Time/Gain Correction (TGC)
Time/Gain Correction (TGC) is the process whereby the amplitude of the ultrasonic waveform is increased or decreased based on the time of the signal. Using TGC, the operator can compensate for signal loss as the sound travels farther into the material of the part.
Setting up TGC
See also: TGC Profile Settings
For A/D boards with or without built-in TGC, the TGC Settings tab of the Digital Oscilloscope contains the TGC controls. This is displayed by pressing the Controls button of the Digital Oscilloscope.
A TGC profile is configured by setting one or more gains that will be applied over a time duration. Thus, the signal gain changes in proportion to these gains: as the gain increases, the amplitude of the signal is increased.
When a TGC profile is active, a line on the scope will show the position and dB gain of the profile point (or points). The color and thickness of this line can be changed in the profile settings. The position of the profile points can be changed using the mouse, similar to the way in which gate settings are changed: click with the left mouse button on any profile point, and drag it to the desired location. Grabbing the first profile point will move the entire profile as a unit.
Basic TGC
For a minimal linear TGC setup, a user can set a a desired end gain and duration. The TGC profile consists of a linear profile, starting with no gain and stopping when the Ending Gain and Duration have been achieved. This will show on the scope as a single rising blue line.
To Create a Basic TGC Profile:
- Select the TGC Settings tab
- Enter the desired starting offset (in microseconds)
- Enter the desired duration (in microseconds)
- Enter the desired dB Gain change from start to end
- Check the "Enabled" box
Once enabled, you should see the profile appear on the scope as a diagonal line.
Custom TGC
If a more complex profile is desired (e.g., changing the gain in one small time region, then restoring the original gain), then the full TGC profile should be used. This consists of a series of times and gains that are used to generate the profile.
To create a Custom TGC Profile:
- Select the "TGC Profile" tab.
- Check "Use Profile". Some additional buttons will appear.
- Enter the desired starting offset (in microseconds).
- Enter the "Desired Peak" value, e.g., if you wish all points to be at 90 %FSH, enter "90.0"
- Move a data gate over the first peak to be adjusted. Note that the gate *must* cross the peak.
- Press the "Add Point" button to add an entry to the TGC Profile. The selected peak should reach the desired %FSH value. Note: adding a point will automatically enable TGC
- Repeat the previous two steps with any additional peaks, moving farther out in time for each one.
Notes:
- When adding points using the "Add Point" button, the peak amplitude of the first available Data gate will be used to set the gain based on the "Desired Peak" setting.
- If there is no available peak and/or the Front Surface follower of the data gate is not crossed, no point will be added to the profile.
- If the data gate is tracking the FSF, the "Track FSF" option will be enabled automatically when the first profile point is added.
- The last point on the profile can be deleted using the "Delete Point" button.
Editing Profiles
To edit a Basic TGC profile, simply change the values on the form. The profile will be reprogrammed automatically.
With the Edit Profile form, the user can enable or disable points on a Custom profile, and adjust the time and gain for each point. Points must be sequential with no gaps. The more points are enabled, the more flexibility there is to design the profile.
Once points are established, they can be adjusted using the mouse cursor on the Digital Oscilloscope.
To reset the profile, press the "Clear Profile" button on the TGC controls or the "Clear" button at the bottom of the Edit Profile form: this will disable and clear all profile points.
Depending on the hardware present, the desired gain may not be achievable: the TGC profile will display the actual gain achieved at the right of the desired gain. By clicking on the column header, the user may see the input range applied at that particular point.
FSF Tracking and Offsets
By default, the TGC profile starts from the Main Bang (or time zero plus the Starting Offset; see below). To have the TGC start track a Follower crossing, click on the "Track FSF" option.
The first point in the profile (whether using the full profile or not) will always begin at the Starting Offset. For example, if tracking a Follower and the Starting Offset is set to 1 us, the first TGC point will be applied 1 us after the Follower crossing. If not tracking a follower, the Starting Offset of the TGC will be measured from the Main Bang.
Hardware limitations do not allow a Starting Offset of less than 0.12 microseconds. This limitation is emulated in software TGC for consistency.
Hardware TGC vs. Software TGC
Some A/D boards (such as the 8250 or 12250 Acquisition Logic digitizers) Have built-in TGC, that allows the signal to be altered before digitization occurs.
For those A/D boards that do not have a hardware TGC setting, Software TGC is used. This will run the TGC profile on the digitized waveform, resulting in similar (but not identical) enhancement to the ultrasonic signal.
The controls used to setup software TGC are identical to those used for hardware TGC.
FocusPX devices also contain hardware TGC. The TGC is applied to all waveforms within a BeamSet.
TGC using Multiple A/D Boards
An alternative method is to use two or more A/D boards to create Time/Gain correction on a signal. By feeding the same ultrasonic signal to multiple boards, different levels of A/D gain can be used to gate on the same signal.