Superheterodyne Receiver

                     In electronics, a superheterodyne receiver (often shortened to superhet) uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency (IF), which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio receivers use the superheterodyne principle.

Block diagram:

               The diagram contains a RF amplifier, a variable frequency local oscillator(LO), a frequency mixer, a band pass filter and intermediate frequency (IF) amplifier, and a demodulator plus additional circuitry to amplify or process the original audio signal (or other transmitted information).For AM The Intermediate frequency(IF) is 455Hz.

principle of operation:

         The principle of operation of the superheterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antennal i.e RF Signal (fs) is filtered sufficiently at least to reject the image frequency (see below) and possibly amplified by RF Amplifier. A local oscillator(LO) in the receiver produces a sine wave( i.e fl ) which mixes with that signal, shifting it to a specific intermediate frequency (IF= fl – fs OR  f_IF = f_LO - f_RF), usually a lower frequency(i.e Mixer performs Down Conversion Here).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).An AF amplifier used to amplify audio signal.

Image Frequency & its suppression:

                   One major disadvantage to the superheterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodyne receiver.

                                  

                                         f_img(Image Frequency) ₌ fs +2* f_IF 

               For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a signal at 580 + 455 + 455 = 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will also appear at the intermediate frequency. This image frequency is within the AM broadcast band.

          Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals; additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images.

Image Rejection Ratio:

                     To determine the suppression factor of tuned ckt image rejection ratio is used. The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. The image rejection ratio is usually expressed in db.

            Mathematically,

where Q=Quality factor, f_img=Image frequency, fs=RF Signal frequency

                                                                  

Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency. Rather, it is the measure of the bandpass characteristics of the stages preceding the IF amplifier, which will consist of RF bandpass filters and usually an RF amplifier stage or two.

NOTE: The image frequency should be suppressed before the mixer stage.Practically IRR Should be as high as possible ,so the tuned circuits are connected in cascade.if X is the IRR Of tuned ckt 1 &Y is the IRR of tuned ckt 2 then IRR of the cascaded stage is  X*Y.To improve the IRR either Q Factor or IF should be increased but to increase IRR we practically prefer to increase IF Because increasing Q Factor causes decrease in Bandwidth.