The use of bandwidths in EMC measurements is important but often confusing. The term bandwidth can have many meanings which may be unrelated: Receiver bandwidth; Resolution Bandwidth or RBW; Video Bandwidth or VBW; 3 dB or 6 dB Bandwidth; Broadband or narrowband bandwidths.
Receiver bandwidth is the frequency range in which the receiver is designed to function. A spectrum analyzer may be specified to function from 9 kHz to 7.5 GHz, which is its receiver bandwidth. An oscilloscope may have a 500 MHz bandwidth, which would be the upper limit of the useful range.
This is different from Resolution Bandwidth or RBW. RBW is the window size in which the measurement is taken. This RBW window is what is swept across a frequency range being measured. The RBW is typically defined as a 3 dB or 6 dB bandwidth. This means that from the center peak of the detection window to either edge, the signal will drop 3 or 6 dB from the maximum amplitude or center location. The width between these points is the resolution bandwidth.
In Figure 1, the green curve shows a 3 dB bandwidth, while the blue curve shows the 6 dB bandwidth. Since the 3 dB curve drops slower than the 6 dB curve, the total energy under the curve will be higher. For energy distributed over a wider frequency range (not a CW signal), this translates to higher readings from a 3 dB bandwidth than using a 6 dB bandwidth of the same bandwidth value. Most receivers and some higher-end spectrum analyzers have the ability to measure using a 6 dB RBW. Most spectrum analyzers have only a 3 dB RBW.
For military and aerospace measurements, the bandwidths required are defined as 6 dB RBW. 3 dB RBW are allowed, but no correction is allowed for using the wider bandwidth. Therefore, care should be taken to ensure the measurement equipment is using the proper style of bandwidth if there is a choice available.
Since some limit lines are reduced to very low amplitudes at specific frequency ranges, commonly called notches, the ability to measure these very low amplitudes may not be possible without reducing the RBW. This is due to Johnson-Nyquist Noise, also known as thermal noise, which appears as broadband energy. For example, at room temperature (about 300° K), a 1 MHz RBW will measure -114 dBm from a disconnected resistor sitting on a bench. If a measurement at 8 GHz is needed, and the antenna factor is 37 dB/m, we have:
This means without considering cable loss, spectrum analyzer noise, instrument noise, or any other sources, and using the required 1 MHz RBW, the minimum noise the system can measure at 8 GHz is 30 dBµV/m even if the equipment is off. However, there have been times when limits of 20 dBµV/m were imposed with a 6 dB margin required, requiring a noise floor of 14 dBµV/m. In other words, your equipment fails when it is off, which does not seem to be the purpose of the test.
A 10 dB improvement in the noise floor can be expected for each reduction of bandwidth of 10 times. Thus, to measure 8 GHz with a noise floor of 14 dbµV/m will likely require using a 1 kHz RBW or less, which is a deviation from the standard’s required bandwidth by 1000 times. Remember, measuring with the reduced bandwidth must be approved by the procuring activity before it can be used. And remember that signal amplifiers will add their own noise to the system and will amplify that thermal noise along with everything else. Also, using a 1 kHz RBW at 8 GHz is a very slow process, 15 seconds/MHz, and may require a great deal of time to take a proper reading. A scan from 8.0 GHz to 8.5 GHz is over 2 hours. In these cases, it may be wise to consider spot checks at specified frequencies, such as harmonics of known clock frequencies.
Video bandwidths, or VBW, are filters that can be applied to the measured signal. They have the effect of smoothing the appearance of the emissions on the spectrum analyzer, lowering the amplitude in the process. When choosing a VBW equal to or less than the RBW, the filter is being applied. This is not allowed for military and aerospace measurements. Thus, the VBW must be three times wider or greater than the value of the RBW.
Resolution bandwidths are now defined and must be used in their designated frequency range. This was done to help eliminate the need for Broadband and Narrowband emission measurements. Broadband measurements were performed using wider bandwidths, commonly at least 10x the narrowband, and the amplitudes measured were normalized as if using a 1 MHz bandwidth. Assume, for example, a conducted emissions measurement used a 10 kHz bandwidth across some range. The readings were corrected by:
These measurements were intended to find energy levels across frequencies, especially when dealing with radios and other receivers that are onboard aircraft. Broadband noise could reduce the sensitivity of the communication systems, which is especially important on overseas flights. Some corporate standards still require this measurement; however, most derive their information from the RBW values currently defined in the standards.
