DDC TR122 - Page 16

To Next Page

To Previous Page

The time recorded in memory will be equal to the number of samples stored in memory

(samples) divided by the sample rate (samples/second). The full depth of the memory is two

million samples. Therefore, at the full sample rate of 200 MS/s the memory will hold

2M/200MS/s or 0.01 seconds of time. At the slowest sample rate of 1 MS/s this expands to 2

seconds of time. The user is probably aware that both times can be considered fairly long

transient events.

In some cases it may not be necessary to record so much time. It may also be the case

that several events are expected to occur in rapid succession. It is possible to configure the

TR122 memory to capture multiple trigger events by dividing its full memory into smaller

segments. The Segment Size is measured in thousands of samples (K samples). The smallest

available segment size can be referred to as a block of memory. The block size of the TR122 is

64K samples. Note that all dimensions suffer from the binary number system employed by the

modern computer, so block and segment sizes will be powers of 2. For example, 64K samples

is 65536 samples. A segment in the TR122 can be sized as a power of 2 times the block size, up

to the size of the full memory. That is, the segment size can be selected as 64K, 128K, 256K,

512K, 1024K, or 2048K samples. The memory can therefore be divided into 32, 16, 8, 4, 2,

or 1 segments respectively. Thus, for short rapidly occurring transient events, up to 32 such

events can be stored in memory before the data must be read or a new recording started.

When a recording is started, the TR122 will immediately begin recording data in the first

segment of memory, filling the segment with samples and returning to the beginning of the

segment to start again in a continuous loop. When a trigger event is detected, the TR122 will

begin counting samples to be recorded after the trigger event, as defined by the user in the

control Post Trigger Samples. When this number of samples has been recorded, the TR122

moves on to the next segment and begins again recording pre trigger samples.

Clearly for sample data in this circular buffer architecture to be sensible, the segment

must be fully written with pre or post trigger samples related to the trigger event. To guarantee

that this happens, the TR122 will not be sensitive to a trigger event until the segment has filled at

least once with pre trigger samples. Once a trigger is received, the TR122 will not be sensitive to

another trigger until all post trigger samples have been recorded and the next segment has been

filled with pre trigger sample data. Therefore, the minimum time between trigger events will be

defined by the memory geometry selected as

Minimum time between trigger events = (segment size / sample rate) + (post trigger samples / sample rate)

Internally to the TR122 software and hardware, the number of Post Trigger Samples can

be defined to any multiple of 2. However, this resolution is rarely needed. The TR122 control

panel allows the Post Trigger Samples control to be adjusted in steps of 512 samples. The limits

will require there to remain at least 1K (1024) pre trigger samples and at least 1K post triggers

samples within the segment. When selecting a segment size, the Post Trigger Samples control is

automatically adjusted to half the segment size. The shortcut buttons under the control allow

setting commonly used values for Post Trigger Samples with one click.