Convert given string into upper case with out using builtin UPPER function

How to convert a given string into upper case string without using builtin UPPER function. Below is the code.

CREATE FUNCTION udfUpper

(

   @InputString NVARCHAR(MAX)

)

RETURNS NVARCHAR(MAX)

AS

BEGIN

   DECLARE @UpperString    NVARCHAR(MAX)  = ''

   DECLARE @Counter        INT            = 1

   DECLARE @Character      CHAR           = ''

   DECLARE @AsciiValue     TINYINT

   DECLARE @NewAsciiValue  TINYINT

   WHILE (@Counter <= LEN(@InputString))

   BEGIN

      SET @Character = SUBSTRING(@InputString,@Counter,1)

      IF ASCII(@Character) BETWEEN 97 AND 122

      BEGIN

         SET @NewAsciiValue = ASCII(@Character) - 32

         SET @UpperString = @UpperString + CHAR(@newAsciiValue)

      END

      ELSE

      BEGIN

         SET @UpperString = @UpperString + @Character

      END

      SET @Counter = @Counter + 1

   END

   RETURN @UpperString

END

GO

SELECT dbo.udfUpper('this is a test STRingdadas@#@#@')