How do I measure the phase noise of an oscillator using a spectrum analyzer?
Phase Noise Measurement
The spectrum analyzer displays the power spectral density of the oscillator output, showing the carrier and its noise sidebands. The phase noise L(fm) at offset fm is the ratio of the noise power in a 1 Hz bandwidth at offset fm to the carrier power. The spectrum analyzer measures the noise in its resolution bandwidth (RBW), which must be normalized to 1 Hz: L(fm) = P_displayed(fm) - P_carrier - 10·log10(RBW) + 2.5 dB (the 2.5 dB corrects for the noise bandwidth of the RBW filter versus its 3 dB bandwidth).
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
Conversion Architecture
The measurement is limited by the spectrum analyzer's own phase noise. The analyzer's LO generates its own noise sidebands that add to the measured signal. If the DUT phase noise is comparable to or better than the analyzer's LO noise, the measurement shows the analyzer noise, not the DUT noise. Always verify that the analyzer specification is at least 10 dB better than the expected DUT phase noise at the offset frequency of interest.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Spurious Performance
Phase noise analyzers (such as Keysight E5052B or Rohde & Schwarz FSWP) use cross-correlation between two independent measurement channels to suppress the instrument's own noise. With enough averaging (increasing cross-correlation count), the instrument noise is reduced by 5·log10(N_correlations), achieving measurement sensitivity below -180 dBc/Hz.
Frequently Asked Questions
What RBW should I use?
Use the smallest RBW your analyzer supports for the cleanest measurement, but consider the sweep time (proportional to 1/RBW²). At 10 kHz offset: use 100 Hz to 1 kHz RBW. At 1 MHz offset: use 10 kHz to 100 kHz RBW. Narrower RBW gives lower noise floor but longer measurement time.
Can I measure close-in phase noise with a spectrum analyzer?
At offset frequencies below ~100 Hz: most spectrum analyzers cannot resolve the noise sidebands due to insufficient frequency resolution and LO phase noise. Dedicated phase noise analyzers with PLL-based or cross-correlation measurement techniques are required for offsets below 100 Hz.
What about AM noise?
A spectrum analyzer measures total sideband power: both AM and PM noise. For most oscillators, PM noise dominates by 20+ dB, so the measurement is essentially PM noise. To separate AM from PM contributions, a quadrature demodulation technique is needed.