Radio and Electronics Fundamentals: Units, Decibels, AC/DC Circuits, Components, Resonance and Filters
Core quantities and units
Candidates must be fluent with SI prefixes and derived units: the volt, ampere, ohm, watt, farad, henry and hertz, and compound forms such as volts per metre for field strength. Know the relationships P = VI, V = IR, and the impedance forms of reactance: X_L = 2πfL and X_C = 1/(2πfC). Traps include mixing peak and RMS values, forgetting that RMS = peak/√2 for a sine wave, and confusing power in a resistive load with apparent power in a reactive circuit.
Decibels
Decibels express ratios: dB = 10 log(P2/P1) for power and 20 log(V2/V1) for voltage or current into the same impedance. Learn the standard ladder: 3 dB ≈ double power, 6 dB ≈ double voltage, 10 dB = ten times power, 20 dB = ten times voltage. Cascaded stages add in dB. A common trap is applying 10 log to a voltage ratio, or forgetting that a loss is negative.
Components and resonance
Understand resistor tolerances and power ratings, capacitor types and temperature coefficients, inductors with Q and self-resonance, and transformers as impedance matches. Series resonance occurs at f = 1/(2π√(LC)), where reactances cancel and current peaks; parallel resonance gives high impedance. Filter questions test recognising low-pass, high-pass, band-pass and notch responses from component placement, plus the meaning of cutoff frequency and roll-off order.
Worked example
An amplifier delivers 5 W where 50 mW was expected. Ratio = 100, so gain = 10 log(100) = 20 dB. If a second stage adds 10 dB, total is 30 dB, i.e. 50 mW × 1000 = 50 W. Always convert to the same units before taking the log.