What phase noise actually is
An ideal oscillator would produce a single, infinitely thin spectral line. Real sources do not. Tiny random fluctuations in the phase of the carrier spread energy into a skirt on either side of the intended frequency. That skirt is phase noise. In the time domain it shows up as jitter. In the frequency domain it shows up as a continuous pedestal of power surrounding the carrier, and it is the single most important measure of a source's spectral purity.
Phase noise matters because it does not stay confined to the source. It mixes, multiplies, and reciprocally couples into every measurement the source touches. A receiver tested with a noisy local oscillator will look worse than it is. A radar built around a noisy reference will struggle to separate a slow target from nearby clutter. The cleaner the source, the more honest the test.
Reading the specification: dBc/Hz at an offset
Single sideband phase noise is quoted in dBc/Hz at a stated offset from the carrier. The unit decodes cleanly once you break it apart. The "dBc" part means decibels relative to the carrier power. The "/Hz" part normalizes the measurement to a one hertz bandwidth, so two instruments measured differently can still be compared. The offset, written as 1 kHz, 20 kHz, 10 MHz and so on, tells you how far from the carrier the measurement was taken.
A figure of -132 dBc/Hz at a 1 kHz offset means that, one kilohertz away from the carrier, the noise power in a one hertz slice sits 132 dB below the carrier. More negative is better. Because the offset is always part of the number, a single phase noise figure is meaningless on its own. A source can be excellent close to the carrier and ordinary far out, or the reverse, so engineers compare sources across a curve of offsets rather than at one point.
Close-in versus far-out
Phase noise behavior changes with offset, and different applications care about different parts of the curve. Close-in noise, roughly 10 Hz to a few kilohertz, is dominated by the reference oscillator and the loop that locks to it. This region governs slow drift, coherence between channels, and the ability to resolve closely spaced tones. Far-out noise, from tens of kilohertz to tens of megahertz, is set by the output stages and the noise floor, and it governs wideband interference and adjacent channel behavior.
A high-quality OCXO reference and a well-designed loop push the close-in numbers down. Clean output amplification and filtering push the far-out floor down. The two are engineered separately, which is why options such as enhanced close-in phase noise (often labeled LN or LN+) target one region without changing the other.
Why it matters: radar, communications, quantum
In radar, reciprocal mixing of local oscillator phase noise onto a strong clutter return can mask a weak moving target nearby in frequency. Close-in phase noise directly limits the smallest Doppler shift a system can resolve, so pulse-Doppler and coherent radars demand the cleanest references available.
In communications, phase noise corrupts the constellation. Higher-order schemes such as 256-QAM pack symbols tightly, and a noisy carrier blurs the boundaries between them, raising error vector magnitude and the bit error rate. Testing a modern transceiver fairly requires a source that is quieter than the device under test.
In quantum work, microwave sources act as local oscillators for I/Q mixers and as pump tones for parametric amplifiers. Phase noise on those tones translates into qubit dephasing and gate error, so the source budget feeds directly into coherence time. Across all three fields the rule is the same: the source must be quieter than the thing it measures.
How Berkeley Nucleonics sources compare
The table below summarizes single sideband phase noise at a 1 GHz carrier for representative models, drawn from current published datasheets. Use it as a starting point and confirm the full offset curve against the latest datasheet for any critical design.
| Model | Frequency range | 10 Hz | 1 kHz | 20 kHz | Notes |
|---|---|---|---|---|---|
| 865B-M | 100 kHz to 40 GHz | verify | -132 dBc/Hz | -145 dBc/Hz | Best close-in figure of the synthesizer family |
| 870A | 10 MHz to 54 GHz | -85 dBc/Hz (-100 with LN+) | -140 dBc/Hz | verify | Widest span; -152 dBc/Hz at 100 kHz |
| 855B | 300 kHz to 42 GHz | -87 dBc/Hz (-100 with LN) | -130 dBc/Hz | -145 dBc/Hz | Multi-channel, phase-coherent |
| 845-M | 10 MHz to 20 GHz | verify | -118 dBc/Hz | -128 dBc/Hz | Compact low-noise module |
Talk to an application engineer
Phase noise requirements are best evaluated against the full offset curve and the carrier you actually operate at, not a single headline figure. Berkeley Nucleonics application engineers can match a source to your radar, communications, or quantum budget and supply the measured curves to back it. For specifications, configurations, and quotations, contact info@berkeleynucleonics.com or call 800-234-7858. The full family overview is on the RF & Microwave Signal Generators documentation page.