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Selectivity

selectivity-1

WHAT IT MEANS

Selectivity is the ability to discriminate between closely spaced stations. There are two common tests: Adjacent channel selectivity  where stations are just 200kHz apart, and Alternate channel selectivity where they are a manageable 400kHz apart. Even the best tuners do a mediocre job with stations just 200kHz apart, which is horrendously close up at 100MHz or so and a rare situation in real life. At 400kHz though, most tuners manage excellent discrimination and have high measured selectivity.


Selectivity is needed in cities with crowded VHF/FM bands, especially when pirate local stations are transmitting at any frequency they choose. However, a multi-element directional aerial is most use here, and a curved dipole ‘all rounder’ is the worst choice. A high gain directional aerial will pickup signals from the transmitter at which it is aimed, and reject all others. An ‘all rounder’ will pickup everything, all at similar strength, making it hard for the tuner to discriminate.


Modern tuners are highly selective, managing over 70dB for Alternate channel spacing, and 30dB for Adjacent channel spacing. That means will receive a wanted station that is 70dB weaker or 30dB weaker than a powerful nearby station with a tolerable degree (-30dB) of interference breaking through. Our diagram depicts this, and gives typical signal levels assuming the tuner is receiving a weak but listenable station of  40µV strength from the aerial. To produce breakthrough interference the Adjacent channel must be strong at 1.26mV; the Alternate channel strength isn’t possible.




leader

Our Leader 3216 Standard Signal Generator represents the wanted station. A Marconi R.F. Generator represents nearby stations.


HOW WE MEASURE IT

Measuring selectivity requires two signal generators, one representing the wanted station, the other the interfering station. The selectivity figure is the difference in strength between them that produces a ‘just tolerable’ amount of interference, -30dB, in the wanted station. It demands a complicated set up of test equipment and delivers an approximate result, but one that is still representative. We use our Leader 3216 Standard Signal Generator as the wanted station and a Marconi signal generator modulated to 100% mono modulation as the interfering station. Noise is measured IEC A weighted in the wanted station, broadly in line with IHF recommendations.

 

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