EST cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series circuits: R_total = R1 + R2 + ...; current is identical through every element; voltage divides in proportion to resistance.

Parallel circuits: 1/R_total = 1/R1 + 1/R2 + ...; voltage is identical across every branch; current divides inversely with resistance.

Two resistors in parallel: R_total = (R1 × R2)/(R1 + R2); equal resistors in parallel give R/n.

Capacitors in series combine like resistors in parallel; capacitors in parallel add directly. Impedance Xc = 1/(2πfC).

Inductive reactance Xl = 2πfL; inductors in series add, in parallel combine reciprocally.

Kirchhoff's voltage law: sum of voltage drops around any closed loop equals zero; current law: current into a node equals current out.

Silicon PN junction forward drop is about 0.6–0.7 V; germanium about 0.3 V. A diode conducts anode-to-cathode only when forward biased.

Resistor color code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9; gold = ±5%, silver = ±10%.

A voltmeter is connected in parallel with the measured element and must have high input impedance; an ammeter is inserted in series and must have low resistance.

An oscilloscope measures amplitude versus time; a spectrum analyzer measures amplitude versus frequency.

Oscilloscope probe compensation is verified against the calibrator square wave; an under- or over-compensated probe distorts high-frequency measurements.

10× probes divide the signal by ten and raise input impedance, but the scope must be set to match or readings are 10× low.

Time-domain to frequency-domain: a periodic signal's fundamental frequency equals 1 divided by its period.

A time-domain reflectometer (TDR) locates faults by timing reflections; distance = velocity of propagation × time / 2.

Continuity and resistance tests are made with power removed from the circuit; measuring resistance in a live circuit gives false readings and can damage the meter.

True-RMS meters are required for non-sinusoidal AC waveforms; averaging meters read low on distorted or pulsed signals.

A dummy load (50 Ω) substitutes for an antenna during transmitter testing to prevent radiation and protect the final amplifier.

Wavelength in free space: λ(m) = 300 / f(MHz). A half-wave dipole is approximately 0.5λ long; a quarter-wave vertical is approximately 0.25λ.

VSWR of 1:1 is a perfect match; 2:1 reflects about 11% of power; 3:1 reflects about 25%. Higher VSWR raises feedline loss and stresses the transmitter.

Return loss (dB) = −20 log|Γ|; reflection coefficient Γ = (ZL − Z0)/(ZL + Z0).

Characteristic impedance of coax depends on the inner/outer conductor diameters and the dielectric constant; common values are 50 Ω (radio) and 75 Ω (video).

Modulation types: AM varies carrier amplitude, FM varies carrier frequency, PM varies carrier phase; digital schemes include ASK, FSK, PSK, and QAM.

Bandwidth of an FM signal is approximated by Carson's rule: BW ≈ 2(Δf + fm), where Δf is peak deviation and fm is the highest modulating frequency.

A repeater receives on one frequency and retransmits on another (offset); duplex operation requires a duplexer to isolate transmit and receive paths.

Free-space path loss increases with both frequency and distance; doubling distance adds about 6 dB of loss.

Decibels: dB = 10 log(P2/P1) for power and 20 log(V2/V1) for voltage; +3 dB is a doubling of power, +10 dB is a tenfold increase.

Single-mode fiber has a core of roughly 8–10 µm and carries one propagation mode over long distances; multimode core is 50 or 62.5 µm.

Common wavelengths: 850 nm and 1300 nm for multimode, 1310 nm and 1550 nm for single-mode. 1550 nm has the lowest attenuation.

Fiber connectors: SC (square push-pull), LC (small form factor), ST (bayonet). Polish styles are PC, UPC (blue), and APC (green, angled 8°).

Never look into a fiber end or connector; use an optical power meter or viewing card. Minimum bend radius and clean connectors prevent most link failures.

Cat 5e supports 100 MHz, Cat 6 supports 250 MHz, Cat 6A supports 500 MHz; 100 m is the maximum horizontal copper run (90 m permanent link plus 10 m patch).

T568A and T568B pinouts differ in the green and orange pairs; both ends must use the same standard for a straight-through cable.

Cable separation: run communications cable at least 2 inches from power conductors and cross at 90° to minimize induced noise.

Fusion splicing gives the lowest loss; mechanical splices and connectors add loss. Always measure with an OTDR or light source/power meter after installation.

Divide-and-conquer: verify power first, then isolate by half-splitting the signal path rather than testing every component.

Substitution is valid only when the replacement part is known good and electrically equivalent; otherwise it introduces a second variable.

Intermittent faults are often caused by thermal drift, cold solder joints, loose connectors, or flexing cables; use freeze spray and flex testing to reproduce.

Signal tracing follows the signal from source to destination; signal injection works backward from the load.

A short reads near 0 Ω and blows fuses; an open reads infinite resistance and produces no current.

Preventive maintenance includes cleaning, connector inspection, torque checks, battery replacement, and calibration interval verification.

Log symptoms, measurements, and environmental conditions before repair; a before/after baseline confirms whether the fix worked.

ESD damage can be latent; use grounded wrist straps and conductive bags when handling boards and semiconductors.

Current, not voltage, causes injury: as little as 10 mA can cause loss of muscle control; 100 mA through the chest can be fatal.

Lockout/tagout isolates energy sources before service; verify absence of voltage with a tested meter after de-energizing.

Use the one-hand rule when probing live circuits; keep the other hand in a pocket and stand on an insulating surface.

Class C fires involve energized electrical equipment; use CO2 or dry chemical extinguishers, never water.

RF exposure limits are frequency dependent; keep antennas away from occupied areas and never transmit into an unmatched or disconnected load without a dummy load.

Grounding and bonding protect people and equipment; a single-point ground prevents ground loops, and the equipment ground conductor must never be lifted.

Laser safety: fiber systems may use Class 1, 1M, or higher sources; treat all unterminated fiber as live and use appropriate eye protection.

Follow applicable codes and standards such as NEC for wiring, TIA/EIA for cabling, and OSHA for workplace safety; local codes take precedence where stricter.