Radio Receiver

A Radio Receiver, or colloquially just Radio, is a device used for the detection of radio signals captured by an antenna, and the extraction of information conveyed by them into a usable form, most typically an audio signal.

Types

Radio receivers can be classified based on how they process the radio frequency signal, as well as the type of demodulator used. A receiver may be built with multiple demodulators for various signal types.

Passive Receivers

These are simple AM receivers with no amplification built in, they are often powered only by the radio waves themselves and use some form of primitive diode as a demodulator (detector).

Some variants are as follows:

• Crystal Radio: Uses a semiconductor crystal in a point contact diode. Traditionally the crystal would be a naturally formed mineral of iron sulfide or lead sulfide though other materials were also used, as well as more modern germanium diodes.
• Foxhole Radio: Variant of the crystal radio using a metal oxide as a semiconductor, usually a blued steel razor blade. Named after its notable use by soldiers in WWII.
• Electrolytic Detector Receiver: Used an electrolytic cell that had diode-like behavior.
• Fleming Valve Receiver: Used a primitive themionic diode vacuum tube. It required a battery to power the filament.

Tuned Radio Frequency (TRF) Receivers

These receivers have one or multiple band pass filter stages, all tuned to the receiving frequency, and chained one after the other before feeding into the demodulator (often called detector) stage. Each stage has a built in amplifier. They were most popular in the 1920s being built with early vacuum tubes to receive AM broadcasts.

The following is a list of some single stage TRF receiver designs:

• Grid Leak detector: A simple vacuum tube receiver or detector stage similar to a fleming valve receiver, but using a three-element triode tube capable of amplifying the signal as well as demodulating. Demodulation was achieved using the diode-like characteristic of the grid impedance. It gave improved sensitivity by amplifying faint signals.
• Plate Detector: Also known as anode detector, it worked similarly to the grid leak detector but it relied on the non linear characteristic of the anode current of the triode tube, when operated near cutoff for demodulation.
• Reflex Receiver: Designed in order to make efficient use of amplifying stages back when they were based on expensive vacuum tubes. A tube is used to amplify a radio frequency signal, before being demodulated by an external detector (usually a crystal detector), and the resulting audio signal is fed back into the same amplifier. The two signals, being at very different frequencies, can be amplified by a single tube due to frequency superposition and separated from each other at the output with simple filters.
• Regenerative Receiver: A triode detector receiver incorporating a positive feedback loop, which greatly increases sensitivity and selectivity using only one amplifying element.
• Superregenerative Receiver: A modification of the regenerative receiver providing even greater sensitivity. The gain of the feedback loop is increased beyond the point where the receiver starts self oscillating, with the oscillation being periodically stopped by an external oscillator of a lower frequency but still above audible range. At the begining of each cycle, the amplitude of the oscillation of the receiver grows exponentially, and the rate at which it grows is effected greatly by the input signal, so that at the time of quenching, the final amplitude will have amplified the input signal by an extremely large gain.

The receivers above may be turned into multi-stage TRF receivers by adding any amount of RF amplifiers and filters at their inputs. It's also possible to combine them, such as incorporating a regenerative feedback loop into a reflex receiver.

Superheterodyne Receivers

Designed as a work around the difficulty of making high frequency tunable amplifiers compared to fixed frequency ones. The superheterodyne receiver is built like a TRF receiver tuned to a fixed frequency called the Intermediate Frequency or IF, which is either above or below the desired band to receive. The receiver is made tunable by using a tunable oscillator and RF mixer to frequency-shift the incoming radio signal into the intermediate frequency and through the subsequent filtering stages and detector, which may be AM, FM, or other.

Direct Conversion Receivers

Also known as homodyne receivers, they work similarly to the front end of a superheterodyne receiver, but the oscillator is set to run at the same frequency as the received signal, thus shifing it down directly into the audible range. They only work with amplitude modulated signals.