Q & A with Vaunix: Lab Brick Attenuators, Windows, and USB

Posted on Tuesday, July 07, 2015
By Scott Blanchard

In our Q & A blog series, we’ve addressed some of the most important and technical questions about our Lab Bricks. We hope these answers give you a better understanding of how our products work.


Q. Can the LDA-302P-2 run from Windows and if so is it possible to guarantee its switching speed or does the Windows GUI just set up the profile and the microcontroller operate on the data? Is it also possible to vary the duty cycle between iterations? For example, at 1 ms we would like 0-dB attenuation and 4 ms at maximum attenuation, repeatedly continuously.


A. The switching speed for the LDA-302P-2 shown on our Web site and on our data sheets is defined as the time it takes to change from one attenuation state to another. It does not include the time required to pass the instruction from the computer to the device.


The minimum timing speed of the attenuator is controlled and determined by its firmware along with constraints posed by how the USB bus works, and how the software is implemented in Windows. With the attenuator at the lowest attenuation setting, the process runs each time there is a USB “start of frame” which occurs at exactly 1 kHz. Consequently, the minimum time between any commands is 1 ms, which is also the minimum attenuator dwell and idle time. The fastest the attenuator can ramp and the fastest it can be changed by directly issuing commands to set the attenuator level is thus 1 ms per step.


That said, a USB host controller and its software can potentially send out commands to the device either faster or slower than 1 ms apart. However, exact timing is difficult to define as it depends on many variables including the type of host controller chip, its driver software, and how the application software issues commands. Writing an application to generate a precisely-timed series of USB command outputs is not trivial, but the result would allow the unit to run at near its 1-kHz maximum rate. The response and status messages from the attenuator will not keep up with the output. Vaunix provides .dll files, and drivers for Linux and LabVIEW for developing custom programming interfaces.


Duty cycle can be controlled with the attenuator’s ramp feature. In your example this could be configured using the attenuator’s dwell and idle times along with the attenuator’s bidirectional feature controlled through the user interface software. In addition, the attenuator’s profile function can be used for more complex cycles and it allows 100 discrete steps to be defined at any setting. The profile can be set to run once or continuously.


If you have any questions about operating or programming Vaunix products or other topics, please send us an email.

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