With the development of modern electronic technology, the integration level of various electronic devices is becoming increasingly higher. The introduction of numerous high-speed buses and RF functions has led to increased electromagnetic radiation issues within these devices. Simultaneously, as users place increasingly stringent requirements on the electromagnetic compatibility (EMC) of purchased products, product design engineers must strive to reduce the EMC interference (EMI) of their designs. Manufacturers are also investing heavily in EMC compatibility testing and certification for their products.
The typical EMC design and certification process can be divided into three stages: pre-test, pre-compliance, and compliance test. Product certification involves stringent standards and requirements for testing equipment and facilities. Many companies complete this stage through third-party professional laboratories, conducting only pre-testing and pre-compliance internally. The traditional design process involves handing the product design over to the product testing department for pre-compliance. If problems are found, the product is returned to the design department for rectification. However, such rectification often requires adjustments to the circuit design, even involving PCB modifications, which is usually complex, time-consuming, and costly. Designers hope to identify and correct problems early in the circuit design process to avoid extensive rectification work later. However, we must face the challenge that EMC equipment is generally expensive, and equipping each design department with dedicated EMC testing equipment is a significant investment. Furthermore, another emerging issue is that with the application of burst communication and high-speed buses, some EMI problems are no longer continuous phenomena; their occurrence is usually related to the interval and rate of burst communication. Traditional EMI receivers have limited ability to capture these problems. More importantly, even if such intermittent EMI problems are discovered at a certain frequency, it does not provide designers with enough information to diagnose the cause.
When faced with these problems, engineers will think of the most commonly used debugging tool—the oscilloscope. If the oscilloscope can provide EMI testing capabilities, it will greatly reduce the overall design cost. If it can also provide diagnostic information on the causes of EMI, the engineer's design and rectification process will be greatly accelerated.
Unfortunately, the vast majority of oscilloscopes on the market are incapable of performing this task. Although almost all oscilloscopes have FFT spectrum analysis capabilities, such functions are often inaccurate, have low update rates, and lack flexible settings, failing to meet the requirements for EMI pre-testing.
The insufficient FFT function of oscilloscopes is mainly due to the following reasons: First, insufficient dynamic range. Oscilloscopes typically have an 8-bit A/D converter, but the actual number of usable bits (ENOB) is often less than 6 bits. In this case, it is impossible to accurately measure minute RF signals. Coupled with the significant noise from the analog front-end, the oscilloscope's dynamic range is only about 40-60 dB. Rohde & Schwarz (R&S) RTO oscilloscopes have an ENOB with a full bandwidth >7 bits, and when combined with an RF instrument-grade analog front-end, the noise is below 100 µV. This ensures that RTO oscilloscopes can provide a dynamic range 15-20 dB greater than comparable products.
Secondly, there are issues with algorithm and function implementation. Ordinary oscilloscopes lack the down-conversion circuitry of spectrum analyzers, making it impossible to set the center frequency and analysis range, and the RBW cannot be adjusted; the displayed spectrum is essentially illustrative. Adding samples for FFT calculations would significantly reduce the spectrum update speed. The R&RTO oscilloscope's FFT function is implemented in hardware. Using a dedicated digital down-converter, users can set the center frequency, analysis range, and RBW like other spectrum analyzers. Hardware-implemented FFT calculations are fast and accurate, unaffected by other functions and settings; even with multiple measurement and analysis functions enabled, the spectrum update speed remains constant. The multi-overlap FFT algorithm, ported from real-time spectrum analyzers, can observe sideband information unseen by similar devices, greatly reducing waveform loss. Another important feature of the RTO oscilloscope's spectrum analysis function is its speed and real-time performance. Rapid spectrum updates ensure the capture and display of occasional EMI problems. No electromagnetic radiation issues in the circuit will escape the RTO oscilloscope's FFT function. With its unique frequency template triggering and MSO functions, users can set an EMI trigger condition. This means that within a specific frequency range, the oscilloscope will trigger and capture any EMI signal that appears. At this point, not only can the EMI signal itself be observed, but by combining this information with the time-domain signals from other analog and digital channels connected to the circuit, we can also analyze what caused the EMI event. Examples include data transmission via a high-speed USB interface and the wake-up status of a 2.4GHz RF module.
A key purpose of using an oscilloscope for EMI pre-testing is to perform tasks similar to those of an EMI receiver. RTO oscilloscopes offer a test interface similar to that of an EMI receiver. Using the MASK test function, we can set a template conforming to EN standards and observe whether the EMI emission status of the entire system meets the standard requirements. For example, we can refer to the EU standard EN55015Q for information technology equipment to perform electromagnetic radiation tests on the product under test from 9kHz to 30MHz.
Essential tools for EMI detection include EMI probes and preamplifiers. R&S offers a complete range of electric and magnetic field probes, allowing users to easily detect EMI issues in various chips, circuits, and interfaces. External preamplifiers with bandwidths up to 3GHz help users capture even minute EMI signals.
Oscilloscopes are among the most frequently used tools for electronic engineers. When using them for EMI pre-testing, oscilloscopes must meet requirements such as high dynamic range, low spurious emissions, high waveform capture rate, and high-speed FFT calculation capabilities. Using template test functions, engineers can also verify relevant standards and diagnose intermittent EMI. R&S' RTO oscilloscopes combine the company's years of experience in RF, communications, and digital design, especially EMC testing. Employing dedicated ASIC circuitry, they possess powerful EMI testing and diagnostic capabilities, making them an invaluable tool for electronic engineers.
