Need To Return X Rows But Select Top Doesn't Work With Variable

I am writing some functions that work on a time series database of prices, ie volatility, correlation. I need to use the SELECT TOP syntax, but cannot do this with a variable, ie 'SELECT @x TOP * from prices'. My solution is to simply have a function for each potential period that will be looked at - 30day_volatility, 60day_volatility, etc. I looked at setting the ROWCOUNT variable but this is not allowed in functions. I haven't posted any DDL because I think the question is general enough - How do I return n ordered rows from a function without using SELECT TOP, or is there a way to use SELECT TOP with a variable that I am not aware of.

Thanks!