Agilent Technologies N5181A/82A User Manual download pdf (Page 205)

Agilent N518xA, E8663B, E44x8C, and E82x7D Signal Generators Programming Guide 195

Creating and Downloading Waveform Files

Waveform Phase Continuity

NOTE If you download I/Q data using a file name that currently resides on the signal generator, it

also overwrites the existing marker file setting all bits to zero and the file header setting all

parameters to unspecified.

Waveform

A waveform consists of samples. When you select a waveform for playback, the signal generator loads

settings from the file header. When the ARB is on, it creates the waveform samples from the data in

the marker and I/Q (waveform) files. The file header, while required, does not affect the number of

bytes that compose a waveform sample. One sample contains five bytes:

To create a waveform, the signal generator requires a minimum of 60 samples. To help minimize

signal imperfections, use an even number of samples (for information on waveform continuity, see

“Waveform Phase Continuity” on page 195). When you store waveforms, the signal generator saves

changes to the waveform file, marker file, and file header.

Waveform Phase Continuity

Phase Discontinuity, Distortion, and Spectral Regrowth

The most common arbitrary waveform generation use case is to play back a waveform that is finite

in length and repeat it continuously. Although often overlooked, a phase discontinuity between the

end of a waveform and the beginning of the next repetition can lead to periodic spectral regrowth

and distortion.

For example, the sampled sinewave segment in the following figure may have been simulated in

software or captured off the air and sampled. It is an accurate sinewave for the time period it

occupies, however the waveform does not occupy an entire period of the sinewave or some multiple

thereof. Therefore, when repeatedly playing back the waveform by an arbitrary waveform generator, a

phase discontinuity is introduced at the transition point between the beginning and the end of the

waveform.

Repetitions with abrupt phase changes result in high frequency spectral regrowth. In the case of

playing back the sinewave samples, the phase discontinuity produces a noticeable increase in

distortion components in addition to the line spectra normally representative of a single sinewave.

I/Q Data

Marker Data

1 Waveform Sample

2 bytes I

(16 bits)

2 bytes Q

(16 bits)

1byte (8 bits)

Bits 4–7 reserved—Bits 0–3 set

5 bytes

1 2 ... 200 201 202 203 204 205 206 207 208 209 210 ... 365 366

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Agilent Technologies N5181A/82A User Manual Page 205