MEDICAL
traces. This reduced space between traces also heightens susceptibility to crosstalk, either through magnetic fi elds inducing voltage on traces or electric fi elds causing parasitic capacitance.
Simulation before fabrication is no longer optional
To address the test challenges of high-speed digital hardware and reduce risk in your medical system, simulating board operation before you send the design to fabrication is critically important. While simulations may not uncover every potential issue, they play a vital role in identifying as many as possible. Simulation before fabrication offers several benefi ts. It helps you improve the actual performance of the board and reduce the number of required hardware prototype iterations, saving time and money. Simulation also accelerates the turnaround time for prototypes, minimising the need for unplanned board modifi cations you need to make, test, and document. It allows you to facilitate fi rmware and software testing with fewer erroneous signals that may be mistaken for bugs. Furthermore, the importance of resolving these issues is amplifi ed in advanced digital logic protocols such as PAM4, PAM8, PAM16, and NRZ, which are less tolerant to signal quality variations.
Use simulation software to provide signal integrity analysis of complex high-speed printed circuit boards (PCBs). See Figure 1 for an example of simulation software analysing the effects of jitter on an eye diagram. You can simultaneously characterise loss and coupling of signal and power nets. Simulation software will also help you extract and seamlessly transfer an accurate electromagnetic (EM) model for use in the ADS transient and channel simulators. You can also do a quick check for trace impedance to save time before committing to an EM extraction. This lets you reduce the risk of errors in your patient’s data or images.
Why it is important to measure signal integrity after PCBA fabrication
Even if you run signal fabrication simulation software and take great care in laying out and fabricating your printed circuit board assembly (PCBA), you should still use an oscilloscope to measure your board’s performance for signal integrity after it comes back from fabrication for many reasons. Variations within components persist despite adherence to specifi cations, which cannot be entirely accounted for by signal integrity software. Additionally, components often possess inherent
Figure 2: Oscilloscope analysing signal integrity issues.
parasitic analogue characteristics, such as impedance, capacitance, and inductance, which lack specifi cation. The surface-mount technology (SMT) component placement process is susceptible to variability, potentially resulting in off-centred components on pads. Also, the solder stencil process may not consistently produce perfect “bricks” of solder, with solder depositions diminishing in size over successive uses. The chemical composition and characteristics of solder may undergo slight alterations over time, impacting the bonding of solder joints between copper pads and SMT components. Moreover, normal variation in the refl ow oven can infl uence solder refl ow behaviour. Even if joints establish electrical connections, they may not be adequately soldered, as evidenced by scenarios
such as gull-wing parts lacking a heel fi llet, billboarded passive SMT resistors, or head-in-pillow ball grid array (BGA) joints. Using an oscilloscope post-fabrication is essential to comprehensively assess and address potential signal integrity issues arising from these factors.
Oscilloscopes are your window into the intricate interactions of complex designs. Look for an oscilloscope that lets you measure up to eight channels simultaneously to quickly fi nd signal integrity issues in your fabricated PCBA as shown in Figure 2. You can use an oscilloscope to conduct logic analysis, real-time spectral analysis (RTSA), serial protocol analysis, waveform generation, frequency response, and phase noise testing. Because the need
for increased data communication speeds in medical systems has led to higher data rates and closely spaced parallel data lanes, electromagnetic coupling, known as crosstalk, is increasing. In addition, power supplies can create interference on the data lanes they drive in the form of noise and jitter, and they are susceptible to data-dependent noise such as simultaneous switching noise (SSN), which leads to ground bounce. Oscilloscopes will detect and quantify the presence of crosstalk, determine which aggressors are primarily responsible, and detect and diagnose jitter, vertical noise, and phase noise.
High-speed digital technology is revolutionising complex medical systems by enabling advanced capabilities in areas including imaging, diagnostics, telemedicine, personalised medicine, and patient care, leading to more effi cient, effective, and patient-centred healthcare services. Delivering high-speed digital medical systems has always been challenging, and with faster data rates, higher resolution imaging, and more edge computing embedded into the system, the challenges associated with maintaining signal integrity appropriate to the application continue to increase. By using simulation software before PCB fabrication and oscilloscopes with powerful software after fabrication, you can reduce risk to patients and get your products to market with the effi cacy and safety required. Lorie Jurkovich, Healthcare Solutions Manager at Keysight Technologies
JUNE 2024 | ELECTRONICS FOR ENGINEERS 37
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