Measurement Computing WBK Options - Manual (Page 169)

WBK30, pg. 6 988296 WBK30, Memory Options
Question: When using a WBK30, how do I calculate the length of the acquisition?
Answer: Before we answer, consider the following analogy: the WBK30 can be thought of as a water
bucket with a hole in it. You fill the bucket at one rate and empty it at another. The time to fill the
bucket is determined by the size of the bucket and difference between these two rates.
Calculating the maximum acquisition length.
This calculation can be broken down into three steps:
Step 1: Determine how long it will take to fill the memory. The following equation is used.
T = Capacity / (RateA – RateB)
Where: T = Time to fill the memory
Capacity = Memory Capacity
RateA = acquisition rate
RateB = transfer rate
The time-to-full memory is simply the memory capacity divided by the difference in rate of filling
and emptying. This value (T) is used in step 2.
Step 2: Determine how much data transfers before the memory is full.
Data Transferred = T * Transfer Speed
The “Data Transferred” value will be used in step three.
Step 3: Determine the acquisition length. This is a combination of the data transferred plus the
data still in the full memory that is waiting to be transferred. The equation is:
Maximum Acquisition Length = Data transferred + Memory Capacity.
Example: You need to acquire 10 Msamples of data at 500K samples / second and are
considering a WBK30/16 because your PC is only capable of transferring
200Ksamples / sec. How much data could you stream to disk before the memory is full
and data collection stops? Is the WBK30/16 adequate?
WBK30/16 holds approximately 8.3 Msamples of data so
[Step 1] Determine how long it will take to fill the memory. In this example:
T = Capacity / (RateA – RateB)
T = 8.3M / (500K – 200K) = 27.6 seconds
[Step 2] Determine how much data will be transferred before the memory is full.
Data Transferred = T * Transfer Speed
Data Transferred = 2 7.6 secs * 200K = 5.52M
[Step 3] Determine the acquisition length.
Maximum Acquisition Length = Data transferred + Memory Capacity.
Maximum Acquisition Length = 5.52M + 8.3M = 13.82 Msamples
Since you needed to acquire 10 Msamples and our Maximum Acquisition Length with the
WBK30/16 calculated out to be 13.82 Msamples the WBK30/16 is seen to be sufficient.
Note that this example does not take into account the data packing capabilities of the
WaveBook/512. With data packing, acquisition lengths would be 25% greater.