Daqarta for DOS Contents
ADVANCED STIMULUS SIGNAL GENERATOR
Page Mode:This determines how the four wave component pages specified for each DAC channel will be deployed, and likewise enables multiple digital output sequences.
Page Sync:If you have set Pg Mode to PAIR or EACH, the stimulus will alternate between traces (N-sample acquisition passes). If you only want to acquire the responses to one of these stimuli, use this Page Sync option to select the one desired. Each time you hit ENTER with the cursor at this option, the Sync selection will advance from page A through D and then to EVERY.
The EVERY option will acquire all traces. This might be hard on the eyes if the responses are very different, but it can be used to average responses from alternating traces. 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.
Note that in PAIR mode the A and B selections give identical results, since these pages are presented simultaneously. The C and D selections behave the same way.
In ALL Pg Mode, there is only one stimulus, so the Page Sync option is ignored, and the cursor will not move to this item. It will remain set to the same page, so if you return to PAIR or EACH mode it will be just as you left it.
In Dynamic RTime mode, the Page Sync option only works for Burst sync on each page, not main wave or modulator sync.
MaxErr:In Dynamic RTime mode with Stim triggering selected, the trace or N-sample acquisition pass begins at the start of the selected Sync source waveform. This is equivalent to a rising slope trigger, with the effective trigger level set to zero.
However, since Dynamic RTime mode allows completely arbitrary waveform frequencies that are unrelated to the sample rate, each cycle of the waveform typically does not start with a sample actually falling on the zero-crossing. For example, consider a 101 Hz sine wave at a 10 kHz sample rate. The first sample starts at 0, and each sample after that is advanced by:
101 cycles/sec × 360 degrees/cycle ---------------------------------- = 3.636 degrees/sample 10000 samples/sec
So after 99 samples we are at 359.964 degrees, and the next sample is at 363.600 degrees... no sample actually landed on zero. With a conventional external signal and Intern trigger mode, if we were waiting for a trigger the 363.600 sample (3.600 degrees relative to zero) would fulfill the trigger requirements. So that trace would start slightly out of alignment compared to a trace that started exactly on zero, and the result as this process continued would be a jittery trace.
On the other hand, if we waited to trigger only on cycles where the initial sample fell exactly on zero, we would only get a trigger every 101 cycles, which may be too infrequent for the desired use. And of course the mismatch could be much worse: Consider that with the signal at 100.001 Hz, the exact zero-crossings would only be every 100001 samples!
So the compromise is to select a Maximum Error (MaxErr) value that gives a reasonable trigger rate, but not too much jitter. This value sets an upper limit on what the Dynamic RTime Stim trigger will accept as a zero-crossing. The default is about 1 degree, but you may wish to change that to suit your needs.
Note that this control only applies to Dynamic RTime Stim trigger mode. It is ignored in all other modes.
Display:This controls the image of the stimulus that appears superimposed over the trace area when you are in waveform display mode. There is a small "trace" for each active DAC channel and for each of the 8 bits of the digital output if DigOut is active.
Repeated ENTERs on this item change it between On, Off, and Expand. The Expand mode is intended for use when the Factor is greater than one. In that case, Expand shows all the output samples, whereas On only shows those that would correspond to each input sample. You would use On to compare the response features and timing to features of the stimulus, whereas Expand would allow you a better view of the stimulus when adjusting its features relative to each other.
Since the stimulus may often be longer than the number of acquisition samples N, especially with the Factor and Expand active, you can use trigger delay to view the later portions.
The Display traces are shown in ALT trace color, which you can change from the main color control menu invoked by CTRL-C.
View:When you are adjusting the individual components of PAIR or EACH modes using the main page Adjust options, the component being adjusted will always be shown if the Stimulus Display is active. When you are NOT adjusting anything, the View option selects which page the Display will show.
Dig Pg:When 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 trace or N-sample acqusition pass 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.
Pulse:This provides a simple digital pulse output as an alternative to the full digital output system. If 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.
The simplest way to do this is usually to make the stimulus longer than the number of data points N acquired in the run. If your desired stimulus is a short tone burst, for example you must pad the end with dead time using the Dwell control.
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 traces or acquisition passes.
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 trace, and the negative with C+D. In EACH mode, there will be positive pulses on the A and C traces and negative on the B and D trace.
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