rs Fig. 2.1-34 Block diagram of a superhet radio receiver. Here, cascade is meant to imply one amplifier following another amplifer and SO on. CHAPTER 2. 1 Basic features of radio communication systems fo-fri fo+fit Frequency Bandwidth Bandwidth Fig. 2.1-35 Image response in superhet receivers. fo -frf = fif (2.1.26) R.F. circuit bandwidths are not critical because re- ceiver selectivity is mainly determined by the i.f. amplifiers. frf - fo = fif (2.1.27) Disadvantages Equation 2.1.26 is more usual for medium-wave The main disadvantages of superhets are as follows. receivers. Typical tuning ranges for a medium-wave Image channel interference is caused by the local os- receiver with fif = 465 kHz are 522-1620 kHz for frf and cillator (fo) combining with an undesired frequency 987-2085 kHz for fo. (fim) which is separated from the desired frequency (frf) by twice the intermediate frequency (fif). Advantages Expressed mathematically The main advantages of the superhet receiver are as follows. fim = =frf=2fif (2.1.28) Better selectivity because fixed bandpass filters with The term 2nd channel interference is another name well defined cut-off frequency points can be used in for image channel interference. the i.f. stages of a superhet. Filters and tuned circuits Image channel interference is more easily understood are also less complex because they need to only operate by substituting some arbitrary values into Equations at one frequency, namely the intermediate frequency. 2.1.26 and 2.1.27. For example, assume that the local In a superhet, tuning is relatively simple. A two-ganged oscillator of a superhet is set to 996 kHz and that its i.f. capacitor can be used to tune the r.f. and oscillator amplifiers operate at 465 kHz. Then, either of two input sections simultaneously to produce the intermediate frequencies, 996 - 465 = 531 kHz (Equation 2.1.26) or frequency for the i.f. amplifiers. 996 + 465 = 1461 kHz (Equation 2.1.27), will mix with 1st R.F. 1st mixer 1st I.F. amplifier frequency frequency section changer amp