Agilent Technologies Figure C-2, Figure C-3, Agilent E1441A Function Generator Tutorial

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.

Figure C-2.

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 (214 = 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 Fclk /248 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-3.

154 Agilent E1441A Function Generator Tutorial Appendix C