FFT Window Controls

 There are two buttons for each channel. The text shown in these buttons changes to indicate the mode being used.

 FFT (OSC off)

When configured to display the FFT decibel response, a range of frequencies is searched to find the largest magnitude signal.  The frequency and magnitude are then displayed.  Mathematicaly frequency resolution depends on the data frames' size used (bigger is better), but is still astonishingly good even for modest frame sizes.  Better than 0.01 Hz accuracy is typical when the peak is sufficiently large to lock in and is uninvolved with other nearby signals (see note below regarding the crystal accuracy). There are four modes for locating the largest magnitude:
  • FMAX - The DC to F-nyquist (1/2 sampling rate) range is searched
  • MOUSE - A left mouse button click sets the measurement point
  • FOUT - The tester's frequency output sets the measurement point
  • LOCK - The measurement point is locked and does not change.

 OSC (FFT off)
In the oscilloscope display mode time and amplitude will be automatically adjusted to fit the selected signal frequency.

  • AUTO - Clicking on the AUTO button will cause the display to be auto ranged.

 Example Usage

An input frequency of 1234.567 Hertz is applied and measured. Even though the third harmonic (Fx3=3703.701 Hz) is 80 dB lower, it is never the less measured as 3703.686 Hz. The 0.025 Hz measurement error is improved by using the next larger FFT size.


Mathematical accuracy is not the same as absolute accuracy. Absolute frequency accuracy is ultimately tied to the crystal time base that derives the codec sampling frequency.  The crystals used in our products have an initial frequency tolerance of +/-30 PPM or 0.03 Hz per 1Khz of measured signal. If you require higher absolute resolution, please contact us.

Similarly, 'math' accurate routines are also used when generating the sine wave outputs.  In this example, the output frequency was set to 1234.567 Hz, and the loop back confirms this.  If, however, two testers are used with the output of one feeding the input of the other, each would have a crystal error of +/-30 ppm.  The resulting final measurement would then have a potential absolute measurement error of +/-60 ppm or 0.06 Hz when a 1 kHz signal is applied. 

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 Oscilloscope View

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