Simple and Superheterodyne Receivers
In radio communications, a radio receiver is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves (em waves) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
The information produced by the receiver may be in the form of sound (an audio signal), images (a video signal) or data (a digital Signal). A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Devices that contain radio receivers include television sets, radar equipment, two way radios, cell phones, wireless computer networks, GPS Navigation Devices, satellite dishes, radio telescopes,bluetooth enabled devices, garage door openers, and baby monitors.
In electronics, a superheterodyne receiver (often shortened to superhet) uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original radio carrier frequency.
The principle of operation of the superheterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at least to reject the image frequency and possibly amplified. A local Oscillator in the receiver produces a sine wave, which mixes with that signal, shifting it to a specific Intermediate Frequency (IF), usually a lower frequency. The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original information (such as audio).
The diagram above shows the minimum requirements for a single-conversion superheterodyne receiver design. The following essential elements are common to all superheterodyne circuits: a receiving antenna; a tuned stage, which may optionally contain amplification (RF amplifier); a variable frequency local oscillator; a frequency mixer; a band pass filter and (IF) amplifier; and a demodulator plus additional circuitry to amplify or process the original audio signal (or other transmitted information).
To receive a radio signal, a suitable antenna is required. This is often built into a receiver, especially in the case of AM broadcast band radios. The output of the antenna may be very small, often only a few microvolts. The signal from the antenna is tuned and may be amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more tuned circuits at this stage block frequencies that are far removed from the intended reception frequency. In order to tune the receiver to a particular station, the frequency of the local oscillator is controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage may use a variable capacitors, or varicap diode. The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator. The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequency oscillator known as the local oscillator (LO). The output of the mixer may include the original RF signal at fRF, the local oscillator signal at fLO, and the two new heterodyne frequencies fRF + fLO and fRF − fLO. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF bandpass filter removes all but the desired IF signal at fIF. The IF signal contains the original modulation (transmitted information) that the received radio signal had at fRF. The received signal is now processed by the demodulator stage where the audio signal (or other baseband signal) is recovered and then further amplified. FM signals may be detected using a discriminator, ratio detector, or phase locked loop. CW (Morse Code) and single sideband signals require a product detector using a so-called BFO, and there are other techniques used for different types of modulation. The resulting audio signal (for instance) is then amplified and drives a loudspeaker.