Agilent Technologies N5181A/82A User Manual download pdf (Page 198)

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

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.

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 (2

− 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.

E9 B7 53 2A

0 1 2 3

I data = bytes 0 and 1

Q data = bytes 2 and 3

Little Endian

Big Endian

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

1 2 ... 193 194 195 196 197 198 199 200 201 202 203 ... 365 366

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Agilent Technologies N5181A/82A User Manual Page 198