Agilent Technologies N5183A MXG, N5181A/82A, E8663B manual Byte Swapping, DAC Input Values

Creating and Downloading Waveform Files

Understanding Waveform Data

Byte Swapping

While the processor for the development platform determines the byte order, the recipient of the data may require the bytes in the reverse order. In this situation, you must reverse the byte order before downloading the data. This is commonly referred to as byte swapping. You can swap bytes either programmatically or by using either the Agilent Technologies Intuilink for ESG/PSG Signal Generator software, or the Signal Studio Toolkit 2 software. For the signal generator, byte swapping is the method to change the byte order of little endian to big endian. For more information on little endian and big endian order, see “Little Endian and Big Endian (Byte Order)” on page 186.

The following figure shows the concept of byte swapping for the signal generator. Remember that we can represent data in hex format (4 bits per hex value), so each byte (8 bits) in the figure shows two example hex values.

Little Endian

Big Endian

01 2 3

E9 B7 53 2A

	B7		E9		2A	53
0		1		2		3

16-bit integer values (2 bytes = 1 integer value)

I data = bytes 0 and 1

Q data = bytes 2 and 3

I Q

To correctly swap bytes, you must group the data to maintain the I and Q values. One common method is to break the two- byte integer into one- byte character values (0–255). Character values use

8 bits (1 byte) to identify a character. Remember that the maximum unsigned 8- bit value is 255 (28

−1). Changing the data into character codes groups the data into bytes. The next step is then to swap the bytes to align with big endian order.

NOTE The signal generator always assumes that downloaded data is in big endian order, so there is no data order check. Downloading data in little endian order will produce an undesired output signal.

DAC Input Values

The signal generator uses a 16- bit DAC (digital- to- analog convertor) to process each of the 2- byte integer values for the I and Q data points. The DAC determines the range of input values required from the I/Q data. Remember that with 16 bits we have a range of 0–65535, but the signal generator divides this range between positive and negative values:

•32767 = positive full scale output

•0 = 0 volts

•−32768 = negative full scale output

Because the DAC’s range uses both positive and negative values, the signal generator requires signed input values. The following list illustrates the DAC’s input value range.

188 Agilent N518xA, E8663B, E44x8C, and E82x7D Signal Generators Programming Guide