Basic Electronics & Communication interview questions

150 questions with answers (ECE-001 to ECE-150). Try each one aloud before reading the answer.

Signals & Systems

ECE-001 · What is a signal?

A function of one or more independent variables, usually time, that carries information. Examples are a voltage waveform, a speech pressure wave or an image's intensity versus position.

ECE-002 · What is the difference between analog and digital signals?

An analog signal is continuous in both time and amplitude. A digital signal takes discrete values at discrete times, which makes it more robust to noise and easier to store, regenerate and process.

ECE-003 · What is bandwidth?

The range of frequencies a signal occupies or a channel passes, measured in Hz. For example, telephone voice is band-limited to roughly 300–3400 Hz.

ECE-004 · State the Nyquist sampling theorem.

A band-limited signal with highest frequency f_max can be perfectly reconstructed from its samples if the sampling rate fs ≥ 2·f_max. The minimum rate 2·f_max is called the Nyquist rate; sampling slower causes aliasing.

ECE-005 · What is the difference between continuous-time and discrete-time signals?

A continuous-time signal x(t) is defined for every instant of time. A discrete-time signal x[n] is defined only at integer sample indices, typically obtained by sampling x(t) every Ts seconds.

ECE-006 · What is the difference between energy and power signals?

An energy signal has finite total energy E = ∫|x(t)|²dt and zero average power, like a single pulse. A power signal has finite non-zero average power and infinite energy, like a sinusoid or periodic wave.

ECE-007 · What is a periodic signal?

A signal that satisfies x(t) = x(t + T) for all t. The smallest such T is the fundamental period T₀, and f₀ = 1/T₀ is the fundamental frequency.

ECE-008 · What is an LTI system?

A system that is both linear (obeys superposition and scaling) and time-invariant (a time shift in the input produces the same shift in the output). An LTI system is completely described by its impulse response.

ECE-009 · What is the impulse response of a system?

The output h(t) when the input is a unit impulse δ(t). For an LTI system, the output for any input is the convolution of that input with h(t).

ECE-010 · What is convolution?

The operation that gives an LTI system's output: y(t) = ∫x(τ)·h(t − τ)dτ, or y[n] = Σx[k]·h[n − k] in discrete time. It sums weighted, shifted copies of the impulse response.

ECE-011 · What is the Fourier transform used for?

It decomposes a signal into its frequency components, X(f) = ∫x(t)e^(−j2πft)dt. It is used to find a signal's spectrum and bandwidth and to analyse filters and channels in the frequency domain.

ECE-012 · What is a causal system?

A system whose output at any time depends only on present and past inputs, never future ones. For an LTI system this means h(t) = 0 for t < 0. All real-time physical systems are causal.

ECE-013 · What is BIBO stability?

A system is bounded-input bounded-output stable if every bounded input produces a bounded output. An LTI system is BIBO stable if its impulse response is absolutely integrable: ∫|h(t)|dt < ∞.

Analog Communication

ECE-014 · Why is modulation needed?

It shifts a baseband signal to a higher carrier frequency, which allows practical antenna sizes (on the order of λ/4), multiplexing of many signals, longer range and better noise immunity.

ECE-015 · What is the difference between AM and FM?

AM varies the carrier's amplitude with the message; it is simple and bandwidth-efficient but noise-prone. FM varies the carrier's frequency; it needs more bandwidth but has much better noise immunity.

ECE-016 · What is the modulation index in AM?

μ = Am/Ac, the ratio of message amplitude to carrier amplitude. It should be between 0 and 1; μ > 1 is overmodulation, which distorts the envelope so an envelope detector cannot recover the message.

ECE-017 · What is DSB-SC modulation?

Double-sideband suppressed-carrier AM transmits both sidebands but no carrier, so no power is wasted on the carrier. It needs coherent (synchronous) detection at the receiver.

ECE-018 · What is SSB modulation and what is its advantage?

Single-sideband transmits only one sideband (upper or lower). It needs half the bandwidth of AM or DSB-SC and less transmitted power, at the cost of more complex generation and coherent detection.

ECE-019 · What is the bandwidth of a standard AM signal?

B = 2·fm, where fm is the highest message frequency, because AM produces an upper and a lower sideband. For a 5 kHz audio signal the AM bandwidth is 10 kHz.

ECE-020 · What is a carrier signal?

A high-frequency sinusoid whose amplitude, frequency or phase is varied by the message. It carries the information through the channel and is chosen to suit the antenna and propagation path.

ECE-021 · What is an envelope detector?

A simple AM demodulator using a diode, capacitor and resistor. The RC time constant must satisfy 1/fc ≪ RC ≪ 1/fm so the output follows the envelope but not the carrier.

ECE-022 · What is the difference between FM and PM?

In FM the instantaneous frequency deviation is proportional to the message. In PM the phase deviation is proportional to the message. FM of m(t) equals PM of the integral of m(t).

ECE-023 · What is frequency deviation in FM?

The maximum shift Δf of the instantaneous frequency from the carrier, Δf = kf·Am. Broadcast FM uses a maximum deviation of 75 kHz.

ECE-024 · What are pre-emphasis and de-emphasis?

Pre-emphasis boosts high audio frequencies before FM modulation, and de-emphasis attenuates them equally after demodulation. Because FM output noise rises with frequency, this improves overall SNR without changing the message.

ECE-025 · What is VSB modulation?

Vestigial sideband transmits one full sideband plus a small part (vestige) of the other. It saves bandwidth compared with DSB while easing filter design compared with SSB, and was used for analog TV video.

ECE-026 · What is signal-to-noise ratio (SNR)?

The ratio of signal power to noise power, SNR = Ps/Pn. In decibels it is 10·log₁₀(Ps/Pn). Higher SNR means better quality and lower error rate.

Digital Communication

ECE-027 · What is digital modulation?

Mapping digital bits onto a carrier by changing its amplitude, frequency or phase in discrete steps. The basic types are ASK, FSK and PSK; QAM combines amplitude and phase.

ECE-028 · What is ASK?

Amplitude-shift keying represents bits by different carrier amplitudes. On-off keying (OOK) is the simplest form. It is simple but sensitive to noise and fading.

ECE-029 · What is FSK?

Frequency-shift keying represents bits by different carrier frequencies, for example f₁ for 1 and f₂ for 0. It is robust to amplitude noise and can be detected non-coherently.

ECE-030 · What is PSK?

Phase-shift keying represents bits by different carrier phases. BPSK uses two phases (0° and 180°) and QPSK uses four. It has better BER performance than ASK or FSK for the same Eb/N0.

ECE-031 · What is the difference between bit rate and baud rate?

Bit rate is bits per second. Baud (symbol) rate is symbols per second. Rb = Rs·log₂M for M-ary signalling, so 16-QAM at 1 Mbaud carries 4 Mbps.

ECE-032 · What is PCM?

Pulse code modulation converts an analog signal into digital by sampling, quantizing each sample to one of L levels, and encoding each level as an n-bit code, where L = 2ⁿ.

ECE-033 · What is quantization error?

The difference between the actual sample value and its quantized level. For a uniform quantizer with step Δ it lies within ±Δ/2 and has mean-square value Δ²/12.

ECE-034 · What is line coding?

Converting a bit stream into a physical waveform for baseband transmission. Common codes are unipolar NRZ, polar NRZ, RZ, AMI and Manchester, chosen for DC content, bandwidth and clock recovery.

ECE-035 · What is Manchester coding?

Each bit has a transition in the middle of the bit period (for example high-to-low for 1 and low-to-high for 0, depending on convention). It has no DC component and is self-clocking, but needs twice the bandwidth of NRZ.

ECE-036 · What is intersymbol interference (ISI)?

Distortion where energy from one symbol spreads into neighbouring symbol periods because of channel band-limiting or multipath. It causes detection errors and is reduced by pulse shaping and equalization.

ECE-037 · What is an eye diagram?

An oscilloscope display of many overlaid symbol periods of a received signal. A wide-open eye means low noise and ISI; the eye's height shows noise margin and its width shows timing margin.

ECE-038 · What is bit error rate (BER)?

The ratio of bits received in error to total bits transmitted. It depends on Eb/N0, the modulation and the coding; typical targets range from 10⁻³ for voice to 10⁻⁹ or better for data links.

ECE-039 · What is a constellation diagram?

A plot of a modulation's symbols as points in the I–Q (in-phase/quadrature) plane. The distance between points determines noise immunity, and the number of points sets the bits per symbol.

Information Theory & Coding

ECE-040 · What is the information content of a message?

Self-information I = log₂(1/p) bits, where p is the probability of the message. Less probable messages carry more information; a certain event carries none.

ECE-041 · What is entropy?

The average information per symbol of a source, H = −Σ pᵢ·log₂pᵢ bits/symbol. It measures uncertainty and sets the minimum average number of bits per symbol for lossless coding.

ECE-042 · What is channel capacity?

The maximum rate at which information can be sent over a channel with arbitrarily small error probability. It is measured in bits per second (or bits per channel use).

ECE-043 · What is the difference between source coding and channel coding?

Source coding removes redundancy to compress data (for example Huffman or JPEG). Channel coding adds controlled redundancy to detect and correct errors introduced by the channel (for example Hamming or LDPC codes).

ECE-044 · What is Huffman coding?

A variable-length prefix code that gives shorter codewords to more probable symbols. It produces the optimal prefix code for a known symbol distribution, with average length within 1 bit of the entropy.

ECE-045 · What is a parity bit?

An extra bit added so that the total number of 1s is even (even parity) or odd (odd parity). It detects any odd number of bit errors but cannot correct them.

ECE-046 · What is Hamming distance?

The number of bit positions in which two codewords differ. The minimum distance dmin of a code determines how many errors it can detect and correct.

ECE-047 · What is code rate?

r = k/n, the ratio of information bits k to total coded bits n. A rate-1/2 code doubles the bit count; lower rates give more protection but use more bandwidth.

ECE-048 · What is the difference between error detection and error correction?

Error detection only identifies that errors occurred, typically triggering a retransmission (ARQ). Error correction locates and fixes errors at the receiver without retransmission (FEC), at the cost of more redundancy.

ECE-049 · What is CRC?

Cyclic redundancy check appends the remainder of dividing the message polynomial by a generator polynomial. The receiver repeats the division; a non-zero remainder signals an error. It detects burst errors very well.

ECE-050 · What is a binary symmetric channel (BSC)?

A channel model with binary input and output where each bit is flipped with the same crossover probability p, regardless of whether it is 0 or 1.

ECE-051 · What is a prefix (instantaneous) code?

A code in which no codeword is the prefix of another, so each codeword can be decoded as soon as its last bit arrives. Huffman codes are prefix codes.

EM Theory & Transmission Lines

ECE-052 · State Maxwell's equations in words.

Electric flux out of a closed surface equals enclosed charge (Gauss). Magnetic flux out of a closed surface is zero. A changing magnetic field induces an electric field (Faraday). Current and changing electric field produce a magnetic field (Ampère–Maxwell).

ECE-053 · What is an electromagnetic wave?

A self-propagating disturbance of electric and magnetic fields. In a plane wave, E and H are perpendicular to each other and to the direction of propagation, and travel at the speed of light in the medium.

ECE-054 · What is the speed of an EM wave in free space?

c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s. In a medium with relative permittivity εr and permeability μr it is c/√(εr·μr).

ECE-055 · What is the intrinsic impedance of free space?

η₀ = √(μ₀/ε₀) = 120π ≈ 377 Ω. It is the ratio of E to H magnitudes in a plane wave in free space.

ECE-056 · What is the Poynting vector?

S = E × H, the instantaneous power flow per unit area in W/m². Its direction is the direction of energy flow, and its time average gives the wave's intensity.

ECE-057 · What is skin depth?

The depth at which current density in a conductor falls to 1/e (about 37%) of its surface value: δ = 1/√(πfμσ). It shrinks with frequency, so copper's skin depth is about 9 mm at 50 Hz but about 2 µm at 1 GHz.

ECE-058 · What is a transmission line?

A structure of two or more conductors (or a waveguide) that guides EM energy from source to load. It must be treated as distributed when its length is a significant fraction of the wavelength.

ECE-059 · What is characteristic impedance?

The ratio of voltage to current of a wave travelling in one direction on a line: Z₀ = √((R + jωL)/(G + jωC)). For a lossless line Z₀ = √(L/C). Common values are 50 Ω and 75 Ω.

ECE-060 · What is the reflection coefficient?

Γ = (ZL − Z₀)/(ZL + Z₀), the ratio of reflected to incident voltage at the load. Γ = 0 for a matched load, −1 for a short and +1 for an open.

ECE-061 · What is wave polarization?

The orientation and path traced by the electric field vector over time at a fixed point. It can be linear (vertical or horizontal), circular (left- or right-hand) or elliptical.

ECE-062 · What are standing waves?

The pattern formed when incident and reflected waves combine on a mismatched line. Voltage maxima and minima appear at fixed positions spaced λ/2 apart, with adjacent maximum and minimum λ/4 apart.

ECE-063 · What are common types of transmission lines?

Coaxial cable, twisted pair, two-wire (parallel) line, microstrip, stripline, coplanar waveguide and hollow metallic waveguides. Choice depends on frequency, loss, power and integration needs.

ECE-064 · What is a quarter-wave transformer?

A λ/4 line section of impedance Z₁ that transforms a load ZL to Zin = Z₁²/ZL. To match a real load to a line Z₀, choose Z₁ = √(Z₀·ZL). It is narrowband.

Antennas & Wave Propagation

ECE-065 · What is an antenna?

A transducer that converts guided electrical energy on a line into radiated EM waves in free space, and vice versa when receiving.

ECE-066 · What is a radiation pattern?

A plot of an antenna's radiated field or power versus direction, usually shown in the E-plane and H-plane. It shows the main lobe, side lobes, nulls and back lobe.

ECE-067 · What is the difference between directivity and gain?

Directivity is the ratio of maximum radiation intensity to that of an isotropic source, ignoring losses. Gain = efficiency × directivity, so it includes ohmic and dielectric losses.

ECE-068 · What is a half-wave dipole?

A centre-fed wire antenna of total length about λ/2. It has radiation resistance about 73 Ω, directivity about 1.64 (2.15 dBi) and a doughnut-shaped omnidirectional pattern around its axis.

ECE-069 · What is half-power beamwidth (HPBW)?

The angular width of the main lobe between the two points where radiated power falls to half (−3 dB) of its maximum. Narrower beamwidth means higher directivity.

ECE-070 · What is an isotropic radiator?

A hypothetical lossless antenna that radiates equally in all directions. It is not physically realizable but serves as the 0 dBi reference for antenna gain.

ECE-071 · What is EIRP?

Effective isotropic radiated power, EIRP = Pt·Gt (in dB: Pt(dBm) + Gt(dBi) − feeder loss). It is the power an isotropic antenna would need to produce the same peak power density.

ECE-072 · What are ground wave, sky wave and space wave propagation?

Ground wave follows the earth's surface (LF/MF). Sky wave is refracted back by the ionosphere (HF, 3–30 MHz). Space wave travels by line of sight, directly or ground-reflected (VHF and above).

ECE-073 · What is the role of the ionosphere in radio propagation?

Its ionized layers (D, E, F1, F2) refract HF waves back to earth, enabling long-distance sky-wave links. The D layer absorbs lower frequencies by day; frequencies above the maximum usable frequency pass through.

ECE-074 · What is the line-of-sight radio horizon distance?

With standard 4/3-earth refraction, d (km) ≈ 4.12·(√ht + √hr) with antenna heights in metres. Two 100 m towers can link about 82 km.

ECE-075 · What does the reciprocity theorem say about antennas?

An antenna's radiation pattern, gain and impedance are the same whether it transmits or receives, provided the medium is linear and isotropic. So patterns can be measured in either mode.

ECE-076 · What is a quarter-wave monopole antenna?

A λ/4 vertical conductor above a ground plane. The ground plane provides an image, so it behaves like half a dipole, with radiation resistance about 36.5 Ω and omnidirectional azimuth coverage.

Microwave Engineering

ECE-077 · What is the microwave frequency range?

Roughly 300 MHz to 300 GHz, corresponding to free-space wavelengths of 1 m down to 1 mm. The upper part (30–300 GHz) is called millimetre-wave.

ECE-078 · What is a waveguide?

A hollow metallic tube (rectangular or circular) that guides microwaves by reflection from its walls. It has low loss and high power handling but only propagates above a cutoff frequency.

ECE-079 · What are TE, TM and TEM modes?

TE (transverse electric) modes have no E field along the propagation direction. TM modes have no H field along it. TEM modes have neither and exist only on lines with two or more conductors, such as coax.

ECE-080 · What is the cutoff frequency of a waveguide?

The lowest frequency at which a given mode can propagate; below it the mode is evanescent. For a rectangular guide, TEmn cutoff is fc = (c/2)·√((m/a)² + (n/b)²).

ECE-081 · What is the dominant mode in a rectangular waveguide?

TE₁₀, which has the lowest cutoff frequency, fc = c/(2a), where a is the broader dimension. Guides are operated where only TE₁₀ propagates to avoid multimode distortion.

ECE-082 · What are S-parameters?

Scattering parameters relate incident and reflected wave amplitudes at the ports of a network. For a two-port, S₁₁ is input reflection, S₂₁ is forward transmission (gain), S₁₂ is reverse transmission and S₂₂ is output reflection.

ECE-083 · What is a directional coupler?

A four-port passive device that samples a fixed fraction of power travelling in one direction on a line while largely ignoring power in the other direction. It is used for power monitoring and reflectometry.

ECE-084 · What is a circulator?

A non-reciprocal three-port (or more) ferrite device in which power entering port 1 exits port 2, port 2 to port 3, and port 3 to port 1. It lets a transmitter and receiver share one antenna.

ECE-085 · What is an isolator?

A two-port non-reciprocal device that passes power in the forward direction with low loss and strongly absorbs power in the reverse direction. It protects sources such as oscillators and PAs from reflections.

ECE-086 · What is a magnetron?

A crossed-field vacuum-tube oscillator in which electrons spiral in perpendicular electric and magnetic fields past resonant cavities. It produces high microwave power efficiently and is used in radar and microwave ovens (2.45 GHz).

ECE-087 · What is a klystron?

A linear-beam vacuum tube that uses velocity modulation and bunching of an electron beam between cavities. Multi-cavity klystrons are high-power amplifiers; reflex klystrons are low-power oscillators.

ECE-088 · What is a cavity resonator?

A closed metallic enclosure that stores EM energy at discrete resonant frequencies set by its dimensions. It has a very high Q and is used in filters, oscillators and wavemeters.

Digital Signal Processing

ECE-089 · What is digital signal processing?

Processing of signals in discrete-time, discrete-amplitude form using algorithms on processors or hardware. Typical tasks are filtering, spectral analysis, compression, modulation and equalization.

ECE-090 · What is the difference between the DTFT and the DFT?

The DTFT gives a continuous, 2π-periodic spectrum of an infinite-length sequence. The DFT samples that spectrum at N equally spaced frequencies for an N-point sequence, so it is computable.

ECE-091 · What is the FFT?

A fast algorithm for computing the DFT. A radix-2 FFT needs about (N/2)·log₂N complex multiplications instead of N² for direct computation.

ECE-092 · What is the Z-transform?

X(z) = Σx[n]·z⁻ⁿ, the discrete-time counterpart of the Laplace transform. It turns difference equations into algebraic equations and reduces to the DTFT on the unit circle, z = e^(jω).

ECE-093 · What is the region of convergence (ROC)?

The set of z values for which the Z-transform sum converges. The ROC determines whether a sequence is causal or anti-causal and whether the system is stable (stable if the ROC includes the unit circle).

ECE-094 · What is aliasing?

When a signal is sampled below twice its highest frequency, higher frequencies fold back and appear as lower frequencies in the sampled data. Once it occurs it cannot be removed.

ECE-095 · What is an anti-aliasing filter?

An analog low-pass filter placed before the ADC to remove frequency content above fs/2. It ensures the sampled signal satisfies the Nyquist criterion.

ECE-096 · What is a digital filter?

An algorithm that modifies a sampled signal's spectrum. FIR filters use only present and past inputs; IIR filters also use past outputs (feedback).

ECE-097 · What are decimation and interpolation?

Decimation reduces the sample rate by an integer factor M after low-pass filtering. Interpolation increases it by L by inserting zeros and then low-pass filtering.

ECE-098 · What is the signal-to-quantization-noise ratio of an ideal ADC?

For a full-scale sinusoid, SQNR ≈ 6.02·N + 1.76 dB, where N is the number of bits. Each extra bit adds about 6 dB, so a 12-bit ADC gives about 74 dB.

ECE-099 · What is a window function in DSP?

A weighting sequence (rectangular, Hann, Hamming, Blackman, Kaiser) multiplied with a finite data block. It reduces spectral leakage and side lobes at the cost of a wider main lobe.

ECE-100 · What is the difference between linear and circular convolution?

Linear convolution of lengths L and M gives L + M − 1 samples. Circular convolution of N-point sequences wraps around modulo N. They match if both are zero-padded to N ≥ L + M − 1.

ECE-101 · What is a MAC unit in a DSP processor?

A multiply-accumulate unit that computes a ← a + b·c in a single cycle. Filtering, FFTs and correlation are dominated by MAC operations, so DSPs are optimized around it.

Wireless & Cellular

ECE-102 · What is a cell in a cellular system?

The geographic area served by one base station, idealized as a hexagon. Splitting coverage into cells allows the same frequencies to be reused in non-adjacent cells.

ECE-103 · What is frequency reuse?

Using the same set of channels in cells separated far enough that co-channel interference is acceptable. It multiplies total system capacity without new spectrum.

ECE-104 · What is handover (handoff)?

Transferring an ongoing call or data session from one cell or channel to another as the user moves or signal quality changes, without dropping the connection.

ECE-105 · What are the main features of 1G to 5G?

1G: analog voice (FDMA). 2G: digital voice and SMS (GSM, TDMA). 3G: mobile data (WCDMA). 4G: all-IP broadband (LTE, OFDMA). 5G: NR with higher rates, low latency, massive IoT and mmWave.

ECE-106 · What is GSM?

A 2G digital cellular standard using FDMA plus TDMA, with 200 kHz carriers, each divided into 8 time slots, and GMSK modulation. It introduced SIM cards and SMS.

ECE-107 · What is a SIM card?

Subscriber identity module, a smart card holding the subscriber identity (IMSI) and secret key used for authentication and encryption. It lets the subscription move between handsets.

ECE-108 · What is multipath fading?

Signal strength fluctuation caused by multiple copies of a signal arriving via different paths (reflection, diffraction, scattering) and interfering constructively or destructively.

ECE-109 · What is MIMO?

Multiple-input multiple-output uses several antennas at both transmitter and receiver. It provides spatial multiplexing (more data streams), diversity (reliability) and beamforming without extra bandwidth.

ECE-110 · What is the difference between uplink and downlink?

Uplink is transmission from the mobile device to the base station. Downlink is from the base station to the mobile device.

ECE-111 · What are BTS, eNodeB and gNodeB?

They are the base stations of different generations: BTS in 2G GSM, eNodeB in 4G LTE and gNodeB in 5G NR. They handle the radio link with mobiles.

ECE-112 · What is the difference between FDD and TDD?

FDD uses separate frequency bands for uplink and downlink simultaneously. TDD uses the same band and alternates in time, which allows flexible UL/DL ratios and channel reciprocity.

ECE-113 · What is roaming?

Using a mobile service in a network other than the subscriber's home network, enabled by agreements between operators. The visited network authenticates the user with the home network.

ECE-114 · What is Doppler shift in mobile communication?

A frequency shift caused by relative motion: fd = (v/λ)·cos θ, where θ is the angle between motion and arrival direction. At 900 MHz and 100 km/h, the maximum shift is about 83 Hz.

Optical Communication

ECE-115 · What is an optical fiber?

A thin strand of glass or plastic with a high-index core surrounded by lower-index cladding. It guides light over long distances with low loss and very high bandwidth.

ECE-116 · What principle guides light in an optical fiber?

Total internal reflection. Light in the core hitting the core–cladding boundary at angles greater than the critical angle is fully reflected, so it stays in the core.

ECE-117 · What is the numerical aperture of a fiber?

NA = √(n₁² − n₂²) = sin θa, where n₁ and n₂ are core and cladding indices and θa is the maximum acceptance angle in air. It measures light-gathering ability.

ECE-118 · What is the difference between single-mode and multimode fiber?

Single-mode fiber has a small core (about 8–10 µm) and carries one mode, giving very high bandwidth over long distances. Multimode fiber has a larger core (50 or 62.5 µm), is easier to couple, but suffers modal dispersion.

ECE-119 · What are the optical transmission windows?

Low-loss wavelength regions around 850 nm, 1310 nm and 1550 nm. 1310 nm has near-zero dispersion in standard fiber, and 1550 nm has the lowest loss.

ECE-120 · What is attenuation in optical fiber?

Loss of optical power along the fiber, expressed in dB/km, caused by absorption, scattering and bending. Standard single-mode fiber has about 0.2 dB/km at 1550 nm.

ECE-121 · What is dispersion in optical fiber?

Spreading of a light pulse in time as it travels, which causes intersymbol interference and limits bit rate × distance. Main types are modal, chromatic and polarization-mode dispersion.

ECE-122 · What optical sources are used in fiber communication?

LEDs, which are cheap and broad-spectrum for short multimode links, and laser diodes (such as DFB lasers), which give narrow linewidth, high power and high speed for long-haul links.

ECE-123 · What photodetectors are used in optical receivers?

PIN photodiodes, which are simple and low-noise, and avalanche photodiodes (APDs), which provide internal gain for higher sensitivity.

ECE-124 · What is WDM?

Wavelength-division multiplexing sends several optical channels at different wavelengths over one fiber, multiplying its capacity. It is the optical equivalent of FDM.

ECE-125 · What is the critical angle?

θc = sin⁻¹(n₂/n₁), the angle of incidence (from the normal) above which total internal reflection occurs at the boundary from a denser to a rarer medium.

ECE-126 · What is the difference between step-index and graded-index fiber?

Step-index fiber has a uniform core index with an abrupt change at the cladding. Graded-index fiber's core index falls gradually from the centre, which equalizes mode delays and greatly reduces modal dispersion.

Data Communication & Networking

ECE-127 · What are the seven layers of the OSI model?

Physical, Data Link, Network, Transport, Session, Presentation and Application. Each layer provides services to the layer above and uses services of the layer below.

ECE-128 · What is the TCP/IP model?

A practical four-layer model used by the Internet: Link (network access), Internet (IP), Transport (TCP/UDP) and Application. Some texts use five layers by splitting Link into physical and data link.

ECE-129 · What is the difference between TCP and UDP?

TCP is connection-oriented and reliable, with acknowledgements, ordering, flow and congestion control. UDP is connectionless with no delivery guarantees but lower overhead and latency, suiting voice, video and DNS.

ECE-130 · What is a protocol?

An agreed set of rules for message format, timing, sequencing and error handling that lets devices communicate. Examples are IP, TCP, HTTP and Ethernet.

ECE-131 · What are simplex, half-duplex and full-duplex communication?

Simplex is one direction only (broadcast radio). Half-duplex is both directions but one at a time (walkie-talkie). Full-duplex is both directions simultaneously (telephone).

ECE-132 · What are common network topologies?

Bus, star, ring, mesh, tree and hybrid. Star is common in LANs because a single link failure affects only one node, while mesh gives the most redundancy.

ECE-133 · What is the difference between a hub, a switch and a router?

A hub repeats bits to all ports (layer 1). A switch forwards frames to the correct port using MAC addresses (layer 2). A router forwards packets between networks using IP addresses (layer 3).

ECE-134 · What is the difference between a MAC address and an IP address?

A MAC address is a 48-bit hardware address used within a local link. An IP address is a logical address (32-bit IPv4 or 128-bit IPv6) used to route packets across networks.

ECE-135 · What is the difference between circuit switching and packet switching?

Circuit switching reserves a dedicated path for the whole call (traditional telephony). Packet switching breaks data into packets that share links and are routed independently, using capacity more efficiently.

ECE-136 · What are LAN, MAN and WAN?

A LAN covers a building or campus, a MAN covers a city, and a WAN spans countries or continents. They differ in scale, speed, ownership and technology.

ECE-137 · What is Ethernet?

The dominant wired LAN technology (IEEE 802.3), defining physical media and frame format. Classic shared Ethernet used CSMA/CD; modern switched full-duplex Ethernet has no collisions.

ECE-138 · What is a modem?

A modulator-demodulator that converts digital data to an analog signal suited to a channel such as a phone line, cable or radio, and back again at the receiver.

RF Circuits

ECE-139 · What does RF mean in electronics?

Radio frequency, the part of the spectrum used for radio transmission, broadly about 3 kHz to 300 GHz. RF circuits must account for wavelength effects, parasitics, impedance matching and noise.

ECE-140 · What is a low-noise amplifier (LNA)?

The first amplifier in a receiver, designed for minimum noise figure with enough gain to make later stages' noise negligible. It must also be well matched and linear enough for strong signals.

ECE-141 · What is a mixer?

A nonlinear or switching circuit that multiplies the RF signal with a local oscillator signal to produce sum and difference frequencies. It is used for up-conversion and down-conversion.

ECE-142 · What is a local oscillator?

An oscillator in a transmitter or receiver that supplies the reference frequency to the mixer. Its frequency sets which RF channel is converted to the IF or baseband.

ECE-143 · What is noise figure?

NF = 10·log₁₀(F), where the noise factor F = SNRin/SNRout. It measures how much a device degrades SNR; an ideal noiseless device has NF = 0 dB.

ECE-144 · Why is 50 Ω a common RF impedance?

It is a compromise for air-dielectric coax: about 30 Ω gives maximum power handling and about 77 Ω gives minimum loss. 50 Ω became the standard for test equipment and most RF systems; 75 Ω is used in video and cable TV.

ECE-145 · What is an RF power amplifier?

The final stage of a transmitter that raises the signal to the power needed at the antenna. Key specifications are output power, efficiency, gain and linearity.

ECE-146 · What is a balun?

A device that converts between balanced (differential) and unbalanced (single-ended) signals, often with impedance transformation. It is used to feed dipoles from coax and in differential RF circuits.

ECE-147 · What is a VCO?

A voltage-controlled oscillator whose output frequency varies with an input control voltage, usually via a varactor in an LC tank. It is the tunable source inside PLL synthesizers.

ECE-148 · What is a PLL used for in RF systems?

A phase-locked loop locks a VCO to a stable reference, multiplied by a divider ratio, to synthesize precise, tunable LO frequencies. It is also used for carrier and clock recovery.

ECE-149 · What is the 1 dB compression point?

The input (or output) power at which an amplifier's gain has dropped 1 dB below its small-signal value. It marks the upper end of the linear range.

ECE-150 · What is a duplexer?

A three-port filter network that lets a transmitter and receiver share one antenna at different frequencies, isolating the receiver from the strong transmit signal. It is essential in FDD radios.

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