Selectivity

Author: J.B. Hoag

The selectivity of a receiver is defined as the ability of the receiver to differentiate between a desired signal and other signals or other disturbances occurring at a different frequency. The overall selectivity of a receiver depends upon the sharpness of the resonance curves of the individual tuned circuits and upon the number of such circuits cascaded one after the other in the receiver. The selectivity curve of Fig. 32 E is much like an inverted resonance curve.

Fig. 32 E. A selectivity curve of a modern receiver

It is obtained by measuring the r.f. input voltage to the receiver needed for the delivery of a standard output voltage at various frequencies in the neighborhood of the carrier frequency. The selectivity curve may be exceedingly sharp (i.e., the band-width can be very small) for c.w. reception, whereas it should be broader for phone reception, where the audio signals use a channel of approximately 5 kHz on each side of the carrier frequency. For the reception of frequency modulated signals the band-widths must be even greater, say 30 or 40 kHz

Let us suppose that the receiver has been adjusted for the reception of signal number 1, but that a second transmitter is operating at the same time on a nearby carrier frequency. The response of the receiver for these two signals will be in proportion to the arrows 1 and 2 shown in Fig. 32 F.

Fig. 32 F. The reception of a desired (1) and an undesired (2) signal

From this consideration, the more selective a receiver, the greater will be its ability to receive the desired signal in goodly strength and to exclude the undesired signal.