwarz SMA100A specifies a typical phase noise of -135 dBc/Hz at the same 20 kHz offset and 1 GHz carrier. A lower dBc/Hz value represents a cleaner signal, which is critical for minimizing system-level measurement errors.

Adjacent channel selectivity (ACS) testing measures a receiver's ability to process a weak wanted signal on its assigned channel in the presence of a strong unwanted signal in an adjacent channel. When performing this test, a signal generator simulates the strong out-of-band interferer.

If the signal generator has high phase noise, its noise sidebands will spill directly into the receiver's passband, raising the receiver's intermediate frequency (IF) noise floor. This phenomenon, known as reciprocal mixing, makes it impossible to distinguish whether the receiver's performance is failing or if the generator is simply too noisy. To perform accurate selectivity and blocking tests, the signal generator's phase noise must be significantly lower than that of the receiver under test.

For higher-frequency microwave applications, instruments like the Keysight 83732B cover up to 20 GHz. However, phase noise typically degrades as carrier frequencies scale upward, making careful consideration of offset performance even more critical at microwave frequencies.