Daqarta for DOS Contents
Xpand) limit settings and the presence of the Xpand bar.
X-axis. Knowing the active N-points value, the extent of the full axis is just the number of points times the time per point.
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:The unusual system timer frequency is a holdover from the original IBM PC, for compatibility reasons. The IBM designers needed several timing references in the PC system, including the CPU clock, the time-of-day timer, and the TV color burst reference for the early CGA Color Graphics Adapter. Of these, only the color burst frequency had an absolute requirement for a fixed value, to maintain compatibilty with standard color television signals if a standard TV was used as a monitor. So the PC ended up with a master oscillator running at 14.31818 MHz, which the CGA divided by 4 to get the standard color burst frequency of 3.579545 MHz. The master oscillator was divided by 3 to get the 4.77 MHz CPU clock (which only had to be less than the 5 MHz limit of the early 8088 chips), and further divided by 4 again for the 1.19318 MHz system timer. The system timer counts up 65536 ticks of this input to generate an interrupt every 54.925 msec, and 65536 of those interrupts to give an hour of 3599.59 seconds... Hmmmm.
Xpand area for the current mode. Separate settings are maintained for waveform, FFT, and X-log FFT displays, as well as for the Y-axis of the spectrogram with N of 1024.
waveform or spectrum. This stretches out a portion of the displayed data for closer inspection... it does not change the actual data points. The portion of the trace to be expanded can be selected via items in the X-axis control menu, or by using the XCurs function to Xpand the area between the trace Cursors. Trace expansion has no effect on any ongoing process... you just get a different view of it. Hit the X-key again to go back to the full view.
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.
waveform or spectrum between the two trace cursors will be expanded to fill the trace. The Xpand option will be shown active, and the Xpand bar will appear below the X-axis just as if you had hit the unshifted X-key to select the Xpand function. The XCurs option will not be shown as active, since you will use the unshifted X-key to toggle Xpand mode back off.
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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