The Definition of Sampling Frequency

Ultrasonic thickness gauge sampling frequency refers to the frequency of the instrument on the ultrasonic echo signal digital acquisition, that is, the number of times per second on the analog signal sampling (unit: Hz or MHz).

According to Nyquist sampling theorem, the sampling frequency must be at least 2 times the highest frequency of the signal in order to restore the original signal without distortion. In ultrasonic thickness measurement, the highest frequency of the signal is mainly determined by the center frequency of the ultrasonic probe (such as common probe frequency of 1-10MHz). Therefore, the sampling frequency of the thickness gauge usually needs to be 5-10 times the probe frequency (e.g. 10MHz probe with 50-100MHz sampling frequency) to ensure that the details of the echoed signal are accurately captured.

Frequency Parameters that are Affected by Sampling Frequency

Time Resolution and Thickness Measurement Precision

• Time Resolution: The higher the sampling frequency, the smaller the time interval between two adjacent sampling points. For example, a 100MHz sampling frequency has a time interval of 10ns, while a 50MHz sampling frequency has a time interval of 20ns.
• Thickness Precision:Thickness calculations are based on the time of flight of the ultrasound through the material. The higher the time resolution, the smaller the measurement time error and the higher the thickness accuracy.

Distance Resolution

Distance resolution is the minimum distance between two neighboring interfaces that the gauge can distinguish. High-frequency sampling allows for more accurate identification of the rising and falling edges of the echo signal, especially when measuring thin-walled materials or multi-layer structures, avoiding false determinations due to echo overlap.

Signal processing capability and noise immunity

• Signal details: High sampling frequency retains more waveform details (e.g., amplitude, phase) of the echo signal, which facilitates subsequent signal processing (e.g., filtering, peak detection, defect identification).
• Noise effects: theoretically, the higher the sampling frequency, the more sensitive to high-frequency noise (e.g. electromagnetic interference), but modern instruments are usually equipped with anti-alias filters, which can filter out invalid high-frequency noise before sampling.

Hardware and data processing limitations

• Hardware Costs: High frequency sampling requires high speed ADCs (analog-to-digital converters), high performance processors, and large amounts of memory, resulting in higher instrument costs.
• Data transfer and storage: Higher sampling frequencies generate larger amounts of data, which may affect real-time display speed or require external storage devices.
• Power Consumption: High-speed hardware typically consumes more power, which affects the endurance of portable devices.

The Applications of High Sampling Frequency

1.Scenarios Where High Sampling Frequencies Are Recommended

Thin-wall measurements (e.g. metal/plastic parts <1mm thick): multiple echoes over a short period of time need to be accurately captured.

High-precision inspections (e.g. aerospace component thickness measurements requiring ±0.01mm accuracy): time resolution directly affects the results.

Complex materials (e.g. coarse-grained materials, multi-layer composite structures): echo signals may be weak or distorted, requiring high-frequency sampling to preserve details.

2.Limitations of High Sampling Frequencies

Thick material measurement: When the thickness is large, the echo time interval is long (e.g. 100mm steel, the flight time is about 40μs), low sampling frequency (e.g. 50MHz) can also meet the time resolution requirements (50MHz sampling interval of 20ns, 40μs can be 2000 points), this time, increase the sampling frequency is limited to improve the accuracy.

Cost and practicality: for routine thickness measurement (e.g. 1-100mm), too high a sampling frequency (e.g. 200MHz or more) may result in excessive performance, increasing operational complexity and equipment costs.

Probe matching: the sampling frequency needs to match the probe frequency (usually recommended ≥ 5 times the probe frequency), if the probe frequency is low (e.g., 1MHz), blindly increase the sampling frequency (e.g., 100MHz) will not improve the resolution (limited by the probe's own bandwidth).

3.The Most Proper Practice

Select according to the measurement range and accuracy requirements: thin-walled/high precision→high sampling frequency (≥100MHz); thick-walled/regular precision→medium sampling frequency (50-100MHz).

Attention to the cooperation between the probe and the instrument: make sure the sampling frequency meets the demand of the probe bandwidth, and at the same time, consider the signal processing algorithms of the instrument (e.g., the optimization of the peak detection and echo recognition algorithms may be more important to the accuracy than simply increasing the sampling frequency).