Case Studies
Laser-Based Position Measurement Systems
THE PROBLEM
In order to test their Laser-Based Position Measurement Systems, the customer needs to characterize an output pulse from his circuitry which lasts for approximately 5 microseconds. According to the customer's estimates, he must use a sampling rate of 50 MSPS or higher in order to be able to analyze the signal completely.
An additional requirement of the test system is that a specific number of successive pulses be digitized and then averaged in order to eliminate system noise. The number of such successive pulses must be variable. The repeat rate of the pulses is 60 Hz. Therefore, the time between pulses is 16.66 milliseconds. Finally, the test software will be written in LabVIEW for Windows 95 (or NT).
GAGE'S SOLUTION
After discussions with the customer, it was determined that the dynamic range requirement of this application makes it impossible to use an 8 bit digitizer. Therefore, 8 bit CompuScopes and/or Digital Storage Oscilloscopes cannot be used.
As such, the customer must use a 12 bit CompuScope card. The sample rate requirement of 50 MSPS means that the customer must use at least a CompuScope 6012 (60 MSPS, 12 bit A/D Card).
The only unknown factor which remains is whether the customer should use an ISA or a PCI bus CompuScope. To narrow that down, we must calculate the bus transfer rate required. If the transfer rate of the ISA bus is sufficient, the customer should use an ISA bus card, otherwise a PCI bus CompuScope is a must.
This application requires 5 microseconds of data to be sampled at 50 MSPS with a repeat frequency of 60 Hz. To calculate how many points will have to be captured and transferred for each pulse, we will use the equation:
Record Length (in points) = Time of Capture (in seconds) * Sample Rate (in Hertz)
= 5 m s * 50 MHz
= (5 * 10-6) * (50 * 106)
= 250 points
We know that the bus throughput across the ISA bus is 1 MegaSample Per Second. Transfer Time can then be calculated by using the equation:
Transfer Time (in seconds) = Number of Points to Transfer / Throughput Across ISA Bus
= 250 / 1,000,000
= 250 microseconds
This calculation shows that there is more than enough time to transfer the data across the ISA bus before the next pulse comes in (16.66 milliseconds later).
An Alternative Method
An alternative method is to use the Multiple Recording feature to capture and store successive pulses in the on-board memory of the CompuScope 6012. With a standard memory depth of 512K points, there is room to store as many as 2000 pulses in the on-board memory. If more pulses are needed, the on-board memory can be expanded to 1 Meg, 2 Meg, 4 Meg, 8 Meg or even 16 Meg.
The advantage of this alternative method is that the customer does not have to write "real time" software under LabVIEW. With reduced complexity of the test software, the customer can get the system up and running within days-a definite advantage.
When reading back the pulses, the customer will be able to read any number of pulses he wants and then average the data and then analyze it. For example, if it is desirable to average 100 pulses, the customer will be able to transfer exactly 100 pulses (100 Multiple Record Acquisitions of 256 points each) into the LabVIEW program, which can then do the averaging and further analysis.
Note:
Normally, if the customer decides to write software under LabVIEW for Windows NT the software timing is not as clear, as under Windows NT, I/O accesses to the hardware can take between 300 microseconds and 1 millisecond per access!!! Under DOS, the same I/O access would have taken 1 microsecond.
LabVIEW makes the situation even more unclear, as it has its own layers of software.
To properly tackle this problem, Gage has written its Windows NT drivers to be "packet driven," i.e. all hardware accesses occur through a kernel mode driver and the user application just passes it a message (thereby not encountering an overhead of 300 µs per access but only one 300 µs delay per message).
PRODUCT RECOMMENDED
CompuScope 6012 -- 12 bit, 60 MSPS A/D card for the ISA Bus
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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