How critical is phase noise for coherent radar systems? What's acceptable for weather radar?

Top Answer by u/SignalIntegrityPhD

↑ 182 • 6 hours ago

Great question. Phase noise is arguably the critical performance limiter for modern coherent pulsed Doppler radar. Let's break it down.

1. The Core Problem: Spectral Leakage and Clutter

A coherent radar relies on a pristine reference signal (the LO) to transmit and then compare the echo against. In an ideal world, your transmitted signal is a perfect spectral line. In reality, phase noise spreads that energy around the carrier frequency. This has a devastating effect on clutter rejection.

Imagine ground clutter (a huge, stationary reflection) coming back at exactly your transmit frequency (0 Hz Doppler). Ideally, your Doppler filter (or MTI - Moving Target Indicator) can notch it out. However, your transmitted signal's phase noise "skirts" get reflected along with the main clutter return. This noise appears at non-zero Doppler frequencies in the receiver's FFT, smearing the clutter into the regions where you're trying to detect weak weather returns. The result is a reduced clutter-to-noise ratio and a higher minimum detectable velocity.

2. Impact on Doppler Measurement Accuracy

Phase noise directly corrupts the phase of your received signal. For a weather radar, the Doppler phase shift from pulse to pulse (Δφ = 2π * f_D * PRT) is how you derive radial velocity. Random phase noise adds uncertainty to this measurement, increasing the variance (the "width") of the Doppler spectrum. This means:

• Velocity estimates become noisier, especially for weak weather echoes.
• Spectral width measurements (used to estimate turbulence and droplet size) are artificially inflated.

3. Typical Specifications & What's Acceptable

There's no one-size-fits-all. It's a trade-off between performance and cost. The key metric is single-sideband (SSB) phase noise (L(f)), typically given at offsets from the carrier (e.g., 1 kHz, 10 kHz, 100 kHz).

• General Purpose / Low-Cost Weather Radar: Might get away with -90 to -95 dBc/Hz at 1 kHz offset. Clutter rejection will be limited (maybe 30-40 dB), and you'll have noticeable spectral broadening. Adequate for basic precipitation detection.
• Quality Meteorological Radar (C-band): This is likely your sweet spot. Look for -100 to -110 dBc/Hz at 1 kHz offset. This enables good MTI/MTD performance (50+ dB clutter rejection) and clean Doppler velocity fields. Many commercial airborne weather radars target this range.
• High-Performance / Phased Array Radars: SAR (Synthetic Aperture Radar), MTI for moving ground targets, and high-fidelity wind profiler radars require -110 dBc/Hz or better at 1 kHz, and critically, very low noise floors at offsets out to the PRF/2. These systems might spec -150 dBc/Hz at 100 kHz. This is where costs skyrocket.

For your C-band weather radar: A good, modern direct digital synthesizer (DDS) + PLL combination can hit -105 dBc/Hz @ 1 kHz without breaking the bank. Don't just look at the 1 kHz number—ask for the phase noise plot out to 1 MHz. The integrated phase noise (or equivalently, the residual FM) over the clutter bandwidth is what matters. A common pitfall is having great close-in noise but a poor noise floor, which can still limit MDS (Minimum Detectable Signal).

Pro Tip: Model it! Take your expected clutter power (e.g., +50 dBsm for ground clutter at your lowest beam elevation), apply your candidate phase noise profile, and calculate the resulting clutter noise power in your Doppler bins. Compare it to your system thermal noise. This will tell you if you've met your clutter rejection specification.