Engineer Radio Electronic Communication flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

FSPL formula

Free-space path loss in dB equals 32.44 + 20log10(f_MHz) + 20log10(d_km). It rises 6 dB per doubling of frequency or distance and assumes isotropic antennas in the far field.

Half-wave dipole impedance

A resonant half-wave dipole presents about 73 + j42 ohms at its feedpoint, with 2.15 dBi gain. Slight shortening to roughly 0.95 of a half wavelength cancels the reactance.

Fresnel zone clearance

The first Fresnel zone radius at midpoint is 17.32 times the square root of d1·d2 divided by f_GHz·D, with distances in km. Keeping 60 percent clear approximates free-space loss.

MUF and critical frequency

The maximum usable frequency for skywave is approximately the critical frequency foF2 divided by the cosine of the incidence angle. Usable skywave requires f below MUF and above the lowest usable frequency.

VSWR and reflection coefficient

Reflection coefficient gamma equals (ZL minus Z0) divided by (ZL plus Z0). VSWR equals (1 plus gamma magnitude) over (1 minus gamma magnitude); a 2:1 VSWR means gamma of one third.

Quarter-wave transformer

A quarter-wavelength line of characteristic impedance equal to the square root of the product of source and load impedances matches two real impedances. It is narrowband and exactly a quarter wavelength at the design frequency.

Cascaded noise figure

Friis formula: total noise factor equals F1 plus (F2 minus 1)/G1 plus later terms divided by preceding gains. First-stage gain and low first-stage noise figure dominate receiver sensitivity.

Skin depth

Skin depth equals the square root of resistivity divided by pi times frequency times permeability. At 1 MHz in copper it is about 2.1 micrometres, so surface plating dominates RF conductor loss.

Carson's rule

FM bandwidth is approximately twice the sum of peak deviation and highest modulating frequency. Modulation index beta equals deviation divided by modulating frequency; beta below about 0.3 is narrowband FM.

AM total power

For amplitude modulation with index m, total power equals carrier power times (1 plus m squared over 2). Envelope detection requires m not exceeding 1 to avoid overmodulation and distortion.

Shannon capacity

Channel capacity in bits per second equals bandwidth times log base 2 of (1 plus signal-to-noise ratio). The Shannon limit of Eb/N0 is minus 1.59 dB for error-free transmission at vanishing rate.

OFDM subcarrier spacing

Subcarrier spacing equals the reciprocal of the useful symbol duration. A cyclic prefix longer than the channel delay spread removes intersymbol interference at a bandwidth cost of prefix length over total symbol length.

Image frequency

With high-side local oscillator injection, the image lies at the RF frequency plus twice the intermediate frequency. The RF preselector must reject it, since the mixer cannot distinguish image from desired signal.

Reciprocal mixing

Local oscillator phase noise mixes with strong adjacent signals and raises the noise floor in the desired channel. Lower phase-noise oscillators and higher first-IF selectivity reduce the effect.

Receiver sensitivity formula

Minimum detectable signal in dBm equals minus 174 plus noise figure plus 10log10(bandwidth in Hz) plus required signal-to-noise ratio. A 12.5 kHz channel with 6 dB noise figure and 10 dB SNR gives about minus 121 dBm.

PA class efficiency

Class A conducts over the full 360 degrees with maximum efficiency 50 percent. Class B conducts 180 degrees at 78.5 percent, Class AB lies between, and Class C below 180 degrees trades distortion for efficiency.

Nyquist sampling criterion

Sampling rate must be at least twice the highest signal frequency. Frequencies above half the sample rate alias into the baseband, so an anti-alias low-pass filter precedes the analogue-to-digital converter.

ADC quantization SNR

For a full-scale sine wave, signal-to-noise ratio is about 6.02N plus 1.76 dB for an N-bit converter. Effective number of bits equals (SINAD minus 1.76) divided by 6.02.

Matched filter

The matched filter is the time-reversed transmit pulse and maximizes output signal-to-noise ratio at the sampling instant. Peak output SNR equals twice symbol energy divided by noise power spectral density.

Raised-cosine bandwidth

With roll-off factor alpha, occupied bandwidth is (1 plus alpha) times symbol rate divided by 2. Alpha of 0.35 is typical, trading excess bandwidth for reduced intersymbol interference.

Shielding effectiveness

Shielding effectiveness in dB equals 20log10 of incident field over transmitted field. It is the sum of absorption loss, reflection loss, and a multiple-reflection correction, with absorption rising with thickness over skin depth.

Third-order intermodulation

Two strong signals at f1 and f2 create products at 2f1 minus f2 and 2f2 minus f1. Odd-order products land in-band and cannot be filtered, so front-end linearity and intercept point matter.

Far-field boundary

Beyond a distance of about one wavelength divided by 2 pi, the field is a plane wave with E over H equal to 377 ohms. Fields then fall as one over distance rather than faster.

Spectrum analyzer noise floor

Displayed average noise level is about minus 174 dBm per Hz plus noise figure plus 10log10 of resolution bandwidth. Halving resolution bandwidth lowers the floor 3 dB but doubles sweep time.

Occupied bandwidth

Per ITU-R SM.328, occupied bandwidth contains 99 percent of mean radiated power. Necessary bandwidth is the minimum needed for a given emission class and appears in the emission designator.

Antenna factor

For a 50-ohm system in free space, antenna factor in dB per metre equals 20log10 of frequency in MHz minus gain in dBi minus 29.79. Field strength adds antenna factor and 107 to the measured dBm level.

EIRP versus ERP

EIRP equals transmitter power plus antenna gain minus cable loss, referenced to an isotropic radiator. ERP is referenced to a half-wave dipole and is 2.15 dB lower than EIRP.

ITU regions

The ITU Radio Regulations divide the world into three regions: Region 1 covers Europe, Africa and the Middle East; Region 2 the Americas; Region 3 Asia-Pacific. Allocations differ by region and footnote.

Primary versus secondary service

Primary services have priority and protection under the regulations. Secondary stations must not cause harmful interference to primary stations and cannot claim protection from interference caused by them.

Emission designator

An emission designator gives necessary bandwidth, class of emission and nature of modulating signal. For example, 16K0F3E denotes 16 kHz bandwidth, frequency modulation, telephony.

Cellular reuse ratio

Cluster size N equals i squared plus ij plus j squared. The co-channel reuse ratio D over R equals the square root of 3N, so smaller N raises capacity but lowers carrier-to-interference ratio.

Erlang B blocking

Erlang B gives blocking probability for offered traffic A equals arrival rate times mean holding time, with blocked calls cleared. Erlang C applies when blocked calls queue instead of being rejected.