Agilent Technologies N5183A MXG - Manual (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