What is the difference between a real-time spectrum analyzer and a swept spectrum analyzer?

What is the difference between a real-time spectrum analyzer (RTSA) and a swept spectrum analyzer (SSA)? The two architectures represent fundamentally different approaches to observing the frequency spectrum, with critical implications for capturing transient and dynamic RF signals: (1) Swept spectrum analyzer (SSA, traditional): how it works: a superheterodyne receiver tunes a narrow bandpass filter (resolution bandwidth, RBW) across the frequency range. At each frequency step: the filter passes a narrow slice of the spectrum, and the detector measures the power. The system sweeps from the start frequency to the stop frequency, building up the spectrum display point by point. Sweep time: depends on the span and RBW. For a 1 GHz span at 1 kHz RBW: sweep time ≈ seconds. During the sweep: the analyzer only sees the frequency it is currently tuned to. If a signal appears at a different frequency during the sweep: the analyzer misses it. (2) Real-time spectrum analyzer (RTSA): how it works: a wideband ADC captures the entire bandwidth simultaneously. An FFT engine processes the captured data block-by-block, producing the spectrum for each time block. Capture bandwidth: 40-800 MHz (depending on the model and ADC speed). Within the capture bandwidth: every signal is captured, with no gaps in time. A signal that exists for as little as one FFT frame (1-50 microseconds) is captured. Key metric: Minimum Signal Duration (MSD) or POI (Probability of Intercept). MSD: 1-50 microseconds (the shortest signal that can be captured with 100% probability). SSA: MSD is essentially equal to the sweep time (milliseconds to seconds), so very short signals are almost always missed. (3) When to use each: SSA: general-purpose spectrum observation (CW signals, stable emissions). EMC pre-compliance (quasi-peak measurements). Narrowband signal analysis (phase noise, spurious). Lower cost for a given frequency range. RTSA: frequency-hopping signals (Bluetooth, Wi-Fi, FHSS). Radar pulse analysis (short pulses, PRF characterization). Interference hunting (intermittent interferers that appear and disappear). Glitch/transient detection. Spectrum monitoring and spectrum management. (4) Modern instruments are converging: many current-generation spectrum analyzers include both swept and real-time modes. Keysight MXA/PXA: swept + RTSA option (up to 510 MHz real-time BW). Rohde & Schwarz FSW: swept + RTSA (up to 800 MHz real-time BW). Tektronix RSA5000: native RTSA (40-800 MHz real-time BW). Siglent SSA3000X Plus: swept + limited RTSA.