Reasons for the modification

Instruments with uncertain trigger timing -- Some ultrasonic systems when externally triggered are not immediately able to generate their main output pulse. These systems include flaw detectors with trigger jitter, multiplexers that need to first switch to the next channel and EMAT systems that first need to build up energy.

This delay can pose a problem when using a Sonix STR81G digitizer under software control since the card starts its delay/acquisition at the same time as it generates its outgoing trigger signal. The digitizer acquisition cycle could theoretically be finished before the external pulser has even had a chance to generate an output pulse. Most external pulsers, when they finally do fire, will generate their own outgoing trigger signal (sync) that we can use to trigger the digitizer.

In these cases, it is necessary to operate the digitizer in a mode where it can first trigger the ultrasonic instrument and then wait for a trigger from the instrument that starts the digitization. This modified operating mode removes trigger jitter, set-up delays and any other time dependency from the A-scan acquisition.

Simultaneously triggering multiple digitizers -- It is not possible to software trigger multiple digitizers at precisely the same time. Since each card is triggered by its own I/O write, a number of computer cycles of time separates each trigger event. This delay is not acceptable for phase sensitive applications.

General description of the modification

In all of the above cases, it is possible to simply modify the Sonix card so that it outputs a pre-trigger enable when the digitizer is armed by a software write.

The digitizer's BNC "Clk IO" connector (which is not need for most configurations) is used to conveniently route the signal off of the card. The modification simply involves the removal of a socketed resistor and the installation of a jumper.

This modification does not in any way affect the normal operation of the Sonix card. If your system did not previously use the Clk IO signal, the modified card will work just as before. It is also very easy to reverse the modification.

You will require:

• An STR81G digitizer
• A small-tipped soldering iron
• 50mm of thin hook-up wire

Modification steps

All modifications are performed on the bottom right-hand side of the digitizer on the component side. All mentioned components are very close to the "Clk IO" connector which is the third from the top of the four BNC connectors.

1. Remove the 20 ohm resistor R502 (it is socketed).
2. Insert a jumper between Pin 2 of J500 (the rightmost pin) and Pin 10 of U408 (It is located 3 rows above the left gold ISA connector). The jumper does not need to have have an optional resistor placed in series since R503 (49.9 ohms) is already in series with the outgoing signal. R503 will stop any external loading of the enable signal from affecting the proper functioning of the digitizer. It is necessary to carefully tack the finished jumper to the side of the leg of Pin 10 being careful not to form a solder bridge.

U408-10 is the closest IC, however you may also connect to the enable (ready) signal on any of the following components: R426-2, R427-1, U408-10, U410-9, U501-6, U503-13, U504-8

How the modified digitizer is used

Two cables are needed to perform jitter-free triggering using the modified digitizer.

The first cable is connected from the modified CLK output on the digitizer to the EXT TRIGGER input of the flaw detector.

The second cable is connected from the TRIGGER OUT (SYNC) output of the flaw detector to the TRIG input on the digitizer.

When used with Winspect, both the digitizer trigger mode and the flaw detector trigger mode should be set to EXTERNAL. When collecting data, Winspect enables the digitizer which then waits to be triggered. The flaw detector is triggered by the newly modified ENABLE signal which appears at the CLK output of the digitizer. When the flaw detector finally fires its pulser, it generates its own TRIGGER OUT signal which starts the digitizer via the digitizer's TRIG input.