Agilent 33120A User's Guide

Chapter 7 TutorialModulationInternal Modulation Source Internally, the function generatorincorporates a second, lower speed and lower resolution DDS arbitrarywaveform generator to produce the modulating signal independent ofthe carrier signal. Internal modulation waveshapes range in length from2,048 points to 4,096 points. User-defined arbitrary waveforms areautomatically expanded or compressed in length as needed to fit withinthe required modulation waveform constraints. Linear interpolation isperformed on user-defined arbitrary waveforms while the lengths ofstandard waveshapes are varied by decimation. Due to the modulationsample rate and waveform size limitations, the best case modulationsignal frequency accuracy is approximately 0.05% of setting.Unlike the main signal output discussed previously, modulationwaveshapes are sampled using a variable “point clock” to sample dataloaded in modulation waveform RAM. Internally, the modulationpoint clock (C) and modulation waveform length are automaticallyadjusted to produce the modulation signal frequency desired. Forfrequencies greater than C/2048, all modulation shapes are sampled upto the maximum modulating frequency. A new modulation waveformis computed and loaded into modulation RAM each time the modulationtype, modulation waveshape, or modulation frequency is changed. Datain standard arbitrary waveform memory is not affected by modulationsignal changes (data is expanded or compressed and loaded directly intoseparate modulation RAM following computation). No expansion orcompression is performed on the modulation waveform data for certainmodulation frequencies.7293