X-AXIS (SAMPLE RATE) CONTROL:

• X-Axis Control Menu
  • Sample Period
  • Sample Frequency (Rate)
  • Xpand Max and Min
  • Xpand Bar Toggle
• eXpand Trace
• eXpand on Cursors
• FFT X-Log Display

X-AXIS CONTROL MENU:

SAMPLE PERIOD:

SAMPLE FREQUENCY (RATE):

Whether you choose to adjust period or frequency, the control is fully "live" (as are almost all Daqarta controls). This allows easy matching of sample rate to a multiple of an input frequency... you can scroll the sample rate while you monitor the waveform for an integral number of cycles, or monitor the spectrum for minimum spread of spectral lines. This is a very poweful technique that can often allow you to avoid using FFT window functions entirely.

Typically, a data acquisition board provides a high frequency "master clock" source and a programmable counter-timer, the output of which initiates each sample. A value loaded into the counter sets the number of "ticks" of the clock between counter outputs, so the sample frequency is just the clock frequency divided by the counter load value. Another way to look at this is that the sample period is the period of the master clock times the counter load. This means the ultimate period resolution of the system can never be finer than one period of the master clock... and depending on the counter chip configuration may often be two periods.

When you set the sample period or frequency you are actually sending a request to the board driver, which sets the closest value it can and reports back what that was. For boards which have no clock or counter, the driver typically uses the main system counter-timer which otherwise would be used by DOS to update the time-of-day counters every 55 msec. The source for this timer runs at the odd frequency of 1.19318 MHz (period of 838 nsec), which results in a strange sample period resolution for these boards.

HISTORICAL FOOTNOTE for engineering types:

XPAND Max and Min:

Bar:

eXPAND TRACE (Xpand):

When the trace has been expanded, the trace grid and X-axis are redrawn with new divisions and labels as needed. Below the X-axis an Xpand Bar will appear in the axis color, which represents the unexpanded X-axis. The portion of the original axis that has been expanded is represented on the bar in the trace screen color. Display of this bar is optional: You may turn it off with the Xpand Bar item in the X-axis menu.

There is no Xpand or XCurs function in spectrogram mode, but when N = 1024, only half of the frequency range is visible on the Y-axis. Use the X-axis menu Xpand functions to slide that portion up or down in frequency over the whole range. Or, use the PgUp/PgDn keys to do this... they still control the Y-axis here, even though that's now frequency instead of the normal trace magnification. Similarly, SHIFT-HOME forces the range to start from zero (or any preset value), and SHIFT-END restores it.

Separate Xpand range settings are maintained for waveform, spectrum, X-log spectrum, and 1024 point spectrogram modes. If you activate Xpand while viewing a waveform and then hit the F-key to view the spectrum, you will not be in Xpand mode there unless you explicitly set it while in that mode.

This is useful for a common problem: If you set the sampling rate high enough to see a nicely spread out waveform with only a few cycles on the trace, then the spectrum will be crowded down at the bottom of the frequency range. Conversely, with the sampling rate set to see a mid-screen spectrum, the waveform will appear so compressed as to be nearly useless. The solution is to shoot for a compromise sampling rate and then apply Xpand as needed to get nice traces.

For waveforms, the maximum expansion allowed is when only 16 data points are shown, regardless of whether you get there by XCurs or by manually setting the Max and Min values in the X-axis menu. These 16 points can be anywhere within the original trace data. If the expanded portion of a waveform is near the start, you can change N-points and still see the same trace... only the Xpand Bar changes to show that you are looking at a different part of the data set with the same start and stop times.

If the expanded portion of a waveform is set to higher trace time points with N = 1024 (for instance), then if you shift to N = 256 those time points may be beyond the trace area, and Daqarta must adjust the expanded portion down to fit. For example, with a 50 µsec sample period (20 kHz rate) you can see 51.2 msec of data when N = 1024, but only 12.8 msec when N = 256. So if you set the Xpand area to run from 10.0 to 20.0 while N = 1024, then when you flip to 256 you will find that the Xpand area is from 10.0 to 12.8 msec.

For spectra, the maximum expansion is based upon N = 1024, where you can have no fewer than 16 data points showing. For N = 256 or 512 you will still see the same frequency range, but with a limit of 4 or 8 lines displayed, respectively.

eXPAND ON CURSORS (XCurs):

Once XCurs has been hit, the new X-axis Xpand range becomes the setting for the normal Xpand function, and the limits will appear in the X-axis menu Xpand Max and Min items. You can thus toggle Xpand off and back on with the unshifted X-key to return to this range.

You can apply XCurs repeatedly to get greater and greater expansions, until as few as 16 data points are displayed in waveform mode, or as few as 4 data points in spectrum mode with N = 256.

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