Test & Measurement Application Case Study - Data Acquisition Electronics / Device Testing

Test & Measurement Applications

Disk Drive Track Scanning

Customer Case

The customer is trying to digitize an 83 millisecond long signal coming out of a disk mounted on a rotary motor. This corresponds to one full revolution of the disk.

The analog signal coming out of the Device Under Test (DUT) is of approximately 2 MHz bandwidth. Currently, the customer is sampling at 10 MSPS and would like to increase the oversampling to get better time resolution.

At present, they are using a CompuScope 225 - 1M, which has sufficient memory to digitize the entire revolution in on-board memory at the 10 MSPS rate, but not at the faster rates.

GaGe Case Solution

Let us first calculate how many data points are needed to sample the entire revolution in on-board memory at different rates.

Number of Points = Record Length (in sec) * Sample Rate (in Hz)
At 10 MSPS rate, Number of points = 83 ms * 10 MHz = 830 Kilosamples
At 25 MSPS rate, Number of points = 83 ms * 25 MHz = 2,075,000 samples = 1.978 Megasamples

There are two solutions for this situation:

Option #1: Use a CompuScope 225 - 2M

The obvious advantage is that the entire revolution can be captured in the on-board memory. This way, the customer's application program can remain exactly as it is. The disadvantage is that it will cost $1,000 extra.

Option #2: Use the CompuScope 225 - 1M with Modified Software

The customer can still use the CompuScope 225 - 1M board they already have and simply modify their current software to capitalize on the fact that the input signal is repeating after every trigger; i.e. while the signal is arbitrary, it is repetitive. The start of the signal is signified by an INDEX pulse which is used as an External Trigger for the CompuScope 225

Disk Drive Track Scanning Example

The idea is to divide up the digitization into two sections: first digitize the first half of the revolution and transfer the data to PC memory; second, digitize the second half of the revolution and transfer data to PC memory.

The "First Capture" is very straightforward. The customer will capture 1 Megasample of Post Trigger Depth. Once the acquisition is finished, they will transfer the data to PC memory and perform the "Second Capture".

In order to perform the "Second Capture", the customer will simply set the Post Trigger Depth of the CompuScope 225 to be 2 Megasamples and perform an acquisition. What this will do is force the on-board memory to be filled up twice before the acquisition is finished.

In other words, they will capture the first half of the revolution and immediately start overwriting it with the second half of the revolution. This does not matter, though, as they have already captured and transferred the first half in the "First Capture".

It should be kept in mind that this scheme will work only if the input signal is repetitive over many cycles. The "Second Capture" will happen many revolutions after the "First Capture".

GaGe Case Recommended Products

Option #1:

  • CompuScope 225 - 2M - GaGe's 8 bit, 50 MSPS A/D card for the ISA bus with 2M of Onboard Memory

Option #2:

  • CompuScope 225 - 1M - GaGe's 8 bit, 50 MSPS A/D card for the ISA bus with 1M of Onboard Memory with Modified Software

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