Theory › Reference
RF Engineering Glossary
130+ RF and microwave engineering terms defined in plain English. From noise figure to VSWR, IIP3 to EIRP — the essential reference for RF engineers at every level.
This glossary covers the most important terms in RF and microwave engineering — from fundamental concepts like impedance and VSWR to advanced topics like IQ imbalance, EVM, and fractional-N synthesis. Each definition is written for practising engineers, not textbooks. Use the search box or category filter to find what you need.
130 terms shown
12 categories
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A
AGC
Automatic Gain Control
System
A feedback loop that adjusts the gain of a receiver chain to maintain a roughly constant signal level at its output, regardless of how strong the input signal is. Prevents the ADC from being overloaded by strong signals while still amplifying weak signals enough to digitise them. AGC range is specified in dB — wider range means the receiver can handle a larger spread of input power levels.
Array Factor
AF
Antenna
The radiation pattern produced by the geometry and phase/amplitude weighting of an antenna array, independent of the individual element patterns. The total array pattern = Array Factor × Element Pattern. Steering the beam is done by adjusting the progressive phase shift between elements. Grating lobes appear when element spacing exceeds λ/2 and the beam is steered off broadside.
Aperture Efficiency
Antenna
The ratio of an antenna's effective aperture (Ae) to its physical aperture area. A theoretically perfect uniformly illuminated aperture has 100% efficiency, but real antennas suffer losses from taper, spillover, blockage, and surface errors. Typical values: 55–75% for reflector antennas, 80–90% for well-designed horns. Related to gain by: G = 4π·Ae·η/λ².
Aperture Jitter
Noise
Uncertainty in the exact moment an ADC samples the input signal. Even a tiny timing error translates directly into a voltage error because the signal is changing at the moment of sampling. The SNR degradation from aperture jitter is: SNR_jitter = −20·log₁₀(2π·f_in·σ_t) where σ_t is the RMS jitter. At 1 GHz with 1 ps RMS jitter, this limits SNR to ~44 dB regardless of ADC resolution.
Attenuation Constant
α
TX Line
The real part of the complex propagation constant γ = α + jβ. It describes how quickly signal amplitude decays per unit length of transmission line, measured in dB/m or Np/m. Has two components: conductor loss (increases as √f due to skin effect) and dielectric loss (increases linearly with f). Total loss = α·length.
B
Bandwidth
BW
System
The range of frequencies over which a component or system operates within a defined specification. Common definitions: −3 dB bandwidth (half-power points), noise bandwidth (equivalent rectangular BW for noise calculations = π/2 × −3dB BW for single-pole filter), and channel bandwidth (allocated spectrum). Noise power scales linearly with noise bandwidth: P_noise = k·T·BW.
Balun
TX Line
A BALanced-to-UNbalanced transformer that connects a balanced transmission line (e.g. a dipole) to an unbalanced line (e.g. coax). Without a balun, the outer conductor of the coax carries return current that radiates and distorts the pattern. Implemented as wound ferrite transformers, sleeve baluns, or λ/4 chokes. Key specs: frequency range, insertion loss, and common-mode rejection ratio.
Blocker / Desensitisation
Noise
A blocker is a strong interfering signal near the wanted channel. Even if it doesn't overlap in frequency, it can drive an LNA or mixer into compression, raising the effective noise figure and reducing sensitivity to the wanted signal — this is called desensitisation. A receiver's blocker performance is characterised by the blocking specification: how large an out-of-channel signal the receiver can tolerate while still meeting its noise figure requirement.
C
Characteristic Impedance
Z₀
TX Line
The impedance of an infinitely long (or perfectly terminated) transmission line: Z₀ = √(L'/C') where L' and C' are the distributed inductance and capacitance per unit length. 50 Ω is the RF industry standard (a compromise between lowest loss ≈77 Ω and maximum power handling ≈30 Ω in coax). Cable TV uses 75 Ω (lower loss, less power). A mismatch between Z₀ and the load causes reflections.
Chebyshev Filter
Filter
A filter approximation that achieves a sharper roll-off than Butterworth of the same order by allowing equal-ripple variation in the passband. Type I ripple is in the passband; Type II (inverse Chebyshev) ripple is in the stopband with a maximally flat passband. The ripple-to-roll-off trade-off is controlled by the ripple parameter ε. Widely used in RF bandpass filters where selectivity matters more than flat passband response.
Compression Point
P1dB
Amplifier
The input (IP1dB) or output (OP1dB) power level at which the amplifier's gain has dropped by 1 dB from its small-signal value. It marks the onset of significant nonlinearity. For a linear amplifier: OP1dB ≈ IIP3 − 9.6 dB. Operating too close to P1dB generates harmonics and intermodulation products that cause distortion and spurious emissions.
Constellation Diagram
Modulation
A 2D scatter plot of a digitally modulated signal's I (in-phase) and Q (quadrature) components. Each discrete symbol maps to a specific point on the constellation. QPSK has 4 points, 16-QAM has 16, 256-QAM has 256. In a real system, each point becomes a "cloud" due to noise, phase error, and IQ imbalance. EVM (Error Vector Magnitude) quantifies the average distance from ideal constellation points.
Conversion Gain / Loss
Mixer
The ratio of IF output power to RF input power in a mixer, expressed in dB. Passive diode mixers typically have 5–8 dB conversion loss. Active mixers (Gilbert cell) can have conversion gain of 5–15 dB. Conversion loss directly adds to the noise figure of the receive chain. Note: conversion gain is defined at the IF frequency, not at RF.
D
Dynamic Range
Noise
The range of signal powers over which a receiver operates correctly. Spurious-Free Dynamic Range (SFDR) = the difference between the minimum detectable signal (noise floor) and the maximum signal before spurious products from nonlinearity rise above the noise floor. Larger SFDR = better. For a two-tone test: SFDR = ⅔(IIP3 − N_floor). Typically 60–100 dB for LNA/mixer combinations.
Directivity
D
Antenna
The ratio of the radiation intensity in the direction of maximum radiation to the average radiation intensity over all directions. Unlike gain, directivity excludes ohmic losses in the antenna. Related by: G = η · D where η is radiation efficiency. A half-wave dipole has D = 2.15 dBi; a small isotropic source has D = 0 dBi.
Doherty Amplifier
PA
A PA architecture that maintains high efficiency even when the signal power is below peak. Uses a main (carrier) amplifier operating class AB, plus a peaking amplifier that turns on only when signal exceeds 6 dB backoff. Load modulation via a λ/4 impedance transformer keeps the main amp near saturation over a wide power range. Dominant in LTE/5G base stations where PAE at average power matters most.
Duplexer
System
A three-port device that connects a single antenna to both a transmitter and a receiver simultaneously in an FDD (Frequency-Division Duplex) system. Uses high-Q bandpass filters to provide >50 dB isolation between the TX and RX ports, preventing the high-power TX signal from saturating the sensitive RX path. Typically implemented as a pair of BAW or SAW filters on a single substrate.
E
EIRP
Effective Isotropic Radiated Power
System
The product of transmit power and antenna gain, referenced to an isotropic radiator: EIRP (dBm) = P_TX (dBm) + G_TX (dBi). EIRP represents the power a hypothetical isotropic antenna would need to produce the same field strength in the direction of maximum radiation. Used in regulatory limits (e.g. FCC Part 15), link budgets, and interference calculations.
EVM
Error Vector Magnitude
Modulation
A measure of modulation quality — the RMS distance between actual and ideal symbol positions on the constellation, expressed as a percentage of the reference signal magnitude. EVM (%) = √(P_error / P_reference) × 100. Caused by phase noise, IQ imbalance, amplitude nonlinearity, and noise. 5G NR requires EVM < 3.5% for 256-QAM uplink. EVM in dB = 20·log₁₀(EVM%).
F
Fade Margin
System
The extra link margin (dB) built into a link budget above the minimum required SNR, to accommodate signal fading from multipath, rain, atmospheric ducting, or antenna misalignment. A fade margin of 10–20 dB is typical for microwave point-to-point links; satellite links need 3–6 dB for clear-sky and 20+ dB for rain-fade at Ka band.
Fractional-N PLL
PLL
A PLL where the feedback divider uses a non-integer (fractional) division ratio, achieved by rapidly switching between two or more integer values using a ΣΔ modulator. This allows fine frequency resolution (much smaller than the reference frequency) while maintaining a wide loop bandwidth. The ΣΔ modulator pushes quantisation noise to high offsets where the loop filter attenuates it. The dominant technique in modern RF synthesisers (5G, WiFi, Bluetooth).
Free-Space Path Loss
FSPL
System
The attenuation of an EM wave propagating through free space with no obstacles. Not absorption — purely geometric spreading of the wavefront: FSPL (dB) = 20·log(d_km) + 20·log(f_MHz) + 32.44. Doubles (adds 6 dB) for every doubling of distance or frequency. A 10× frequency increase adds 20 dB — which is why 28 GHz mmWave 5G has much shorter range than 700 MHz LTE.
Frequency Cutoff
f_c
TX Line
The minimum frequency below which a particular waveguide mode cannot propagate. For rectangular waveguide TE₁₀ (dominant mode): f_c = c / 2a where a is the wide dimension. Below cutoff, the wave is evanescent and decays exponentially. For microstrip, the cutoff frequency marks the onset of higher-order modes that cause radiation loss and dispersion.
Friis Noise Formula
Noise
The formula for the total noise factor of a cascaded chain of components: F_total = F₁ + (F₂−1)/G₁ + (F₃−1)/(G₁G₂) + …. Each subsequent stage contributes less to total noise because it is divided by the cumulative gain of all preceding stages. This is why the first LNA dominates the receive chain NF — it sees the raw antenna thermal noise with no prior gain. Maximise LNA gain and minimise LNA NF first.
G
Gain (Antenna)
G, dBi
Antenna
The ratio of an antenna's radiation intensity in its peak direction to that of a lossless isotropic radiator radiating the same total power. Unlike directivity, gain includes ohmic losses. Expressed in dBi (dB relative to isotropic). A 10 dBi antenna radiates 10× more power in its peak direction than an isotropic source would with the same input power.
Gain Compression
Amplifier
The reduction in an amplifier's gain as input power increases into the nonlinear regime. At low power, gain is constant (small-signal regime). As power increases, gain starts to drop — 1 dB compression (P1dB) is the standard reference point. Beyond P1dB, the output power saturates. Gain compression causes AM-AM distortion and is described by the odd-order terms of the device's power series.
Grating Lobe
Antenna
An unintended main lobe that appears in an array pattern when element spacing exceeds λ/2 and the beam is steered. Grating lobes are as strong as the main beam and radiate (or receive from) an unintended direction. They appear in the visible region when d/λ · (sin θ_scan + sin θ_grating) = ±1. Rule of thumb: keep d ≤ λ/2 for scan angles up to ±90°.
Group Delay
S-Param
The negative derivative of phase with respect to angular frequency: τ_g = −dφ/dω. It represents the time delay experienced by the envelope of a narrowband signal. Flat group delay = linear phase = no distortion of wideband signals. Filters with sharp roll-offs (Chebyshev, elliptic) have severe group delay ripple near the band edge. Bessel/Thomson filters are designed for maximally flat group delay.
H
HPBW
Half-Power Beamwidth
Antenna
The angular width of the main beam between the two points where power density drops to half (−3 dB) of the peak. Wider HPBW = lower gain but easier pointing. HPBW ≈ 102°/N·(d/λ) for a broadside array of N elements with spacing d. For a reflector dish: HPBW ≈ 70·λ/D degrees where D is diameter. HPBW and gain are inversely related.
Heterodyne / Superhet
System
A receiver architecture where the RF signal is multiplied (mixed) with a local oscillator (LO) to produce an intermediate frequency (IF): f_IF = |f_RF − f_LO|. The IF is fixed regardless of RF frequency, allowing high-performance fixed filtering and amplification. The main challenge is the image frequency — the mirror frequency 2·f_IF away from the LO that also produces f_IF. Requires a preselector filter before the mixer to reject images.
I
IF
Intermediate Frequency
System
The fixed output frequency of a mixer in a heterodyne receiver: f_IF = |f_RF − f_LO|. The IF stage provides the receiver's channel selectivity (via IF filters) and most of its gain. Common IF frequencies: 455 kHz (AM radio), 10.7 MHz (FM), 70 MHz (satellite), 140 MHz (microwave backhaul). Modern software-defined radios often use zero-IF (direct conversion) or low-IF architectures to simplify hardware.
Image Frequency
Mixer
The unwanted mirror frequency that also downconverts to the same IF as the wanted signal: f_image = f_LO − f_IF (low-side LO) or f_image = f_LO + f_IF (high-side LO). Separated from the wanted RF by exactly 2 × f_IF. Must be rejected by a preselector bandpass filter before the mixer. Image rejection ratio (IRR) specifies how well the image is suppressed. IQ (quadrature) architectures provide inherent image rejection.
IIP3 / OIP3
Input/Output 3rd-Order Intercept
Noise
A figure of merit for linearity. In a two-tone test, the 3rd-order intermodulation products grow at 3× the rate of the fundamental (in dB). IIP3 is the extrapolated input power at which the IM3 products would equal the fundamentals — a hypothetical point, always above P1dB. Higher IIP3 = better linearity. Rule of thumb: IIP3 ≈ P1dB + 9.6 dB. Cascaded IIP3 is dominated by late high-gain stages.
IQ Imbalance
Modulation
Error in a quadrature (IQ) system where the I and Q paths are not perfectly orthogonal or equal in amplitude. Phase imbalance: the two LO paths differ from 90°. Amplitude imbalance: the I and Q mixers have different gains. Both cause the image signal to "bleed" into the wanted channel and degrade EVM and image rejection. Corrected by digital IQ calibration algorithms in modern transceivers.
Impedance Matching
Matching
The process of transforming a source impedance to equal the complex conjugate of a load impedance to maximise power transfer. Techniques: L-network (2 elements, narrowband), π/T networks (3 elements, tunable Q), λ/4 transformer (single frequency, real impedances), stub matching (distributed, wideband). On a Smith chart, matching moves the load point to the centre (50 Ω).
Intermodulation Distortion
IMD
Noise
When two or more signals enter a nonlinear device, their harmonics mix to produce new frequencies at m·f₁ ± n·f₂. The most troublesome are 3rd-order products at 2f₁−f₂ and 2f₂−f₁, which fall in-band close to the original signals and cannot be filtered out. IMD is characterised by the IIP3 figure. The "order" of a product = m+n; higher-order products are weaker but may still cause issues with large blockers.
J
Jitter
PLL / Clock
Short-term, random deviations in the timing of a clock edge from its ideal position. Expressed as RMS picoseconds. Phase jitter is the time-domain equivalent of phase noise — they are related by: σ_t = (1/2πf₀) · √(2 · ∫L(f)df) where L(f) is the single-sideband phase noise in dBc/Hz and the integral is over the offset frequency range. Critical for ADC performance: 1 ps RMS jitter limits SNR to 50 dB at 1 GHz input.
K
K (Rollett Stability Factor)
S-Param
A measure of amplifier stability derived from S-parameters: K = (1 − |S₁₁|² − |S₂₂|² + |Δ|²) / (2|S₁₂||S₂₁|) where Δ = S₁₁S₂₂ − S₁₂S₂₁. For unconditional stability (stable for any passive source/load): K > 1 AND |Δ| < 1 (or equivalently μ > 1). K < 1 means the amplifier is potentially unstable and may oscillate with certain source/load impedances.
L
Link Budget
System
An accounting of all gains and losses in a communication link from transmitter to receiver: P_RX = P_TX + G_TX − L_path + G_RX − L_cable − L_other. The received power is compared to the minimum detectable signal (receiver sensitivity) to determine link margin. A positive margin means the link works; negative means it fails. Link budgets are the fundamental design tool for any wireless system.
Leeson's Equation
Oscillator
An empirical model for oscillator phase noise: L(Δf) = 10·log[2FkT/P_s · (f₀/2Q·Δf)² · (1 + Δf₁/f/|Δf|)]. Key takeaways: phase noise improves with higher Q (steeper resonator), higher signal power, and lower noise figure (F). The 1/f corner frequency Δf₁/f divides the 1/f³ (close-in, flicker-dominated) region from the 1/f² (thermal noise) region. Directly predicts the −20 dB/decade slope seen in real oscillator phase noise plots.
LNA
Low Noise Amplifier
System
The first active stage in a receiver chain, directly after the antenna and any preselect filter. Its job is to amplify the weak received signal while adding as little noise as possible. Because it is the first stage, its noise figure dominates the cascaded NF of the whole chain (Friis formula). Design trade-off: low NF vs. high IIP3 vs. high gain — pushing noise figure down typically hurts linearity.
Loop Bandwidth
PLL
The −3 dB bandwidth of a PLL's closed-loop transfer function. Sets the trade-off between reference noise suppression (wider BW passes more reference noise) and VCO noise suppression (narrower BW leaves more VCO noise in the output). Typical loop BW is 1–5% of the reference frequency. Also determines lock time: faster locking requires wider loop BW.
M
MDS
Minimum Detectable Signal
System
The weakest signal a receiver can detect with acceptable quality, set by the noise floor: MDS (dBm) = −174 + NF + 10·log(BW) + SNR_min. At room temperature, the noise floor is −174 dBm/Hz. A 10 MHz BW, 5 dB NF receiver needs SNR_min = 10 dB has MDS = −174 + 5 + 70 + 10 = −89 dBm. Also called receiver sensitivity.
MIMO
Multiple Input Multiple Output
System
A technique using multiple antennas at both transmitter and receiver to improve capacity and/or reliability. Spatial multiplexing (multiple independent data streams) increases capacity by min(N_TX, N_RX) times. Transmit diversity improves reliability. Beamforming increases range. 5G massive MIMO uses 64–256 antenna elements at the base station for simultaneous multi-user MIMO (MU-MIMO).
Mixer
Mixer
A three-port nonlinear device that multiplies RF and LO signals to produce sum and difference frequencies at the IF port: f_IF = f_RF ± f_LO. Real mixers also produce harmonics: all products m·f_RF ± n·f_LO appear at the output with amplitudes that decrease for higher orders. Key specs: conversion loss, NF, IIP3, LO-RF isolation, LO-IF isolation.
Microstrip
TX Line
A planar transmission line consisting of a conducting strip on one side of a dielectric substrate with a ground plane on the other. The dominant mode is quasi-TEM (not pure TEM, unlike stripline) because the fields are partly in air and partly in the substrate — giving a frequency-dependent effective dielectric constant. Width controls Z₀; thinner trace = higher Z₀. The workhorse of microwave PCB design.
Modulation
Modulation
The process of encoding information onto a carrier wave by varying its amplitude (AM), frequency (FM), or phase (PM). Digital modulation schemes use discrete symbol alphabets: BPSK (2 points), QPSK (4), 16-QAM (16), 256-QAM (256). Higher-order QAM packs more bits per symbol (better spectral efficiency) but requires higher SNR and tighter linearity.
N
Noise Figure
NF
Noise
A measure of how much noise a component adds to a signal, expressed as the degradation in signal-to-noise ratio from input to output: NF (dB) = SNR_in (dB) − SNR_out (dB). An ideal noiseless component has NF = 0 dB. A real LNA might have NF = 1–2 dB; a passive mixer with 7 dB conversion loss has NF ≥ 7 dB. Noise figure is defined at a reference temperature of T₀ = 290 K.
Noise Floor
Noise
The minimum signal level set by thermal (Johnson) noise at a reference temperature of 290 K: N_floor = kT₀B = −174 dBm/Hz + 10·log(BW_Hz). At 1 MHz bandwidth: −114 dBm. This is the absolute lower limit — no passive network can have a lower noise floor, and any active component makes it worse. Adding a receiver's NF raises the effective noise floor.
Nyquist Theorem
ADC
To perfectly reconstruct a bandlimited signal, the sample rate must be at least twice the highest frequency component: f_s ≥ 2·f_max. Sampling below the Nyquist rate causes aliasing — higher frequencies fold back and appear as spurious lower-frequency signals. In RF design, intentional undersampling (bandpass sampling) is used to downconvert IF signals directly to baseband using a carefully chosen sample rate.
O
OFDM
Orthogonal Frequency-Division Multiplexing
Modulation
A multicarrier modulation scheme that splits the channel into many closely spaced orthogonal subcarriers, each modulated with QAM or PSK. The orthogonality prevents inter-carrier interference (ICI). Highly efficient use of spectrum, robust against multipath (via cyclic prefix). Used in: 4G LTE, 5G NR, WiFi 802.11a/g/n/ac/ax. Main drawback: high PAPR (10–12 dB) which stresses PA efficiency.
Oscillator / VCO
Oscillator
A circuit that generates a periodic signal at a specific frequency. A VCO (Voltage-Controlled Oscillator) tunes its frequency via a control voltage, characterised by its tuning sensitivity K_VCO (MHz/V). Key specs: phase noise (dBc/Hz at a given offset), tuning range, pushing (frequency vs supply voltage), pulling (frequency vs load impedance). Crystal oscillators achieve very low phase noise by using a high-Q piezoelectric resonator.
P
PAE
Power-Added Efficiency
PA
The efficiency of a power amplifier accounting for both DC power consumption and RF input power: PAE = (P_out − P_in) / P_DC. This is the most meaningful PA efficiency metric because it captures the "added" RF power relative to the DC consumed. A typical GaN PA might have PAE = 40–60% at peak power. PAE drops sharply at power backoff — the main challenge in modern communication systems with high-PAPR signals.
PAPR
Peak-to-Average Power Ratio
Modulation
The ratio of peak instantaneous power to average power in a signal, expressed in dB. OFDM has high PAPR (~10–12 dB for 5G NR) because many subcarriers occasionally add in phase. High PAPR forces the PA to operate well below its saturation power (backoff) to avoid clipping/distortion, reducing efficiency drastically. Mitigated by techniques: crest factor reduction (CFR), clipping, envelope tracking (ET), or Doherty PAs.
Phase Noise
Noise
Short-term, random fluctuations in the phase of an oscillator output, expressed as single-sideband (SSB) power spectral density relative to the carrier: L(Δf) in dBc/Hz at offset Δf. Plotted as a curve from close-in offsets (~1 Hz) to far offsets (~10 MHz). Degrades system performance by: reciprocal mixing (a strong blocker's phase noise covers the wanted channel), EVM degradation, and ADC SNR floor. Specified at key offsets (e.g. −90 dBc/Hz at 100 kHz).
PLL
Phase-Locked Loop
PLL
A feedback control system that locks the output phase (and frequency) of a VCO to a reference oscillator. Components: Phase-Frequency Detector (PFD) → Charge Pump → Loop Filter → VCO → Frequency Divider (÷N) → back to PFD. Output frequency = N × f_reference. Used everywhere: RF synthesisers, clock generation, data recovery, FM demodulation, and carrier recovery.
Polarisation
Antenna
The orientation of the electric field vector of a radiated wave. Linear: horizontal or vertical. Circular: field rotates in a helix — RHCP or LHCP. Elliptical: intermediate case. Polarisation mismatch between TX and RX causes up to 3 dB loss (linear to circular) or complete null (cross-polar: RHCP to LHCP). GPS uses RHCP to be rotation-invariant to the receiver orientation.
Port
S-Param
A terminal pair of a network where power can enter or exit. Most RF components are characterised as 2-port networks (input and output) but couplers, combiners, and circulators are 3- or 4-port. S-parameters fully describe a linear N-port network's behaviour: S_ij = transmitted/reflected wave at port i when port j is driven and all other ports are terminated in Z₀.
Q
Q Factor
Quality Factor
Filter
A dimensionless ratio measuring how "good" a resonator is: Q = f₀/BW₃dB = 2π × Energy Stored / Energy Lost per Cycle. Higher Q = narrower bandwidth, lower insertion loss, better phase noise in oscillators, and better selectivity in filters. Surface mount inductors: Q ≈ 20–60. On-chip inductors: Q ≈ 5–15. Crystal resonators: Q ≈ 10⁴–10⁶. Cavity resonators: Q ≈ 10⁴–10⁵.
QAM
Quadrature Amplitude Modulation
Modulation
A modulation scheme that encodes information in both amplitude and phase of a carrier, using independent amplitude modulation of an I (cosine) and Q (sine) carrier. 16-QAM = 4 bits/symbol, 64-QAM = 6 bits/symbol, 256-QAM = 8 bits/symbol. Higher-order QAM needs higher SNR (closer constellation points → more susceptible to noise). 5G NR supports up to 1024-QAM in ideal conditions.
R
Radar Range Equation
Radar
Relates the maximum detection range of a monostatic radar to its system parameters: R_max = [P_t·G²·λ²·σ·n / ((4π)³·k·T₀·B·NF·SNR_min·L)]^(1/4). The R⁴ dependence means doubling range requires 16× more transmit power. Key insight: every dB of loss or NF costs 0.25 dB of range (because of the ¼ exponent). σ is the target's radar cross section in m².
Reflection Coefficient
Γ (Gamma)
Matching
The complex ratio of reflected to incident voltage wave at a discontinuity: Γ = (Z_L − Z₀)/(Z_L + Z₀). |Γ| ranges 0 (perfect match) to 1 (total reflection). Related to: VSWR = (1+|Γ|)/(1−|Γ|), Return Loss = −20·log|Γ| dB, Mismatch Loss = −10·log(1−|Γ|²) dB. On the Smith chart, Γ is plotted as a point in the complex plane with the outer boundary |Γ| = 1.
Return Loss
RL
Matching
The ratio of reflected to incident power expressed as a positive dB value: RL (dB) = −20·log|Γ| = −S11 (dB). Higher return loss = better match: 0 dB = total reflection, ∞ dB = perfect match. Rules of thumb: RL > 10 dB is acceptable (VSWR < 1.92), RL > 20 dB is good (VSWR < 1.22). Connectors and adapters typically have RL > 30 dB.
S
Sensitivity
Noise
The minimum input signal power for which a receiver meets its performance specification (e.g. BER < 10⁻³). Calculated as: S = −174 dBm/Hz + NF_dB + 10·log(BW_Hz) + SNR_min_dB. Every 1 dB improvement in NF gives 1 dB better sensitivity. Every halving of bandwidth gives 3 dB better sensitivity. Critical in link budget analysis — receiver sensitivity defines the maximum usable path loss.
SFDR
Spurious-Free Dynamic Range
Noise
The range between the noise floor and the highest spurious signal in a receiver or ADC output. For a receiver: SFDR = ⅔(IIP3 − NF − 10·log(BW) + 174). For an ADC, SFDR is measured in a single-tone test as the ratio of fundamental to highest harmonic or spur, expressed in dBc or dBFS. Better SFDR = cleaner spectrum = less interference between channels.
Skin Effect / Skin Depth
General
At high frequencies, current in a conductor crowds toward the surface, flowing in an effective shell of thickness δ: δ = √(2ρ/ωμ) = √(ρ/πfμ). For copper at 1 GHz: δ ≈ 2.1 μm. This increases the effective resistance (R ∝ √f) and therefore increases conductor loss. Conductors thinner than ~3δ have significantly elevated resistance. Key design rule: copper plating must be >3× skin depth at the operating frequency.
Smith Chart
S-Param
A graphical tool for visualising complex impedances and performing matching network design. The entire complex impedance plane (from 0 to ∞ Ω) is mapped onto a unit circle using the reflection coefficient Γ. The centre = Z₀ (perfect match). Circles of constant resistance run vertically; arcs of constant reactance run horizontally. Moving along a lossless transmission line traces a clockwise circle centred at the chart centre.
S-Parameters
Scattering Parameters
S-Param
A set of parameters that describe the electrical behaviour of a linear N-port network in terms of power waves. For a 2-port: S11 = input reflection (mismatch), S21 = forward transmission (gain/loss), S12 = reverse isolation, S22 = output reflection. All measured with the other ports terminated in Z₀. Used universally at RF/microwave frequencies because voltages and currents are difficult to measure accurately — power ratios are not.
Superheterodyne
System
See Heterodyne. The dominant receiver architecture for the past century. The "super" prefix refers to the use of supersonic (above audio) intermediate frequencies. Modern alternatives: direct conversion (zero-IF) — eliminates IF by mixing RF directly to baseband, avoiding image issues but introducing DC offset and IQ imbalance problems. Low-IF — small non-zero IF (e.g. 1–2 channel spacings) as a compromise.
Spurious Response / Spur
Mixer
Any unintended output signal from a nonlinear component. In mixers, spurs are all the m×f_RF ± n×f_LO products besides the wanted IF. In oscillators and synthesisers, reference spurs appear offset from the carrier at multiples of the reference frequency due to charge pump leakage. In ADCs, spurs appear at harmonics or intermodulation frequencies. Characterised by their level relative to the fundamental (dBc).
T
Taper / Amplitude Weighting
Antenna
In antenna arrays, reducing the amplitude of excitation at the array edges relative to the centre to lower sidelobe levels at the cost of a wider mainbeam. Common windows: Uniform (SLL −13.3 dB), Taylor (equiripple, best SLL/BW trade-off), Chebyshev (equiripple sidelobes), Hamming (SLL −42.7 dB), Blackman (SLL −58 dB). Choosing the right taper is the fundamental array design trade-off.
Thermal Noise
Noise
Noise generated by the random thermal motion of charge carriers in any resistive element. Power spectral density is white (flat with frequency): N = kTB where k = 1.38×10⁻²³ J/K, T = temperature (K), B = bandwidth. At room temperature (290 K), this gives −174 dBm/Hz. Thermal noise is irreducible — it cannot be filtered away because it exists at all frequencies.
Transmission Line
TX Line
A two-conductor structure designed to guide electromagnetic waves from one point to another while maintaining a defined impedance. Types: coaxial, microstrip, stripline, CPW, waveguide, two-wire. Characterised by distributed parameters: R', L', G', C' per unit length. Transmission line effects become significant when the line length exceeds ~λ/10. Below that, lumped-element models are adequate.
V
VSWR
Voltage Standing Wave Ratio
Matching
The ratio of maximum to minimum voltage on a transmission line with standing waves caused by a mismatch: VSWR = (1+|Γ|)/(1−|Γ|). Ranges from 1.0 (perfect match) to ∞ (total reflection). Rules of thumb: VSWR < 1.5 (RL > 14 dB) = acceptable; VSWR < 1.2 (RL > 20 dB) = good; VSWR < 1.1 (RL > 26 dB) = excellent. Power reflected = |Γ|² × incident power.
VCO
Voltage-Controlled Oscillator
PLL
An oscillator whose output frequency is controlled by an input voltage: f_out = f_free + K_VCO × V_tune. K_VCO (MHz/V or rad/s/V) is the tuning sensitivity. Higher K_VCO = wider tuning range but more phase noise (the VCO is more sensitive to control voltage noise). VCOs are implemented as LC oscillators (better phase noise, narrower range) or ring oscillators (worse phase noise, wide range, CMOS friendly).
W
Waveguide
TX Line
A hollow metallic tube that guides EM waves without a centre conductor. Unlike coax, waveguides support only transverse electric (TE) and transverse magnetic (TM) modes — the dominant mode is TE₁₀. Waveguides have a cutoff frequency below which they don't propagate. Advantages: extremely low loss at high frequencies, high power handling. Used at millimetre-wave and above. Standard sizes designated as WR-xx (WR-90 = 8.2–12.4 GHz X-band).
X
XPD
Cross-Polarisation Discrimination
Antenna
The ratio of the wanted co-polarised signal to the unwanted cross-polarised component in the same direction, measured in dB. Higher XPD = better polarisation purity. Typical values: well-designed horn or patch antenna: XPD > 25 dB. Dual-polarised base station antenna: XPD > 30 dB in the mainbeam. Poor XPD causes interference between dual-polarised MIMO streams.
Y
Y-Factor Method
Noise
The standard technique for measuring noise figure. Two noise sources at different temperatures (hot: T_h, cold: T_c) are connected to the DUT and the output noise power is measured: Y = P_hot/P_cold. Then: NF = ENR − 10·log(Y−1) where ENR (Excess Noise Ratio) is the calibrated level of the noise source. Implemented in noise figure analysers and spectrum analysers with noise source accessories.
Z
Zero-IF / Direct Conversion
System
A receiver architecture where the LO is tuned to the RF carrier frequency, mixing the RF signal directly to baseband (DC). Eliminates image frequency problems (the image is the signal itself, mirrored to negative frequencies — rejected by the low-pass baseband filter). Challenges: DC offset (LO self-mixing, saturates baseband circuits), IQ imbalance, and 1/f (flicker) noise from CMOS devices falling in the signal band. Dominant in modern single-chip RF transceivers.
Z₀ (Characteristic Impedance)
TX Line
See Characteristic Impedance. The standard reference impedance for RF/microwave systems is 50 Ω (telecom and general RF) or 75 Ω (cable TV, video). All S-parameters are normalised to this reference impedance. Mismatches between 50 Ω and other impedances (antenna feeds, transistor ports) must be corrected with matching networks.
About This RF Glossary
This RF engineering glossary covers the essential terminology used in RF and microwave engineering, from classic transmission line theory to modern 5G system design. Terms are organised alphabetically and categorised across 12 topic areas: Noise & Dynamic Range, Matching & VSWR, S-Parameters, Transmission Lines, Filters, Amplifiers, System & Link Budget, Modulation, Antennas, PLL & Oscillators, ADC/DAC, and General.
Key reference sources used in compiling these definitions include: Balanis "Antenna Theory" 4th Ed., Pozar "Microwave Engineering" 4th Ed., Razavi "RF Microelectronics" 2nd Ed., Mailloux "Phased Array Antenna Handbook" 2nd Ed., and IEEE standard definitions. Each definition is written for practising engineers — precise enough to be technically accurate, plain enough to build real intuition.
Every term links to the relevant calculator or theory page on RFLab where you can go deeper. Use the search box to find any term instantly, or browse by category to explore a topic area.