What is the noise floor of a spectrum analyzer and how does it limit my measurement range?
Spectrum Analyzer Noise Floor
Understanding the spectrum analyzer noise floor is essential for making accurate measurements of weak signals, spurious emissions, and noise-like signals. The DANL is the most fundamental sensitivity specification of a spectrum analyzer.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
- 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
Frequently Asked Questions
How do I tell if a signal is real or a noise artifact?
Three tests: (1) RBW test: change the RBW by 10×. A real signal changes by 0 dB (its power is concentrated at a fixed frequency). Noise changes by 10 dB (noise power is proportional to bandwidth). A spurious response (internal to the SA) changes by 0 dB. (2) Attenuation test: increase the input attenuation by 10 dB. A real signal drops by 10 dB. An internally generated spurious drops by more than 10 dB (typically 20-30 dB for IM3). The noise floor rises by 10 dB. (3) Frequency shift test: change the center frequency slightly. A real signal moves with the frequency. An internally generated spur may stay at the same position (if related to the SA LO harmonics).
Should I use the internal preamplifier?
Enable the SA preamplifier when: measuring weak signals (near the noise floor), performing noise figure measurements, measuring spurious emissions below -70 dBm, or any measurement where sensitivity is critical. Do NOT use the preamplifier when: strong signals are present (risk of compressing the preamplifier, which has lower P1dB than the SA mixer), measuring high-power signals (> -10 to 0 dBm), or when the best intermodulation performance (SFDR) is needed (the preamplifier limits the IIP3 of the receiver chain). Many SAs have a "preamp on" and "preamp off" switch with indicator: the preamplifier gain (20-30 dB) is automatically accounted for in the displayed amplitude.
What limits the noise floor below -150 dBm?
Below approximately -150 to -160 dBm/Hz: the noise floor is limited by: (1) SA local oscillator phase noise: the LO phase noise mixes with the LO itself to produce noise at the IF frequency. This sets a noise floor independent of RBW. Typical SA LO phase noise floor: -150 to -160 dBm/Hz. (2) ADC quantization noise: the digital IF section of modern SAs uses 14-16 bit ADCs. The quantization noise floor is approximately -174 + 6.02×N_bits + 10×log10(BW). For 14 bits at 100 MHz sampling: -174 + 84 + 80 = -10 dBm noise power (well above quantization floor; not a limitation). (3) Spurious signals: SA internal spurs (from clock signals, power supply harmonics, etc.) set a spurious noise floor of typically -90 to -110 dBm. These are fixed-frequency spurs, not broadband noise, but they limit the minimum detectable signal at specific frequencies. (4) Thermal noise of the input attenuator: at 0 dB attenuation, the thermal noise is -174 dBm/Hz. The SA noise figure adds 15-25 dB. This is the fundamental limit for the SA noise floor.