Selectivity (electronic)

Selectivity (also near selection ) refers to the ability of a receiver to separate a desired service from a closely spaced in the frequency band transmitter.

The most important variable is the range between the upper and lower cut-off frequency. Other influencing factors are:

• Through loss
• Kling - tilt
• Shape factor
• Signal delay
• Ripple

Preselector

Latin: pre = before, selector = Selector (r )

In modern receivers, the selectivity is " far forward ", ie determined by filter different kind in the ZF possible directly behind the mixing stage, so as not to burden the steps with signals that are outside the desired frequency segment.

Some wideband receiver concepts, including RACAL, work with mixers directly as an input stage and without any further pre-selection, the mixer being might very high field strengths exposed outside the desired range. This only works satisfactorily in heavy payloads, selective antenna as possible and extremely durable, thus highly linear, complex mixers. If the reception of weak desired signals required despite other, strong incident transmitter, a receiver sufficiently large signal strength ( sa receiver dynamic, intermodulation, cross-modulation, ... ) is required to interference effects to entgegenen possible.

Reinforcing, working without selection, precursors exacerbate these problems and often provide the basis for overburdened by total signal power mixers dar.

Remedy precursors ( or, better selective pre- selectors ) provide little or no gain and suppressed by belt filter, low - or high-pass filters, resonant circuits or other suitable screening means the unwanted signals as possible. They can be integrated into the receiver or connected as an independent device between antenna and receiver.

AFC

At a high selectivity stability of the frequency-determining components is particularly important. Since a temperature - dependency and aging is unavoidable, the receivers are now equipped with an AFC circuit.