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How does the MXG N5182B achieve 160 MHz of bandwidth with a sample rate of 200 MSa/sec?

The MXG and EXG baseband generator (BBG) has an arbitrary fractional re-sampling ASIC that will up-sample and interpolate the user sample rate setting from the front panel. For example the front panel on the MXG allows user sample rates from 100 Hz to 200 MHz. The re-sampling ASIC will always up-sample to a fixed intermediate rate of 200 MHz before the DAC (independent of the user sample rate setting). After the ASIC, the signal gets passed to the DAC. The DAC in the MXG and EXG then uses a built-in fixed 4x up sampling and interpolation. This sets the final sample rate of the DAC to a fixed 800 MHz. As a result of the large amount of oversampling, only one analog reconstruction filter is required to filter off the sampling images. The analog reconstruction filter is a low pass filter with knee frequency of 80 MHz on MXG and EXG. The sampling images are harmonic multiples of the final DAC rate. In this case, the first image will be centered at 800 MHz. This large amount of oversampling also enable a very flat sin(x)/x amplitude response over the baseband bandwidth supported by the MXG (80 MHz). For the MXG this is less than 0.1 dB. Most amplitude error for the IQ system is now dominated by analog baseband and RF hardware (real-time factory channel corrections eliminates a majority of this error).

In the past, people who created waveforms for arbitrary waveform generators (AWG’s) had to consider the max sample rate supported by the DAC together with the reconstruction filters available to determine the final oversampling ratio (OSR) needed without aliasing. By leveraging the MXG and EXG up-sampling ASIC, this task has been greatly simplified. The user no longer has to build in a large amount of oversampling into the waveform, the ASIC and DAC will do this for them. The waveform developer only has to follow the Nyquist rule that dictates an OSR = 1 to accurately re-produce the signal. In this case due some hardware specific limitations in the MXG/EXG OSR = 1.25.

  • For example the MXG N5182B flat RF bandwidth = 160 MHz.
    Therefore sample rate: 160 x 1.25 = 200 MHz
  • Another way to look at this:  The baseband I and Q channel each generate an 80 MHz signal. Nyquist states sample rate must be at least 2x above highest frequency you want to accurately reproduce.
    Therefore: 80 MHz x 2 = 160 MHz, this represents an OSR = 1
  • Because the MXG/EXG hardware is not perfect we need a little extra to make this work.
    Therefore: 80 MHz x 2.5 = 200 MHz, this represents an OSR = 1.25

 

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