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Daqarta for DOS
Data AcQuisition And Real-Time Analysis
Shareware for Legacy Systems
(Use Daqarta for Windows with modern systems)
From the Daqarta for DOS Help system:
 
DAQARTA - STIM3A
ADVANCED STIMULUS SIGNAL GENERATOR

WAVE TYPE:

Wave:

Noise:

Sine Wave:

Standard sinusoidal waveform. There is no separate submenu. Use the Freq menu to control the phase if you want both sine and cosine waves.

Triangle Wave:

Standard triangular waveform, with no submenu. Sometimes called a "sawtooth", but that makes it easy to confuse with Ramp waves. The normal Triangle wave is bipolar, running between positive and negative full-scale values. If you need a positive unipolar wave, just set the Level to 50% and Offset to 50%. For a negative wave, set Offset to -50%.

Ramp Wave:

This has its own submenu. As noted for the triangle, this is a bipolar wave, but it can be converted to all positive or negative by setting Level to 50% and Offset to +50% or -50%, respectively.

Ramp Rise %:

This controls the percentage of the cycle that the wave spends rising; the remainder is falling. If Rise is set to 50%, then the wave is a triangle.

Ramp Stairs:

When this is set to a non-zero value, the simple continuous ramp is converted to a staircase with the specified number of steps, plus a baseline or 0th step, for each slope.

For a 16-bit system, there are a total of 65535 steps possible. A 12-bit system only allows 4095 distinct steps, and an 8-bit allows 255, but this control does not change; if you set more steps than the DAC supports you will simply get a "smooth" ramp, as far as that DAC can produce.

Ramp Mod:

This toggles the Phase Modulation option to become Slope Modulation. With Phase Modulation the waveform simply slides back and forth, whereas Slope Modulation effectively modulates the Rise value. If you set Rise to 50% you get a triangle wave, and with Slope Modulation this tilts to the right and left. At 100% modulation, the wave smoothly changes between an effective Rise of 0% and 100%. This effect is easier to see at lower frequencies, where there are only a few cycles on the trace.

Square Wave:

Standard bipolar square wave. Use the Level and Offset controls to make it unipolar. This wave has a fixed 50% duty cycle; use the Pulse wave to create other duty cycles (rectangular waves).

Pulse Wave:

Has its own submenu to allow creation of monophasic or biphasic pulses with adjustable levels. The first pulse is called A and the second B. The B pulse always starts immediately after the A pulse ends.

Note that although this is usually by far the most convenient method for generating and controlling pulses, you may also use the DigOut system if you have a lab-type board. Getting controllable biphasic pulses is more difficult, but sometimes that is the most appropriate approach.

Pulse A Width:

Depending on the Width Mode setting, this width can be given in percent, degrees, samples, or time. If A Width is set to 0 then the B pulse is the only pulse, and vice-versa.

Pulse B Width:

Similar to A Width. However, since it is often desired to have the two widths of a biphasic pulse track each other, the Pulse B Width control has a special Track option: When this is active, any changes made to the A Width will automatically be made to the B Width, which will show only 'Track'.

To set the Track option, begin by hitting ENTER at the item as though you were going to adjust it normally, then ALT-Up or ALT-Dn will activate Track mode.

To exit Track mode, simply enter any B Width value normally.

If you want to create a monophasic pulse, you would typically set B Width to 0. This gives a rectangular wave with variable duty cycle, level, and offset.

Pulse A Level %:

Sets the level of Pulse A. The default is +100%, with B Level at -100% for a biphasic pulse that starts positive. If you want a biphasic pulse that starts negative, just reverse these values.

If you set A Level to 0 (or to the 0 Level value) you will get a delayed Pulse B. Use A Width to adjust the delay time.

There is nothing to prevent you from setting both A Level and B Level to the same polarity. There is probably no reason to set identical levels, since you would simply get one longer pulse that required two different Width adjustments. But you might want to set different levels to get a two-step pulse.

Pulse B Level %:

Similar to Pulse A Level. However, since it is often desired to have the positive and negative pulses of a biphasic pair mirror each other, the Pulse B Level control has a special Mirror option: When this is active, any changes to A Level will cause the actual B Level value to be the negative of the A Level. While this is active, the B Level will show only 'Mirror'.

To set the Mirror option, begin by hitting ENTER at the item as though you were going to adjust it normally, then ALT-Up or ALT-Dn will activate Mirror mode.

To exit Mirror mode, simply enter any B Level value normally.

Setting the B Level to 0 (or to the 0 Level value) will give what appears to be a single monophasic pulse, but this is usually not the best way to do that. It's better to set B Width to 0, so that A Width will give complete control over the duty cycle. However, there may be occasions where you want to deliberately limit the range of duty cycles, and for that you can set B Level to 0 and use B Width to set the minimum Off (dwell) time.

Pulse 0 Level %:

This sets the baseline for the pulse. It effectively allows full-scale offset adjustment without driving the pulse peaks out of range.

This is especially useful for monophasic pulses where you want to control the average (DC) value of the overall wave. For example, if the A Level is 100% and the A Width is 10%, the average level would be 10%. You can balance this out by setting the 0 Level to -11.111%. In general, for a pulse of width W and level A, the balancing 0 Level value Z will be: 

              A × W
    Z  =   -----------      (All units assumed to be %)
            (100 - W)
Note that this approach does not keep the same base-to-peak height as set by the A Level value; as the base goes down the effective pulse size rises. If you are more concerned about keeping that constant, use the page Offset control instead. In the above example, you would simply set -10% to balance out the +10% average of the pulse. The pulse would then go from a baseline of -10 to a peak of +90.

Pulse Width Mode:

This control toggles between Pct, Deg, Smpls, and Time on successive hits. It determines the units that are used for entry and display of the two Width values. You may choose whichever is most relevant for your application, keeping in mind the effects upon Modulation Type, below.

Pulse Modulation Type:

This is not really a control, but a status indicator that changes according to the above Pulse Width Mode control. It indicates that either Phase or Width modulation is available in the main component Modulation menu. Pct and Deg allow Width modulation, while Smpls and Time allow Phase modulation.

Pulse Width Modulation (PWM) is commonly used in many areas. The output signal can be purely digital, on or off, but the average value changes with the modulation. The digital nature makes it easy to send the signal over a simple interface (such as an optoisolator), yet it is easy to recover the modulating signal via a simple low-pass filter.

PWM is the traditional method used in high-efficiency Class D amplifiers. Each output device (transistor) is always in either the full-on or full-off state, neither of which dissipates much power in the device itself, delivering it instead to the load. For audio amplifiers, the basic pulse frequency must be high enough so that it is out of the range of human hearing (well above 20 kHz). A simple low-pass filter at the output is typically made from an inductor and a capacitor.

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