aaron_do
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Hi,
its probably a matter of image rejection. Your band is from 9.4 MHz to 9.9 MHz right? That means to mix directly to 0.455 MHz, you could use a tone from 8.945-9.445 MHz. But doing this, your image frequencies would be between 8.49 and 8.99 MHz. So before filtering, you would need a filter which passes 9.4-9.9 MHz while rejecting 8.49-8.99 MHz (with some rejection, say 60 dB, but I don't really know what you need). Obviously the requirements on such a filter are quite stringent, so its easier to mix down to 4 MHz first. In that case the image frequencies are much further away and can be easily rejected.
cheers, Aaron
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