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154 Agilent E1441A Function Generator Tutorial
Appendix C
waveform (whose data values are loaded in
RAM). The frequency of the voltage
waveform is proportional to the rate at which the
RAM addresses are changed.
The Agilent E1441A represents amplitude values by 4,096 discrete voltage levels
(or 12-bit vertical resolution). Waveforms may contain between 8 points and 16,000
points of 12-bit amplitude values. The number of points in
RAM representing one
complete cycle of the waveshape (or 360
°) is called its length or horizontal
resolution. Each
RAM address corresponds to a phase increment equal to 360° /
points, where points is the waveform length. Therefore, sequential
RAM addresses
contain the amplitude values for the individual points (0
° to 360° ) of the waveform.
Direct digital synthesis (DDS) generators use a phase accumulation technique to
control waveform
RAM addressing. Instead of using a counter to generate sequential
RAM addresses, an “adder” is used. On each clock cycle, the constant loaded into
the phase increment register (PIR) is added to the present result in the phase
accumulator (see below). The most- significant bits of the phase accumulator output
are used to address waveform
RAM — the upper 14 bits (2
14
= 16,384 RAM
addresses) for the Agilent E1441A. By changing the
PIR constant, the number of
clock cycles required to step through the entire waveform
RAM changes, thus
changing the output frequency. When a new
PIR constant is loaded into the register,
the waveform output frequency changes phase continuously following the next clock
cycle.
The Agilent E1441A uses a 48-bit phase accumulator which yields F
clk /2
48
or
approximately 142 nHz frequency resolution internally. The phase accumulator
output (the upper 14 bits) will step sequentially through each
RAM address for
smaller
PIR values (lower frequencies). However, when the PIR is loaded with a
larger value, the phase accumulator output will skip some
RAM addresses,
automatically “sampling” the data stored in
RAM. Therefore, as the output frequency
is increased, the number of output samples per waveshape cycle will decrease.
In fact, different groups of points may be output on successive waveform cycles.
Figure C-2.
Figure C-3.