Daqarta for DOS Contents
Pg Mode option to produce alternating stimulus sweeps:
The "EVERY" option will acquire all sweeps. This might be hard on the eyes if the responses are very different, but it can be used to average responses from alternating sweeps. One typical use for this is in recording a response that has two components, one of which mirrors the stimulus phase and polarity, and one which is independent of these and only responds to frequency and amplitude. If you use PAIR mode and set the two stimuli to be tone bursts that are opposite in phase but otherwise identical in all respects, then any response that mirrors the stimulus will average to zero with the "EVERY" option, leaving only the other response.
In ALL Pg Mode, there is only one stimulus, so the Sync option is fixed to "ALL".
Display is active. When you are NOT adjusting anything, the View option selects which page the Display will show.
Pg Mode is set to PAIR or EACH there are two or four complete sets of 8 digital bit pages that must be configured for DigOut operation. However, you may want each set to be the same, or nearly the same. Setting Dig Pg to "Lock A" copies the settings from pages A0 through A7 to the other page sets, and when you go to the DigOut Adjust submenus you will find only pages A0-A7 available... if you use the CTRL-ALT-Pg keys you will get an alert and the message
'Dig Out locked to A.'Any changes you make to an A bit page will also set the corresponding B, C, and D bit pages identically.
This option would typically be used when you want to alternate between different DAC stimuli, and also provide a digital output pulse at the start of each sweep to synchronize some external device like an oscilloscope.
When you set Dig Pg to "Ind", then each A-D set of bit pages may be adjusted independently, depending on Pg Mode. If you want MOST of the bit outputs to be the same across sets, use "Lock A" for the initial configuration, and then switch to "Ind" to make any additional changes.
(0.5 - 0.5 × cos(theta))^nwhere theta goes from 0 to 180 degrees on the rising portion of the burst, and from 180 to 0 on the falling portion. The exponent is thus applied after an ordinary cosine is shifted and scaled to the 0 to 1.0 range, rather than the normal -1.0 to +1.0 range. (Otherwise, odd exponents would flip the polarity of the result.) The cosine of 0 degrees is 1.0, so at the start of the rise the above formula becomes 0.5 - 0.5 or 0, no matter what the exponent. Similarly, the cosine of 180 degrees is -1, so at the top of the rise the above formula becomes 1.0, again independent of exponent. The tone thus always rises smoothly from 0 to 100% of the Level value, but the selection of the exponent controls the steepness of this shape.
The Rise and Fall durations for individual tone components in the DAC 0 or DAC 1 Adjust submenu also control the steepness of this shape by setting the width of the rise/fall areas. Many investigators standardize on one exponent like the cos^2 default used here, and specify only rise/fall durations.
To see the effect of Shape and the interaction with rise/fall durations, perform the rise/fall experiment with different Shape exponents.
The Shape used here is really a "window function" applied to the stimulus tone, similar to the functions applied to acquired data with the Windo option. The purpose of each is the same: to reduce the "spectral splatter" that would be introduced by a sudden onset or offset of a signal, either a signal being generated (as here) or one being analyzed, whose "onset" and "offset" are artifacts of a finite number of analysis samples.
To compare the cosine-power Shape window to the standard Windo option analysis functions, set the Rise and Fall durations to half the total number of samples N, and set Start and Sustain to zero samples. In this condition, the cos^1 Shape is identical to a Hanning window applied to the same wave having Rise and Fall durations set to zero and Sustain duration set to N samples. Verify this for yourself by observing the spectra of these two conditions.
Since applying a window to a continuous wave reduces the signal energy seen by a subsequent FFT, Daqarta automatically applies the appropriate correction factor to the spectrum when the Windo option is active. In the case of the Hanning window, this is a factor of exactly two. (Note that this correction is different from the optional bandwidth correction used for broadband signals.) Thus, if you are comparing the spectrum of a windowed continuous tone to a shaped tone burst, you might want to cut the amplitude of the continuous tone in half for a direct comparison.
You can facilitate comparisons by recording each response in trace Memory, so you can flip quickly back and forth. An even better comparison can be made by using a Memory Array of traces, but for this purpose you will need to save spectral averages, not instantaneous spectra. That's because Daqarta always saves unaveraged traces, including spectra, as WAVEFORMS with no windowing, to allow you to later flip back and forth between waveform and spectrum views, and apply any window you wish. Spectral averages, however, are saved with the windowing included... even if the "average" is of just a single sweep.
DigOut is active, or if RTime is active, this option is ignored. This is exactly the same Pulse source option that may be selected from the main Trigger menu when the Stimulus Generator is inactive in Sequential mode, but here it may be used in conjunction with either or both of the DAC outputs.
Unlike the complete DigOut system, no stimulus buffer memory is used by Pulse. Whereas DigOut updates the digital output port once per sample from the stimulus buffer, Pulse only updates the output line when there is a change... at the start and end of the DAC stimulus portion of the sweep. For critical work on systems with poor isolation of the digital output strobe line from the signal input, the Pulse mode will insure that only a single glitch contaminates the input, at a time before any response to the stimulus has taken place.
The simplest way to do this is usually to make the stimulus longer than the number of data points N acquired in the sweep. If your desired stimulus is a short tone burst, you must pad the end with dead time. To do this, use a separate tone component for padding, with its Frequency set to ZERO. (You can't use the "obvious" method of setting the Level to zero, since the component would be automatically shut off.) Now use any combination of Start, Rise, Sustain, and Fall samples to get the desired total.
When Pulse is active, the main Trigger menu will show Source Pulse instead of Stim, and the Slope options will be present. As with non-stimulus operation, you may select the polarity of the pulse via the Slope controls, or have it alternate polarities on alternating sweeps.
If the stimulus generator Pg Mode is set to PAIR, setting the Pulse SLOPE to Alt will make the positive pulse coincide with the A+B sweep, and the negative with C+D. In EACH mode, there will be positive pulses on the A and C sweeps and negative on the B and D sweeps.
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