of components, 2.1.9.1 Radio frequency section Ideal selectivity curve A r.f. section is designed to have the following properties. Selectivity Receiver selectivity is a measure of the ability of a radio receiver to select the desired transmitted signal from Skirt Skirt other broadcast signals. An ideal selectivity response curve for an AM broadcast receiver centred on a desired carrier frequency (fo) is shown in Figure 2.1-29. Two main Frequency (kHz) points should be noted about the ideal selectivity response Fig. 2.1-29 Ideal selectivity curve for an AM medium wave curve. First, it should have a wide enough passband (9 kHz broadcast receiver. Basic features of radio communication systems CHAPTER 2. 1 Loss (dB) F (kHz) +F (kHz) 18.49 15.17 33.80 29.30 (Fi is measured relative to fo) Log frequency (kHz) Fig. 2.1-30 Typical selectivity curve of a commercial AM six transistor receiver. particularly inductors, varies with frequency. Note also a receiver should be wide enough to pass all modulation that the 3 dB bandwidth is only 3.28 kHz and that the frequencies without discriminatory frequency 6 dB bandwidth points are approximately 4.82 kHz attenuation. apart. The 60 dB points are 63.1 kHz apart. Consider the case of a carrier signal (fo) modulated Adjacent channel selectivity with two inner sideband frequencies fil, fiu (+1.64 kHz) A graphical comparison of the selectivity curves of and two outer sideband frequencies f2L, f2U (+2.4 kHz) Figures 2.1-29 and 2.1-30 is shown in Figure 2.1-32. away from the carrier. The frequency spectrum of this From these curves, it can be seen that the practical se- signal is shown in Figure 2.1-31(a). When this frequency lectivity curve does not provide complete rejection of spectrum is passed through a receiver with the selectivity signals to stations broadcasting on either side of the de- response shown in Figure 2.1-30, the inner sidebands sired response channel. The breakthrough of signal from fil, fiu (+1.64 kHz) and the outer sidebands f2L, f2U adjacent channels into the de