A TDR instrument generates a fast-edge step function (typically 20-200 ps rise time) and sends it down the transmission line under test. At every impedance discontinuity, a portion of the signal reflects back. The TDR samples the returning waveform and displays impedance vs. distance. The reflection coefficient at each point: rho = (Z_L - Z_0)/(Z_L + Z_0). From the TDR display: Z(t) = Z_0 * (1 + rho(t))/(1 - rho(t)). Distance to discontinuity: d = v_prop * t/2, where v_prop = c/sqrt(er_eff). Resolution (minimum feature size that can be resolved): delta_d = t_rise * v_prop / 2. A 35 ps system in FR-4 (v_prop = 0.5c): resolves 2.6 mm. Modern high-end TDRs achieve 10-15 ps rise time, resolving sub-millimeter features for IC package characterization.

What can TDR diagnose?

(1) PCB trace impedance: verify controlled-impedance traces meet 50 +/- 5 ohm spec. See exactly where impedance deviates (connector pad, via transition, routing change). (2) Cable fault location: find opens, shorts, water ingress, and crimp problems in installed cables. Distance accuracy: better than 0.1% for cables with known velocity factor. (3) Connector characterization: see the impedance profile through each connector section (pin, dielectric, transition). Used for connector design optimization. (4) IC package modeling: characterize bondwire inductance, lead frame impedance, via transitions in BGA packages. Requires < 20 ps rise time. (5) Differential pair characterization: differential TDR (two simultaneous pulses) measures differential and common-mode impedance simultaneously. Essential for high-speed digital (USB, HDMI, PCIe).

TDR vs VNA for impedance measurement?

TDR gives impedance vs. distance (spatial profile). Intuitive for locating problems. Limited bandwidth (set by rise time). Good for: cable testing, PCB impedance verification, fault location. VNA gives S-parameters vs. frequency (reflection and transmission). Higher dynamic range and accuracy. Better for: component characterization, filter tuning, amplifier matching. Mathematically equivalent: TDR and VNA data can be converted between each other via Fourier transform. A VNA with time-domain option can compute the TDR response from S11 frequency data. Modern instruments often combine both: Keysight PNA with TDR option, Tektronix DSA8300 with S-parameter extraction. For production testing: TDR is faster (single pulse, real-time display). For design characterization: VNA is more accurate (error correction, calibration).

Test & Measurement

Time Domain Reflectometry (TDR)

Q: TDR vs VNA for impedance measurement?

TDR gives impedance vs. distance (spatial profile). Intuitive for locating problems. Limited bandwidth (set by rise time). Good for: cable testing, PCB impedance verification, fault location. VNA gives S-parameters vs. frequency (reflection and transmission). Higher dynamic range and accuracy. Better for: component characterization, filter tuning, amplifier matching. Mathematically equivalent: TDR and VNA data can be converted between each other via Fourier transform. A VNA with time-domain option can compute the TDR response from S11 frequency data. Modern instruments often combine both: Keysight PNA with TDR option, Tektronix DSA8300 with S-parameter extraction. For production testing: TDR is faster (single pulse, real-time display). For design characterization: VNA is more accurate (error correction, calibration).

/tee-dee-ar/

Fast-edge pulse reflects from impedance discontinuities. ρ = (Z_L-Z₀)/(Z_L+Z₀). Distance: d = v_prop×t/2. Resolution = t_rise×v_prop/2. 35 ps rise in FR-4: resolves 2.6 mm. Opens: +ρ. Shorts: -ρ. Used for PCB impedance verification, cable fault location, connector/package characterization.

Rise time: 20-200 ps

Resolution: sub-mm to cm

Accuracy: <0.1% distance

Understanding TDR

TDR is the "radar for cables." It sends a pulse down a transmission line and listens for echoes. Every impedance change creates a reflection proportional to the mismatch. By plotting the reflected signal vs. time (converted to distance), TDR reveals the exact location and nature of every discontinuity: connector transitions, via stubs, trace width changes, and faults. It is indispensable for signal integrity engineering.

TDR Equations

Distance to fault:
d = v_p·t/2
v_p = c/√ε_eff

Impedance from TDR:
Z = Z₀(1+ρ)/(1−ρ)
ρ = reflected/incident amplitude

Rise time ↔ bandwidth:
BW ≈ 0.35/t_r

TDR Application Comparison

Application	Rise Time	Resolution	Target	Key Measurement
Cable fault	200 ps	~2 cm	Installed cables	Fault distance
PCB impedance	35-50 ps	~3 mm	Controlled-Z traces	50±5Ω verify
Connector	20-35 ps	~1.5 mm	SMA, N-type	Impedance profile
IC package	10-15 ps	<1 mm	BGA, bondwire	Parasitics
Differential	35 ps	~3 mm	USB, PCIe pairs	Z_diff, Z_cm

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Rise time	BW	Resolution	Application	Instrument
35 ps	10 GHz	1.8 mm	MMIC/IC	Sampling scope
150 ps	2.3 GHz	7.5 mm	PCB trace	TDR/VNA
500 ps	700 MHz	25 mm	Cable test	Handheld TDR
5 ns	70 MHz	250 mm	Coarse cable	Field tester
50 ns	7 MHz	2.5 m	Long cable	OTDR/TDR

Common Questions

Frequently Asked Questions

How it works?

Fast-edge step down the line. Each impedance change reflects. rho = (ZL-Z0)/(ZL+Z0). Display: Z vs distance. Resolution = rise_time * v_prop / 2. 35 ps in FR-4 = 2.6 mm. Open = +rho, short = -rho.

What it diagnoses?

PCB trace impedance (50±5Ω). Cable faults (opens, shorts, water). Connector profiles. IC package parasitics. Differential pairs (Z_diff, Z_cm). Sub-mm resolution with 10-15 ps rise time.

TDR vs VNA?

TDR: impedance vs distance, intuitive, fast, production. VNA: S-params vs frequency, more accurate, design. Mathematically equivalent (Fourier transform). Modern instruments combine both. TDR faster for fault finding; VNA better for design characterization.

Related Terms

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