Look up filter 'Q'.
When the bandwidth of the filter is forced to be very small relative to the filters center frequency (f0), i.e. it has a high 'Q', the filter's BW and f0 become very sensitive to slight variations in the components. Parasitic inductance and capacitance of the components and their connections become significant. Thus the practicality of the superhetrodyne radio receivers in which the the RF is first mixed down to some lower IF frequency where the tuning filters can be constructed reliably and easily.
As said above that is the point of having superhets..
The IF filter defines the selectivity of the receiver and can be a sharp sided crystal filter. RF filters may be needed at a variety of frequencies and may be tunable and are only required to prevent overload and reduce spurious responses such as the image frequency.
LOL The easiest way to learn is to do the math for yourself if you can here is a link to my rf filter http://www.oscilent.com/catalog/Category/rf_saw_filter.htm page let me know if you find what you are looking for.... Good luck
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