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Arbitrary Waveform Generation
Introduction
Arbitrary (Arb) waveforms are generated by sequentially addressing the RAM containing the
waveform data with the arbitrary clock. The frequency of the arb waveform is determined both by
the arb clock and the total number of data points in the cycle.
In this instrument an arb waveform can have up to 65536 horizontal points. The vertical range is
−
2048 to +2047, corresponding to a maximum peak
−
peak output of 20 Volts. Up to 100
waveforms can be stored in the 256k non
−
volatile RAM and each given a name; the number that
can be stored depends on the number of points in each waveform.
Arb waveforms can be created using basic front panel editing capabilities (particularly useful for
modifying existing standard or arb waveforms) or by using waveform design software that
enables the user to create waveforms from mathematical expressions, from combinations of
other waveforms, or freehand, see Appendix 4.
Arb Waveform Terms
The following terms are used in describing arb waveforms:
•
Horizontal Size.
The number of horizontal points is the time component of the waveform. The
minimum size is 4 points and the maximum is 65536 points.
•
Waveform Address.
Each horizontal point on an arb waveform has a unique address.
Addresses always start at 0000, thus the end address is always one less than the horizontal
size.
•
Arb Frequency and Waveform Frequency.
The arb frequency is the clock rate of the data RAM
address counters and has a range of 0·1Hz to 40MHz on this instrument. The waveform
frequency depends on both the arb frequency and horizontal size. A 1000 point waveform
clocked at an arb frequency of 40MHz has a waveform frequency of 40e6
÷
1000 = 40kHz.
•
Data Value.
Each horizontal point in the waveform has an amplitude value in the range
−
2048
to +2047.
•
Arb Waveform Amplitude.
When playing arb waveforms the maximum output amplitude will
depend on both the range of data values and the output amplitude setting. A waveform that
contains data values ranging from
−
2048 to +2047 will produce a maximum output which is
100% of the programmed peak
−
to
−
peak amplitude; if the maximum range of the data values
is only
−
1024 to +1023, for example, the maximum output will only be 50% of the programmed
level.
Arb Waveform Creation and Modification – General Principles
Creating arb waveforms with the instrument alone consists of two main steps:
•
Creating a new blank waveform, or a copy of an existing one, and giving it a size and a name
•
Modifying that waveform using the various editing capabilities to get exactly the waveform
required.
These steps are fully described in the Creating New Waveforms and Modifying Arbitrary
Waveforms sections which follow.
Waveform creation using waveform design software also consists of two steps:
•
Creating the waveform using the software on a PC.
•
Downloading the waveform to the generator via the RS232 or GPIB interface.
This process is described in Appendix 4.
Certain constraints apply to the overall operation of the generator during creation and
modification of an arb waveform on the instrument; these ensure proper management of the arb
waveforms and avoid contentions, particularly in multi
−
channel instruments. The constraints are
mentioned in the individual sections which follow but are summarised here.
•
No arb creation or modification is possible unless
all
channels are running in continuous
mode; summing and modulation of channels is allowed.
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