Receiver purpose and block diagram
A receiver selects a wanted signal, amplifies it, recovers the information and delivers it to a speaker or data system. Common stages include RF selection, amplification, frequency conversion, IF filtering, detection and audio amplification.
TRF receiver
A tuned radio-frequency receiver amplifies the signal at its incoming frequency. It is simple, but tuning several RF stages together can be difficult. Its selectivity may vary across the tuning range.
Superheterodyne receiver
A superheterodyne receiver mixes the incoming signal with a local oscillator. The mixer produces sum and difference frequencies. The receiver selects a fixed intermediate frequency for amplification and filtering.
Mixer and local oscillator
The local oscillator sets the conversion frequency. A mixer is a nonlinear circuit that creates new frequency components. For high-side injection, fLO = fRF + fIF. For low-side injection, fLO = fRF − fIF.
Intermediate frequency
The IF is the fixed frequency used after conversion. Common historical examples include 455 kHz for many AM broadcast receivers and 10.7 MHz for many FM broadcast receivers. Actual equipment may use other values.
Image-frequency interference
An image is an unwanted input frequency that also converts to the selected IF. RF preselection and good mixer design help reject it. With high-side injection, image frequency is fRF + 2fIF.
Adjacent-channel interference
A strong nearby signal can enter the receiver passband or overload its stages. Good IF filters, front-end selectivity and suitable gain control reduce this problem.
Sensitivity
Sensitivity is the ability to receive a weak signal with a defined output quality. It is often specified in microvolts or dBm under stated conditions. Lower minimum detectable signal generally means better sensitivity.
Selectivity
Selectivity is the ability to separate a wanted signal from nearby signals. It depends on filter bandwidth and shape, IF design and receiver architecture.
Fidelity
Fidelity describes how accurately the receiver reproduces the original information. Distortion, poor audio response and overloading reduce fidelity.
AVC and AGC
Automatic volume control or automatic gain control changes receiver gain as signal strength changes. It helps keep output more even and can prevent strong signals from overloading later stages.
Squelch
Squelch mutes audio when the receiver detects no signal above a set threshold. It is common in VHF and UHF FM equipment.
Signal-to-noise ratio
SNR compares wanted signal power with noise power. A higher SNR usually gives clearer reception.
SNR(dB) = 10 log10(Psignal/Pnoise)
CW reception
A CW signal is an unmodulated carrier that is keyed on and off. A beat-frequency oscillator or product detector creates an audible tone. The pitch depends on the difference between the received signal and BFO frequency.