etects the positive peaks of the modulated carrier and filters the r.f. out, SO that the remaining signal is the inital Block diagram low frequency modulation frequency. The audio section uses audio amplifiers which serve to A block diagram of a superheterodyne (commonly called superhet) receiver is shown in Figure 2.1-34. amplify the signals to operate a loudspeaker. This section This receiver features an r.f. section which selects the is similar to the amplifier in your home which is used for playing compact disks (CDs) and cassettes. desired signal frequency (frf). This signal is then mixed with a local carrier at frequency (fo) in a frequency changer to produce an intermediate frequency (fif) Advantages which retains the modulated information initially carried The main advantages of TRF receivers are that they are by frf. The intermediate frequency (fif) then undergoes relatively simple, easy to construct, and require a mini- intensive amplification (60-80 dB) in the intermediate mum of components. A complete TRF receiver can be frequency (i.f.) amplifiers to bring the signal up to constructed using a single integrated circuit such as a suitable level for detection and subsequent application a ZN414 type chip. to the post-detection (audio) amplifiers. R.F. amplifiers are sometimes included in the r.f. section in order to make the noise figure of the receiver as small as Disadvantages possible. Frequency changers have comparatively larger TRF receivers suffer from two main disadvantages, noise figures (6-12 dB) than r.f. amplifiers. gain/bandwidth variations and poor selectivity. The in- The frequency of the local oscillator (fo) is always set evitable change of gain and bandwidth as the receiver is SO that its frequency differs from the desired frequency tuned through its frequency range is due to changes in (frf) by an amount equal to the intermediate frequency the selectivity circuits. (fif), i.e. Mixer/ Intermediate amplifier frequency frequency Detector section changer amplifiers Audio Oscillator amplifie