Replace A Function By Pure T-SQL

I have 2 tables
1) One With ExchangeRates
a. CurrencyCode ValidFromPeriod Rate
b. EX1 200801 1
c. EX1 200803 2
d. EX1 200805 3
2) One with just OrderHeaders
a. ID CurrencyCode Period
b. 1 EX1 200801
c. 2 EX1 200802
d. 3 EX1 200803
e. 4 EX1 200804
f. 5 EX1 200805

The Idea is to link these orderheaders with their correct ExchangeRate. Which I only managed to do by using a function. Is there a beter (faster) way to do it?




Code Snippet
SELECT OrderHeaders.ID, OrderHeaders.CurrencyCode, OrderHeaders.Period, dbo.GetExchangeRate(OrderHeaders.CurrencyCode, OrderHeaders.Period) AS ExchangeRate FROM OrderHeaders

FUNCTION GetExchangeRate
@CurrencyCode VarChar(3),
@Period int
BEGIN
DECLARE @VarResult Float
SET @VarResult =0
SELECT TOP 1 @VarResult=Rate FROM ExchangeRates WHERE CurrencyCode=@CurrencyCode AND ValidFromPeriod<=@Period ORDER BY ValidFromPeriod DESC
Return @VarResult
END