The global medical electronics market is vast and has experienced rapid growth in recent years. Medical electronics range from highly complex systems like MRI machines to simple devices such as home blood pressure monitors. Manufacturers of medical electronic equipment aim for the highest possible measurement accuracy, but in reality, accurately measuring human body indicators is extremely challenging.
During testing, engineers often input known test signals into a device under test. These signals are real-world analog signals, and the device's response is observed. This method applies equally to testing medical devices. However, when these devices are used in actual clinical settings, signals must be acquired directly from the human body, introducing additional complexity.
Two main challenges arise in this context:
1. There is no strictly repeating (periodic) signal source. For example, each heartbeat or neuron electrical pulse is slightly different from the previous one and cannot be fully replicated.
2. The signal-to-noise ratio (SNR) of human body signals is generally worse than other types of electrical signals.
In medical devices, measurement accuracy is crucial. From a testing perspective, when noise levels are high, the instrument should ideally capture and measure a single signal with high precision—rather than averaging multiple non-repeating signals.
**Generating Test Signals**
Before a medical device reaches the clinic, it undergoes extensive testing during its development phase. Even in the design stage, engineers simulate signals that the device will eventually capture from the human body. These simulations ensure that the device functions correctly before it’s delivered to users.
Figure 1 shows a test signal generated by an ArbStudio arbitrary waveform generator. The signal was captured using a digital oscilloscope (DSO) and contains several pulses. Each large amplitude pulse is preceded and followed by smaller pulses, mimicking an electrocardiogram (ECG). The larger pulse is called the “R wave,†while the smaller preceding pulse is the “P wave.†This waveform is modulated slowly and then output through ArbStudio, which can generate both arbitrary and standard function waveforms.
ArbStudio allows users to output waveforms in single, multiple, or continuous cyclic modes. It also enables editing and modifying real waveforms, such as adding or subtracting noise or glitch signals. Teledyne LeCroy offers two arbitrary waveform generators: ArbStudio for long storage and complex waveforms, and WaveStation for short storage and basic waveform generation.
**Measuring Signals**
If we focus on the small P wave in the ECG signal shown in Figure 1, the lower curve represents the amplified version of that section. This waveform is typical when measuring noise levels in specific parts of the signal. Oscilloscopes provide parameters like peak-to-peak and RMS noise measurements. However, if the goal is to measure the baseline of the signal, which may be buried in noise, traditional methods like averaging are not effective due to the aperiodic nature of the signal.
Another approach is to use filters. If the noise frequency is much higher than the signal of interest, a low-pass filter can help reduce noise. Many digital oscilloscopes come with built-in low-pass filters, and Teledyne LeCroy offers the Digital Filter Package (DFP), allowing users to customize cutoff frequencies with various filter types.
In Figure 2, the same signal is displayed in Hi-Res mode, which reduces noise but alters the waveform significantly. This makes it unsuitable for precise clinical analysis.
In contrast, Figure 3 shows the same signal captured using a 12-bit high-resolution HDO oscilloscope. The noise level is dramatically reduced, and the waveform closely resembles the original. Further filtering in Figure 4 reveals the signal’s baseline, highlighting an important feature at the end of the P wave—an undershoot that can be critical for medical diagnosis.
**Summary**
Testing medical electronic devices often involves capturing and analyzing single, noisy signals. Accurate measurement requires understanding the true noise level of the signal, not just noise added during the process. Teledyne LeCroy’s HDO 12-bit high-resolution oscilloscope is ideal for this task, offering superior noise reduction and signal clarity. Combined with ArbStudio, which can simulate real human signals, engineers can capture and replay real physiological data for thorough testing.
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