Appendix A: Glossary of RTSA Terms - Nuts and Bolts of Real-Time Spectrum Analysis

The working vocabulary used throughout this book. Definitions are written for the bench, not the lecture hall: enough to read a datasheet and know what a specification means for your measurement.

ADC (Analog-to-Digital Converter): Component that samples analog voltage at periodic intervals and produces a digital number representing each sample.
ADS-B: Automatic Dependent Surveillance-Broadcast. Aircraft positioning system on 1090 MHz.
AGC (Automatic Gain Control): Feedback loop that adjusts amplifier or attenuator gain based on input signal level.
AOA (Angle of Arrival): The direction from which a signal is received, typically measured in azimuth and elevation.
Aaronia: German manufacturer of real-time spectrum analyzers (SPECTRAN V6 family) and direction-finding antennas (IsoLOG 3D DF).
Aliasing: Spectral artifact that occurs when a signal is sampled at less than twice its highest frequency component.
Amplitude Imbalance: Difference in gain between I and Q paths in a quadrature receiver, producing image artifacts.
Analytic Signal: Complex-valued signal with a one-sided spectrum, constructed from a real signal via the Hilbert transform.
ASK (Amplitude Shift Keying): Digital modulation that encodes data in carrier amplitude.
Bandwidth: The frequency span occupied by a signal or analyzer.
Beamforming: Signal processing technique that creates a directional response pattern from a multi-element antenna array.
BLE (Bluetooth Low Energy): Low-power short-range wireless protocol using GFSK in the 2.4 GHz ISM band.
BPSK (Binary Phase Shift Keying): Digital modulation with two phase states (0 and pi).
Carrier Aggregation: Cellular technique combining multiple frequency bands for higher throughput.
Channelization: Splitting a wideband digitized signal into multiple narrowband sub-channels via parallel decimating filters.
Chirp: Signal with frequency that changes monotonically over time. LFM (Linear Frequency Modulation) is the most common form.
Constellation Diagram: Scatter plot of received I/Q symbols against ideal modulation points; reveals modulation quality.
Counter-UAS: Counter-Unmanned Aircraft System. Detection, tracking, and where legal, neutralization of unauthorized drones.
Cross-Correlation (Phase Noise): Technique in dedicated phase noise analyzers that runs two independent reference-and-detector chains and correlates their outputs. The DUT's noise is common to both and survives; each chain's own noise averages down as 5·log10(N), allowing floors near −180 dBc/Hz.
dBc/Hz: Decibels relative to the carrier, per hertz of bandwidth. The unit of single-sideband phase noise: the ratio of noise power in a 1 Hz bandwidth at a given offset to total carrier power.
dB (Decibel): Logarithmic unit of power ratio (10 log10) or amplitude ratio (20 log10).
dBc: Decibels relative to the carrier; used to specify spurious or noise levels.
dBm: Decibels relative to 1 milliwatt absolute power.
DF (Direction Finding): Determining the angular direction from which a signal arrives.
DFT (Discrete Fourier Transform): Mathematical operation transforming N time-domain samples into N frequency-domain values.
Direct-Spectrum Method: Measuring phase noise by reading the carrier-to-sideband ratio on a spectrum analyzer, normalizing the resolution bandwidth to 1 Hz, and correcting for detector statistics. Simple and fast, but floor-limited by the analyzer's own local oscillator.
DRFM (Digital Radio Frequency Memory): EW jammer that captures, modifies, and retransmits incoming victim signals.
ERP (Effective Radiated Power): Transmitter power multiplied by antenna gain in a given direction, referenced to a dipole; the quantity broadcast regulators license and drone surveys verify, typically to 1 to 2 dB overall uncertainty.
EVM (Error Vector Magnitude): RMS error between received and ideal constellation points, normalized by reference power.
ENBW (Equivalent Noise Bandwidth): The noise bandwidth equivalent to a window function's frequency response.
ENOB (Effective Number of Bits): Realistic ADC resolution after accounting for thermal and quantization noise.
EW (Electronic Warfare): RF operations to detect, characterize, jam, or otherwise affect enemy electromagnetic systems.
FFT (Fast Fourier Transform): Efficient O(N log N) algorithm for computing the DFT.
FHSS (Frequency Hopping Spread Spectrum): Modulation that rapidly hops the carrier across many frequencies in a pseudorandom pattern.
Flowgraph: A GNU Radio program expressed as a directed graph of processing blocks: sources produce samples, sinks consume them, and DSP blocks transform them in between. The runtime schedules the blocks and moves sample buffers automatically.
FMT (Frequency Mask Trigger): RTSA capability that captures I/Q whenever the spectrum exceeds a user-defined envelope.
FSK (Frequency Shift Keying): Digital modulation that encodes data in carrier frequency.
FR1: 5G NR Frequency Range 1 (sub-6 GHz, up to 7.125 GHz).
FR2: 5G NR Frequency Range 2 (mmWave, 24.25 to 71 GHz).
Friis's Formula: Equation describing how the noise figure of a multi-stage receiver depends primarily on the first stage.
GFSK (Gaussian FSK): FSK with Gaussian pulse shaping for narrower spectral occupancy.
GNSS (Global Navigation Satellite System): Generic term covering GPS, Galileo, GLONASS, BeiDou, QZSS.
GNU Radio: Free, open-source software development toolkit providing signal processing blocks for building software radios. Used across academia, industry, government, and the hobby community, and the foundation of many satellite ground-station modems.
GPSDO (GPS-Disciplined Oscillator): Reference oscillator locked to GPS for high-precision frequency and time.
GRC (GNU Radio Companion): GNU Radio's visual editor. Blocks are dragged onto a canvas, wired together, and parameterized; GRC then generates and runs a Python program implementing the flowgraph.
Harmonic Distortion: Unwanted signal components generated at integer multiples of a signal's fundamental frequency (2f0, 3f0, ...) by nonlinearity in a device's transfer function. The nth-order term of the power-series nonlinearity produces the nth harmonic.
Hilbert Transform: Mathematical operation that produces a 90-degree phase shift across all frequencies.
HRP (Horizontal Radiation Pattern): An antenna's radiated field strength as a function of azimuth, measured in the horizontal plane at the aperture. Broadcast licenses specify the HRP; drone orbits at aperture height now measure it directly.
IF Overflow: Error condition raised when input signal level exceeds the headroom of the instrument's digitizer at the current reference level. Instead of silently clipping, the streaming driver reports the overflow and expects the user to raise the reference level.
IIP3 (Input Third-order Intercept Point): Theoretical input power at which third-order intermodulation products equal the input signals.
I/Q (In-phase and Quadrature): Pair of signals representing the real and imaginary parts of a complex baseband signal.
IQ Streaming API: A documented programming interface through which an instrument exports its calibrated I/Q sample stream to external software, turning a closed measurement appliance into a programmable front end for the open SDR ecosystem.
ISM (Industrial, Scientific, Medical): Unlicensed RF bands available worldwide for general use; common bands at 2.4 GHz, 5.8 GHz, 433 MHz.
IsoLOG 3D DF: Aaronia's direction-finding antenna family with sectored arrays and elevation capability.
KML (Keyhole Markup Language): XML format for geographic data, commonly used for Google Earth.
Leeson Model: The classical model relating an oscillator's phase noise curve to physical noise sources. Its slope regions (from −40 dB/decade random-walk FM through flicker FM, white FM, and flicker phase to the flat white-phase floor) let an engineer read mechanism from measured slope.
LFM (Linear Frequency Modulation): Chirp with linearly varying frequency.
LNA (Low-Noise Amplifier): First active stage in a receiver, optimized for low noise figure.
LO (Local Oscillator): Reference signal mixed with the input to translate frequency.
LoRa: Low-power wide-area protocol using chirp spread spectrum.
MIMO (Multiple Input Multiple Output): Wireless technique using multiple antennas at transmitter and receiver.
Mixer Level: The signal power actually reaching a spectrum analyzer's first mixer: input power minus input attenuation. The controlling variable for internally generated distortion; the optimum mixer level sets internal distortion equal to the noise floor.
Modulation: Encoding information onto a carrier wave by varying amplitude, frequency, or phase.
Monitoring Node: A receiver built from compact RTSA hardware, packaged for fixed installation with Ethernet, a remote-control API, and typically a GNSS-disciplined reference, deployed in numbers to form a distributed monitoring grid.
NF (Noise Figure): Ratio of input SNR to output SNR of a stage; expressed in dB.
Normalization: The scalar substitute for VNA calibration: storing a through-connection trace as a 0 dB reference and subtracting it from subsequent sweeps to remove cable loss and tracking generator flatness ripple. Corrects frequency-response error but not mismatch error.
Nyquist: Sampling theorem; minimum sample rate is twice the highest signal frequency.
OFDM (Orthogonal Frequency Division Multiplexing): Wideband modulation using many orthogonal narrowband subcarriers, each modulated with QAM.
OOT Module (Out-of-Tree): A package of custom GNU Radio blocks distributed independently of the main GNU Radio tree, written in Python or C++, with descriptors that make the blocks appear in GRC like native ones. Examples include gr-adsb, gr-dab, and gr-ieee802-11.
PDW (Pulse Descriptor Word): Fixed-format record describing a single pulse: TOA, PW, frequency, amplitude.
Phase Noise: Random fluctuations in oscillator phase, expressed as dBc/Hz at offset frequencies.
Phase Noise Skirt: The sloping pedestal of noise sidebands on either side of a real oscillator's carrier line. The skirt rises toward the carrier, and its shape versus log offset is diagnostic of the underlying noise mechanisms.
POI (Probability of Intercept): Minimum signal duration captured with near certainty by an analyzer. SPECTRAN V6 PLUS achieves 10 ns POI.
PRF (Pulse Repetition Frequency): Number of pulses transmitted per second by a pulsed system.
PRI (Pulse Repetition Interval): Time between consecutive pulse starts; PRI = 1/PRF.
PSD (Power Spectral Density): Power per unit bandwidth, commonly expressed in dBm/Hz.
PSK (Phase Shift Keying): Digital modulation that encodes data in carrier phase.
QAM (Quadrature Amplitude Modulation): Digital modulation combining amplitude and phase, e.g., 16-QAM, 256-QAM, 1024-QAM, 4096-QAM.
QPSK (Quadrature Phase Shift Keying): Four-state PSK encoding 2 bits per symbol.
RBW (Resolution Bandwidth): The width of one frequency bin in an FFT, equal to sample rate divided by FFT length.
Return Loss: The ratio, in dB, of power reflected from a port to power incident on it, measured in a scalar setup with a directional bridge or coupler between the tracking generator and the device. Convertible to VSWR.
RF Heat Map: A map rendering of GNSS-tagged spectrum captures, coloring each location by measured signal strength (channel power, RSSI, or occupied bandwidth) so coverage and interference hot zones are visible at a glance.
RMS Jitter: The time-domain equivalent of integrated phase noise: the band-limited integral of L(f), converted to RMS phase and divided by the angular carrier frequency. Meaningless without its integration band; the common convention is 12 kHz to 20 MHz.
RTBW (Real-Time Bandwidth): Width of spectrum that an RTSA digitizes simultaneously with no time gaps.
RTSA (Real-Time Spectrum Analyzer): Instrument that captures wide spectrum continuously without sweeping.
Scalar Network Analysis: Measurement of a network's magnitude response versus frequency (gain, insertion loss, ripple, rejection) using a tracking generator and spectrum analyzer, without phase information or vector error correction.
Scalloping Loss: Variation in measured amplitude as a tone moves between FFT bin centers.
SDR (Software Defined Radio): Radio system where most signal processing is implemented in software.
SDR++: Cross-platform, open-source SDR receiver application with a live spectrum and waterfall, click-to-tune operation, and a modular plugin architecture separating sources, DSP, decoders, and sinks. The de facto standard open front end for SDR hardware, including the latest generation of USB-format RTSAs.
SFDR (Spurious-Free Dynamic Range): Difference between strongest signal and strongest analyzer-generated spur.
SHI (Second Harmonic Intercept): Extrapolated figure of merit for second-order nonlinearity: the fictitious input level at which the second harmonic would equal the fundamental. For an analyzer, internally generated second harmonic in dBc equals SHI minus mixer level.
SigMF (Signal Metadata Format): Open standard for I/Q file metadata; JSON sidecar paired with a raw data file.
SoapySDR: A vendor-neutral abstraction layer for SDR hardware. A single SoapySDR driver exposes a device to every SoapySDR-aware application, including SDR++, GQRX, CubicSDR, GNU Radio, and rtl_433.
SPECTRAN V6 PLUS: Aaronia's flagship USB real-time spectrum analyzer family (250XA, 500XA, 2000XA models).
Spot Noise: The value of L(f) at a single standard offset (for example 1 kHz, 10 kHz, 100 kHz, or 1 MHz), reported as a table rather than a full curve. The usual format for datasheet limits and production-test pass/fail checks.
Spread Spectrum: Modulation technique that spreads signal energy across a wider bandwidth than data alone requires.
Spectrogram: Time-frequency-amplitude plot, commonly displayed as a waterfall.
Subclutter Visibility: A Doppler radar's ability to detect a small moving target beneath much stronger stationary clutter. Limited directly by the phase noise of the radar's oscillator chain at offsets equal to the Doppler frequencies of interest.
SWaP (Size, Weight, and Power): The three budget constraints that decide whether an instrument can ride a given platform. Drone-mounted spectrum measurement became practical when compact RTSAs dropped to the few-hundred-gram, 15 W class.
TCXO (Temperature-Compensated Crystal Oscillator): Reference oscillator with temperature compensation, typically 1-5 ppm accuracy.
TDOA (Time Difference of Arrival): Geolocation technique using arrival time differences across spatially separated sensors.
THD (Total Harmonic Distortion): Single-number distortion summary: the ratio of the root sum of squares of all harmonic amplitudes to the fundamental amplitude, expressed as a percentage or in dB. Less diagnostic than per-harmonic dBc measurements.
TOI (Third-Order Intercept): Extrapolated intercept for third-order distortion products, which rise 3 dB per dB of fundamental. Also written IP3.
Tracking Generator (TG): A signal source phase-locked to a spectrum analyzer's sweep so that its output frequency always equals the analyzer's tuned frequency. Routed through a device under test, it enables stimulus-response (scalar network) measurements.
Type Approval: Regulatory certification confirming a wireless device meets emissions requirements before authorization.
UAS (Unmanned Aircraft System): Drone or other unmanned aerial vehicle.
VFO (Variable Frequency Oscillator): In SDR software, an independently tunable receive channel within the captured bandwidth. Multi-VFO operation lets one capture serve several simultaneous listeners or decoders.
VRP (Vertical Radiation Pattern): An antenna's radiated field strength as a function of elevation angle. Cannot be sampled at aperture height from the ground; drones fly vertical slices to reconstruct it.
VSG (Vector Signal Generator): Test instrument that generates arbitrary modulated waveforms.
Waterfall: Spectrogram display with frequency on the X-axis, time on the Y-axis, and amplitude as color.
Window Function: Tapering function applied to FFT input to reduce spectral leakage.
Wrapped Spectrum: Aaronia patented display that stacks the frequency axis across multiple lines for high-resolution wide-span monitoring.