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how is multi-tone internally generated in E4438C?

Question asked by hudla on Oct 25, 2011
Latest reply on Oct 26, 2011 by tabbott
Dear all,

I have a problem to generate user-defined multi-tones using E4438C generator. The generator only allows 64 tones and I would like to generate more (I know, Signal Studio for Multitone Distortion exists, yet I want to do it by myself). Which multi-tone definition is used in E4438C?

I tried to generate a 64-tone signal using E4438C (from the front panel) and then to capture the I, Q components of the signal using E4440A spectrum analyzer, B7J Basic option. At the same time I generated an "ideal" multi-tone signal in Matlab (the same sampling frequency, the same number of samples as captured by E4440A):

m(t) = SUM (exp(j*phi)*exp(j*k*2*pi*df*Ts)), where k = 1, ..., Number_Of_Tones (equal tone amplitudes, all phases = 0), df = tone spacing, phi = tone phase, Ts = sampling period

The baseband I and Q components (ideal and captured by E4440A) do not have the same shape nor spectrum (I also tried several other multi-tone definitions), so apparently some other definition must be used. Or is the multi-tone internally created in the frequency domain and then converted back to the time-domain using IFFT?

Thank you,
Martin  

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