Doug Smith has a diagram called the Measurement Confidence Factor, which is shown here. It shows that most engineers' confidence in a measurement is monotonically related to the cost of the test equipment. However, this is not always true. Even with expensive equipment, measurement mistakes can occur. These mistakes include errors when making contact measurements (such as probing using a 10X passive probe) where the device under test is loaded by the probing activity itself. Sometimes, capacitive loading of a circuit can occur simply because an engineer gets too close to the circuit. A human hand getting close to a circuit can divert RF current from the DUT to earth.

The point here is that perhaps you don't always need expensive equipment (although expensive equipment often provides better accuracy and quality). Instead, it's crucial to understand the principles of high-frequency measurement to avoid the pitfalls of high-frequency measurement.

Two case studies are demonstrated in the video below:

Typically, a small to medium engineering firm may have some basic test equipment for developing their products, much of which may not be suitable for EMI diagnosis. However, in today's market, with a budget of around 10,000GBP, companies can establish a reliable EMC pre-compliance test set-up, which enables engineers to perform both emission and immunity checks before sending the product to an accredited EMC lab. The affordability of equipment, such as spectrum analysers, oscilloscopes, and pre-compliance EMC kits, has significantly contributed to this accessibility. As an example, a budget-friendly benchtop spectrum analyser with a frequency range of 1.5GHz can now be acquired for under 1,000GBP. Similarly, an oscilloscope boasting a bandwidth of 300 MHz is available for less than 1,000GBP as well. Additionally, several manufacturers have substantially reduced the prices of equipment like LISNs, CDNs, and RF amplifiers.

A valuable resource for selecting EMC pre-compliance test kits is Ken Wyatt's EMC workbench troubleshooting trilogy bookset, starting with the first volume titled "Create Your Own EMC Troubleshooting Kit." Another useful reference is Tekbox's application notes on pre-compliance EMC. For video demonstrations, check this Youtube list out.

Spectrum Analysers vs Oscilloscopes

A spectrum analyser is always essential for serious EMI troubleshooting work. Some engineers familiar with the advanced FFT functions in certain types of oscilloscopes have successfully troubleshooted EMI issues by using the FFT function of a scope. However, this method has many limitations. For instance, the input impedance of a 10:1 passive probe is in the MΩ range, whereas the input impedance for EMC work is often 50Ω. The spectrum analyser's resolution bandwidth (essentially a moving window filter) is designed for spectrum analysis, and the user interface is also designed to show the spectrum much clearer than an oscilloscope. This is not to say that an oscilloscope is less useful. Oscilloscopes are essential for time domain measurement and troubleshooting work (e.g. ESD, EFT, surge, etc).

When selecting a spectrum analyser for troubleshooting work, the following characteristic need to be considered:

•Acquisition type – swept or real-time? A swept-type spectrum analyser is usually adequate; but if your products include wireless, digital modulations or otherwise include intermittent emission peaks, a real-time spectrum analyser will be more useful. For real-time spectrum analysers, the real-time IF bandwidth is the most important feature.

•Frequency range – Most spectrum analysers in the market cover beyond 1 GHz, which is often good enough for benchtop EMI tests. Higher frequency range is preferred, but it comes with added cost.

•Resolution bandwidth (RBW) – without additional features (paid upgrade), most spectrum analysers cannot do 9kHz, 120kHz RBW as defined in EMC standards. But the 10kHz and 100kHz RBW are often close enough. If you have the budget and want to get some good pre-compliance results, then an upgrade is worth considering.

•Additional features – A tracking generator (TG) function is extremely useful for assessing filter circuits, antenna structure, etc. It can also act as an RF signal source to feed into RF amplifiers for immunity tests.

•Software compatibility – For troubleshooting and quick tests, you don't need any additional software. However, if you want to perform a full scan sweep and generate a report to share with your colleagues, it's worth considering an easy-to-use software bundle.

The figure shows a typical benchtop test set-up consisting of a spectrum analyser, an RF current probe, and LISNs.

When selecting an oscilloscope for EMI troubleshooting, the most important factors are bandwidth and sampling rate. We recommend models with at least 500 MHz bandwidth for EMC work. The reasons are:

•With transistors switching faster, we need higher bandwidth and sampling rate to capture a fast rise time (less than 5 ns).

•To troubleshoot transient events such as an ESD, where the current discharge time is less than 1 ns.

A built-in 50Ω input impedance in each channel is also extremely useful for EMC work. One can, of course, purchase some feed-through 50Ω terminators. However, caution needs to be taken, as some low-cost terminators have very high stray capacitance, which can affect measurement results, especially at high frequencies. Figure 2 shows a 300 MHz battery-powered portable oscilloscope used for troubleshooting EFT/Burst failures in the field. Three RF current probes are connected to Channels 1-3, all with built-in 50Ω impedance. Current readings can be calculated using the known transfer impedance of the probes.

Table 1 lists the functions of spectrum analysers and oscilloscopes in EMI troubleshooting. As one can see, the input devices include some of the most useful troubleshooting tools in EMC engineering, which we will discuss in detail in the following chapters.