The bit error rate is the number of error bits that occur over the total number of transmitted bits in a signal.
Adhering to strong layout practices ensures that the bit error rate is a random process rather than an indicator of design issues.
Testing for the bit error rate has to balance time and confidence concerns.
Communication services require a low bit error rate to promote reception quality.
People increasingly rely on the quality and reliability of signal transmissions for moment-to-moment device communication. Whether updating social media feeds or receiving GPS coordinates, networks need considerable data transmission volume with minimal error that can disrupt the integrity of the conveyed information. The bit error rate (BER) is a common quantitative tool that establishes the maximum accepted transmission error rate and serves as a quality check for board layout and component selection.
Factors Affecting the Bit Error Rate
The Effect of Board Layout on the Bit Error Rate
The BER analyzes the occurrence of bit errors over the total number of transmitted bits; the metric is an important measurement of complete system reliability from input to output. Over the length of the transmission path, best practices limit the occurrence of bit errors, assuring the signal that arrives is identical as transmitted (or nearly so). No component or transmission medium maintains perfect transmission quality: common circuit elements like amplifiers, filters, and converters (ADCs and DACs) introduce some error to the signal. The designers aim to simulate thoroughly, collaborating with test engineers on the prototype after production to keep the error rate acceptable.
The primary culprit of errors on the signal transmission line is noise, which reduces the signal-to-noise ratio (SNR). Noise can originate from many different sources during circuit layout:
Load switching - Traces with high current or voltage change (reported in most datasheets as dI/dt and dV/dt, respectively) can inductively or capacitively couple to nearby traces. Often, these signals are part of power circuitry; designers will want to provide ample space between nearby lines to promote signal integrity.
Component quality - Bit errors can arise during signal reconstruction due to the inherent limitations of ADC and DAC edge detection resolution capabilities. Filters and modulation networks can also experience bit errors related to the accuracy of the components.
Noise spectral density (NSD) - The BER is proportional to another rate: the per-bit energy over the NSD or the noise of a 1 Hz bandwidth. Reducing the bandwidth reduces the NSD (and, thus, the BER), but the Nyquist sampling theorem, which defines the minimum rate for effective signal reconstruction, limits this method. Similarly, increasing the energy per bit is possible, but this can incur additional coupling and interference due to stronger E/M fields. Reducing the total bit rate also increases the energy per bit, but this reduces total data throughput. Designers must carefully balance the data's accuracy, bandwidth, and volume to achieve the optimal implementation.
Although measures are necessary to minimize the overall prevalence of the BER, individual occurrences are a random process. This factor can complicate testing feasibility: while the limit of the BER approaches a value after an infinite number of transmissions, how many transmissions are necessary to determine this value practically? Simulation models can accurately predict this occurrence in a minimal amount of time while also simplifying the network construction for testing the device (i.e., only the necessary features of the radio network must be present).
Balancing Time and Confidence Requirements in BER Testing
System designers will instead utilize a confidence level to assuage the quality of the transmission. This approach offers a concrete value for testing accuracy and duration: for example, a 1 Gbps bit-rate system that requires a confidence interval of 99% and a BER of 10-12 would have a testing cycle of approximately 75 minutes. While increasing speeds can reduce the total test time necessary, the period is still excessive for most purposes, and the need for increased reliability with a smaller BER can rapidly extend this time frame. Instead, testing can lean on the association between the BER and SNR for a close approximation of the BER based on the assumption that random thermal noise is the dominant source of bit errors (i.e., the board layout is not contributing to bit errors).
A standard BER test setup transmitter sends a generated test pattern that travels through the device under test into a receiver against a test pattern that transmits directly from the transmitter to the receiver. The SNR method interpolates an attenuator between the direct path of the receiver and transmitter, assuming that the signal, not the noise, will be subject to attenuation. In effect, testing can attenuate the acceptable BER by the same level as the SNR, vastly reducing the testing time; however, the confidence level for this extrapolation method is not necessarily valid for the original duration of the target confidence level. For this reason, testing should beware of using extrapolation for anything more than ballpark checks of system BER and lean on the more extensive testing methods to confidently grade PCBA design.
Cadence Solutions Offer Robust Simulation for Signal Integrity
The bit error rate is a fundamental assessment of system reliability for signal transmission, and designers and testers will want to align design goals to meet modern reliability requirements, especially in the case of electronics where communication integrity is central. BER is only a random process when the design does not meaningfully account for noise contributions, usually the result of poor layout decisions contributing to EMI. Designers can trust Cadence PCB Design and Analysis Software suite for a comprehensive, constraint-driven analysis that catches manufacturing defects before they happen. Combined with OrCAD PCB Designer, ECAD for accelerated production schedules has never been easier.
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