28 August 2011

Basic Receiver Circuit


      The diagram shows the minimum requirements for a single-conversion super heterodyne receiver (sometimes shortened to superhet) design. The following essential elements are common to all superhet circuits: a receiving antenna, a tuned stage which may optionally contain amplification (RF amplifier), a variable frequency local oscillator, a frequency mixer, intermediate frequency (IF) amplifier, and a demodulator plus additional circuitry to amplify or process the original audio signal.

      In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio and television receivers use the superheterodyne principle. A "heterodyne" refers to a beat or "difference" frequency produced when two or more radio frequency carrier waves are fed to a detector.

         To receive a radio signal, a suitable antenna is required. The signal from the antenna is tuned and may be amplified in RF amplifier. 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 mixer uses a non-linear component to produce both sum and difference beat frequencies signals. The output of the mixer may include the original RF signal at fd, the local oscillator signal at fO, and the two new frequencies fd+fO and fd-fO. The undesired signals are removed by the IF band pass filter, leaving only the desired offset IF signal at fIF which contains the original modulation (transmitted information) as the received radio signal had at fd

        Once the signals have passed through the IF stages of the super heterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered. The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker. 

       The diagram above shows a very basic version of the superhet or superheterodyne receiver. Many sets these days are far more complicated. Some superhet radios have more than one frequency conversion, and other areas of additional circuitry to provide the required levels of performance. However the basic superheterodyne concept remains the same, using the idea of mixing the incoming signal with a locally generated oscillation to convert the signals to a new frequency.