Used Phase Noise Analyzers & Signal Source Analyzers

Phase noise is the short-term, random frequency instability of an oscillator or signal source, expressed as dBc/Hz (power relative to the carrier, per Hz of bandwidth) at a series of offset frequencies from the carrier — typically 1 Hz to 10 MHz offset. A phase noise analyzer plots this as an L(f) curve. Lower (more negative) dBc/Hz numbers mean a cleaner, more stable source. The measurement matters directly for radar range resolution, communications channel spacing, and clock jitter in digital systems, since phase noise integrated over a bandwidth converts directly to timing jitter.

A single-channel phase noise measurement is limited by the noise floor of the measuring instrument's own internal reference. Cross-correlation instruments — the Keysight/Agilent E5052A, E5052B, and E5053A signal source analyzers — use two independent measurement channels and correlate the results, which cancels out uncorrelated noise from each channel's own reference. This pushes the effective noise floor down 15-20 dB or more below what either channel could measure alone, which is what makes it possible to characterize very clean sources like OCXOs and low-noise synthesizers accurately.

The HP 3048A is a phase-lock-loop-based system: it phase-locks a reference source to the device under test, then uses an FFT analyzer (via the 11848A phase noise interface) plus software to compute the noise spectrum. It requires a lockable, tunable reference near the test frequency, and setup is manual and time-consuming. The Keysight E5052A/B and E5053A use built-in cross-correlation receivers and largely automate the process, cover wider frequency ranges natively, and reach lower noise floors without needing an external lockable reference. The 3048A remains a capable, well-understood system for labs already set up around it.

Yes — most phase noise systems double as VCO testers when paired with a bias/tuning supply, and instruments like the Rohde & Schwarz FSWP are explicitly built and marketed for both phase noise and VCO characterization (tuning sensitivity, harmonics, output power vs. tuning voltage) in one setup. This is a common use case for oscillator and synthesizer manufacturers qualifying parts before they go into a production design.

A spectrum analyzer with a phase noise measurement option can give a reasonable single-sweep estimate for sources with moderate-to-high phase noise, but its noise floor is set by its own local oscillator, so it cannot accurately characterize very clean sources — the measurement floors out at the analyzer's own noise rather than the device under test. A dedicated cross-correlation system (E5052A/B, E5053A, FSWP) is needed when qualifying low-noise references, OCXOs, or any source where the true noise floor matters, such as radar and precision timing applications.