Spectrum analyzers sweep across the frequency range in discrete steps. Figure 2 below shows an example of such a step where the RBW was changed from 9 kHz in the first part of the sweep to 120 kHz after 12 MHz:įigure 2: Transition from 9 kHz to 120 kHz RBW at 30 MHz – Note steps in the base noise level.įigure 3: Spectrum analyzer DANL level versus resolution bandwidth. You may already have observed steps in test house plots, which are caused by switching filter bandwidth. The base noise level is a function of the measurement instruments Displayed Average Noise Level (DANL), the measurement transducer/antenna/probe, and the environmental RF present during the measurement. The smaller the resolution bandwidth (RBW), the lower the base noise level. Table 1: CISPR frequency range and filter bandwidth setting. In Figure 1 below, there is a comparison between the Gaussian and CISPR filter shapes:įigure 1: Gaussian and CISPR filter shapes.īesides specifying filter shape, impulse response, and sidelobe suppression, CISPR specifies frequency bands and the corresponding filter bandwidths that have to be used: Many analyzers use a Gaussian filter by default, so the user should select the EMI filter option if required. In order to be compliant with CISPR standards, the spectrum analyzer must additionally provide so-called CISPR-filters. Typically, the resolution bandwidth filters on spectrum analyzers use Gaussian shaped Intermediate Frequency (IF) filters with adjustable bandwidths that follow a 1 -3 -10 sequence, e.g. Standardized transducers such as Line Impedance Stabilized Networks (LISNs), Coupling/Decoupling Networks (CDNs), RF current probes, and others are used to establish interfaces with defined impedance, in order to connect 50 Ohm measurement equipment. Consequently, emission limits are predominantly specified in and amplitude units. In EMC pre-compliance applications, the impedance of EUTs and power supply sources is hardly predictable. is a logarithmic power unit, which makes sense, as input and output impedance of RF building blocks are typically designed to 50 Ohm. In RF applications, is the predominant amplitude unit. Standard related requirements 3.1 Amplitude units This document is mainly focused on the CISPR 16 standard to keep this application note as compact as possible.ģ. There are additional relevant standards, such as CISPR 25, Mil-461, DO 160, and more. Most prominent are the CISPR 16 and EN 61000-4 series. Several standards specify EMC test setups and requirements for measurement equipment. Measurement plots documented in this application note are created using a Siglent SSA3021X Plus, an entry-level EMI- spectrum analyzer with an excellent price-performance ratio. Optimizing the instrument is necessary to achieve a good compromise between high sensitivity and low distortion. Radiation limits and transducer characteristics also affect the required settings. EMC standard-related requirements determine the correct instrument settings of the RBW filter, video bandwidth (VBW), detector type, frequency span, and sweep time. Spectrum analyzers offer a wide range of parameter settings and need to be set up correctly in order to make measurements as close as possible to the requirements of the specific EMC standards that apply to the product’s design and end-use. Analyzers with EMC-specific features have become very affordable in recent years and these are usually sold as “EMI-options” that typically include CISPR filters and Quasi-Peak (QP) detectors in addition to the standard features of spectrum analyzers. There are also sensitivity considerations and tips for the protection of the spectrum analyzer during tests.Ħ.2 Conducted noise testing with RF current probesĦ.3 Radiated noise testing with TEM-cellsĪ spectrum analyzer is a key instrument for conducting EMC testing. In this note, we discuss EMC pre-compliance test parameters and how different settings affect your measurements.
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