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[Question] poor channel selectivity?

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hebu

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poor selectivity

what will be effected by poor channel selectivity?
We assume this is due to bad channel selection filter in IF stage.
the BER will be effected? or the Signal will be distorted by adjacent channel power?

Any idea? I would like to know how good filter I need.
 

If a receiver has poor channel selectivity, the interferers that are out of the channel, will be amplified by the IF amplifier and will go into the demodulator, interfering with desired signal.
In a digital system BER will be affected.
You have to check the requirements for selectivity of your system (adjacent and alternate channel selectivity). If doesn’t have one, you can write your own, doing a system analysis.
If the match of your filter is not well designed the selectivity will be affected, even if the filter is ok for that application.
 

Allow some tolerance in the analog filter and include sharp filter in the digital domain.
 

What is the channel selectivity ,does that mean the sensitivity ?
 

With poor selectivity, your system is hard to reject blockers and their intermodulation terms. It is not only resulted from insufficinet IF filtering, but also many related characteristics like RF filtering(for heterdyne), image rejection, LO phase noise, dynamic range, spurious free dynamic range, etc. Another case of insufficient selectivity resulted from LO leakage or Tx leakage, these two leakage produce blockers (interferrer) once your transceiver is powered on.

Sure, the BER is effected by insufficient selectivity. But the extreme case is that the whole receiver totally fails while there are some in-band and out-band blockers.
 

TO mckinson


selectivity is abt channel freq selection
sensitivity is abt ckt behavior ie at particular freq,ckt will have more gain(one freq is amplified a lot).it depends on gain of amplifier
i hope i had thrown a some light on it
 

Check the TX/RX switch. IF there is no enough isolation between the TX/RX, the TX signal could leak into the RX system and introduce the extra distrotion due to tthe non-linearity of LNA, Mixer.
 

Best regards ? If the selectivity is poor , what performace maybe fail ?
 

mckinson said:
Best regards ? If the selectivity is poor , what performace maybe fail ?
Click to expand...

Your receiver is not able to sustain blockers if its selectivity is poor.
Namely, blocker can disable your receiver. Or your receiver treat blocker as the desired signal and down convert it to the baseband.
 

Who decide the selectivity? channel select filter? demodulator?
 

If you do open loop modulation and if your VCO has an offset regarding to center frequency when PLL is opened, demodulation filter ( e.g. channel filter ) will attenuate the signal depending on offset range due to symmetric parabola behaviour of the demodulator.
In other hand, channel selectivity can be perturbed by many factors like interferences, IP3 of the blocks of the RX chain etc...
 

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