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Using the 81150A as an arbitrary waveform generator

Although the 81150A is primarily an arbitrary function generator (AFG), it can be used as an arbitrary waveform generator (AWG) with some limitations. The main difference between an AFG and an AWG is the way how data for a digital-to-analog converter (DAC) is internally generated. While an AFG typically has a fixed sample clock to drive the DAC and uses the direct digital synthesis (DDS) principle to read from arbitrary locations in its internal sample memory, the AWG uses an user adjustable sample clock but reads consecutive locations from its sample memory. If you follow a few simple rules for the number of samples and the frequency/period, you can force the 81150A to also read consecutive memory locations from its internal memory and thus behave similar to an AWG.

The 81150A uses always a fixed 2 GSa/s DAC sample rate. In Arbitrary Waveform Mode, the waveform memory access rate is 1 GSa/s. Note: there is a one step linear interpolation between two memory samples. In order to force the DDS engine to read consecutive memory locations one or more times, the period must be set to an integer multiple of (1 ns * number of samples). If you want to have jitter as low as 50ps p-p and equidistant sample intervals, the number of samples must be a power of 2. As an example: If you download 16384 (=2^14) samples you can set the period to 16.38400 us (= 1 * 1ns * 2^14) or for example to 81.92us (= 5 * 1ns * 2^14) to get all your samples generated in equidistant intervals. If you download 512k (=2^19) samples, you must set the period to 524.28800 us (= 1ns * 2^19) or an integer multiple thereof to achieve the same effect. Make sure that you enter a period and not a frequency (double click on the "Frequency" softkey to switch to Period if necessary) since the number of valid digits is not sufficient to enter the correct number as a frequency in many cases. If the application can tolerate a jitter of 1 ns p-p, any period (frequency) can be adjusted. Also, please note that the analog bandwidth of the 81150A is limited at approx. 240 MHz, so don’t expect to generate 500 MHz square wave signal, just because your sample rate is 1 GHz…