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Lines Frequency Entry Step Mode

Controls: Gen Dlg >> Stream >> Tone Freq >> Step Mode >> Lines
Macros: Fstep, FstepLine

This mode may be selected from the Frequency Entry Step Mode dialog, which opens via the Step mode button that indicates the current mode. That button is located under many frequency entry controls in the Generator. The mode setting applies to most Generator frequency entry controls. The mode only affects values during entry or adjustment; it does not change previously-entered frequencies.

Lines mode forces frequencies to take on values related to the FFT spectral line spacing. If the step size is set to 1.0000 line (default), changes to any frequency value will cause it to fall exactly on a spectral line for a 1024-sample (512 line) FFT.

The spectral line spacing is equal to the sample rate divided by the number of samples in the FFT data run, which is 1024. Example: For a 48 kHz sample rate, it would be

48000 / 1024 = 46.875 Hz.

However, you can set Lines to get waves that land between the lines, just by setting a fractional step value.

The ability to place frequency components exactly on spectral lines is a very powerful tool, because it can eliminate the need for windowing: There are no spectral leakage "skirts" whatsoever when viewing these frequencies. And if a fundamental frequency falls exactly on a line, then all harmonics will fall exactly on lines as well. What's more, the intermodulation distortion products generated by two primary tones will also fall on spectral lines. This can greatly increase the ability to resolve low-level products that would otherwise be buried in leakage skirts, even when reduced by windowing.

If you set Lines to 0.125 or any multiple thereof, it will insure that one cycle of a frequency spans an exact integer number of samples. This will guarantee that there will perfect jitter-free display sync on every update, without ever needing to wait for a sync point that meets a certain Maximum Sync Error limit; with an integer number of samples per cycle, there will never be any sync error.

Macro Notes:

Fstep=Line or Fstep=2 sets Line Step Mode.

FstepLine=2 sets the step size to 2 lines.

See also Frequency Entry Step Mode, Tone Frequency Dialog


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