Rustu Kaya wrote:
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>Yes, if the input frequency varies in wide >range, a simple band-pass filter fails to >make such conversion.

Anyway you can play with switched capacitor filters such as MF10 from Maxim and LMF100 from National (up to 40kHz) to build the band-pass filter.

Quoting from the National page:

'The center frequency of each filter stage is tuned by using an external clock or a combination of a clock and resistor ratio. Up to a 4th-order biquadratic function can be realized with a single LMF100.'

Maxim has a lot of info about filter ICs here:
http://dbserv.maxim-ic.com/Filters.cfm


Regards,
Enrique.