DocID10909 Rev 2

9/13

AN2052

Analysis and possible further development

13

Figure 13. Configuration “B” peak

One remarkable point is that the peak value of the three tested configurations is almost the

same. The major difference between peak and average measurement is in the filter and in

the measurement time. Wide-band filter and short measurement time are used to capture

the peak value; while narrow-band filter and relatively long measurement time are used to

capture the average value. Through web search engines it is possible to find the definitions

for the terms “peak” and “average” as “spectrum peak” and “spectrum average”,

respectively. These definitions are different from the “time-peak” and “time-average”

defined in this report. Instead, the term “frequency modulation” can be found on the web

as “spread spectrum”. The spectrum peak is the highest component of the frequency

spectrum. In EMI test data of SMPS, it corresponds to a low frequency.

The spectrum average is the average noise over the whole frequency spectrum. The

time peak is the “peak” data measured by most EMI test lab. Noise, in terms of dBuV/m,

is measured through a wide-band filter within a short period of time. Analysing such

measurements, the fact that FM does not bring benefit is highlighted. The

measurements obtained are very similar to the spectrum envelope of Figure 11.

The time average is the “average” data measured by EMI test lab. Noises are

measured through a narrow-band filter, over a relatively long period of time.

Note that there is no regulation on the measurement average time.