Gain Adjustment
Adjusting the gain is one of the most common tasks when shaping the ultrasonic waveform. By increasing the gain, low-amplitude features can be made visible that would otherwise be lost in the signal. Conversely, and increase in gain will also increase any noise present. Using the right combination of transducers, pulser, A/D input range, and focus is critical to generate a proper signal with minimal noise.
Gain Adjustment using the A/D Input Range
Most A/D boards supported by ODIS have an adjustable Input Range value. This changes the voltage range at which the A/D board will sample. By adjusting this range, the user can create an effective increase in gain of 24 dB or more, depending on the initial Input Range setting.
The input range is adjusted via the Setup screen (Basic Mode), or the Board Settings tab of the Digital Oscilloscope (Advanced Mode).
Every time the input range is halved (e.g., going from 1 V to 500 mV), the gain of the signal is increased by 6 dB.
The A/D does not add any additional energy to the signal when adjusting the Input Range: it only changes the range in which it will sample. Thus the input range adjustment minimizes noise when enhancing a signal.
Gain Adjustment using a Pulser/Receiver
Digital or Analog Pulser/Receivers contain some form of gain adjustment as well. This boosts the signal returning from the transducer in order to enhance low amplitude results, similar to the way the volume setting on a speak increases or decreases the loudness of the audio.
Different pulsers have different means of adjusting gain: if the pulser is digitally controlled, a simplified gain setting will be available in Basic Mode. Full control over the gain settings can be found in Advanced Mode, where each pulser/receiver has a separate interface.
See also: Pulser/Receiver Settings.
Adjusting the gain increases the sensitivity of the receiver, thus increasing the apparent amplitude of the signal: however, this can also introduce noise into the resulting waveform. If too much gain is used, the useful signal can be swamped.
Gain Adjustment on a FocusPX Device
When using the FocusPX Phased Array device, gain can be defined using the Focal Law or applied manually using the Analog Gain and the Sum Gain. Together, Analog Gain and Sum Gain cannot exceed 80 dB.