PDF available

EMI Measurements Using A Preselector

Tektronix 2706 Stepping RF Preselector shown with a 2712 Spectrum Analyzer.

What Is A Preselector?

A preselector is a filter arrangement set in front of the input to a spectrum analyzer that permits some signal components to get to the spectrum analyzer while stopping others (pre-selects signals by frequency). The usual purpose of the preselector is to control input signals in such a way as to minimize, and ideally prevent, the generation of spurious responses within the spectrum analyzer.

An EMI preselector must do more than just eliminate spurious responses. Two additional objectives are important in EMI. The most critical function for EMI measurements is for the preselector to limit the input spectrum width so as to increase the measurement dynamic range, especially when performing quasi-peak (QP) measurements. This is discussed in more detail later. The EMI preselector is also useful in blocking large ambient signals when doing open-site measurements. Therefore, a preselector for EMI applications is more stringent in performance specifications than a preselector for spectrum analysis in general.

The EMI Preselector

The primary difference between a spectrum analysis preselector in general and a preselector aimed particularly at EMI measurements is in the preselector bandwidth. A general-purpose preselector aims to prevent mixer conversion spurious responses. This permits quite a wide preselector bandwidth as long as spurious generating signals, such as the conversion image, are restricted. A preselector optimized for EMI measurements, on the other hand, needs a relatively narrow bandwidth in order to reduce the input signal drive level into the spectrum analyzer. Therefore, even spectrum analyzers that are "preselected" using a low-pass filter below 1 GHz need to use a separate "preselector" for some EMI measurements.

The ideal preselector for EMI applications would provide a signal bandwidth just a bit wider than the specified spectrum analyzer measurement bandwidth, such as 120 kHz at 400 MHz input frequency. This is the narrowest possible preselector bandwidth that will not interfere with regulatory measurement bandwidth requirements. Such a narrowband filter is almost impossible to build, and it would be of prohibitively high cost. Various solutions aimed at different price/performance compromises are currently available.

The simplest EMI preselection procedure consists of several individual filters that are used as needed. A band-stop filter, for instance, can be used to eliminate a high-level signal that interferes with the measurement. This procedure costs very little but it's cumbersome and time-consuming.

The highest performance EMI preselectors currently available are of the swept filter variety, providing filter bandwidths of between 20% and 5% of center frequency. Thus, a 50 MHz bandwidth would be typical at 400 MHz. This provides overload protection for almost all possible signal combinations. Also, the preselector is automated and easy to use. But spectrum analyzer overload is still possible in rare cases and the measurement dynamic range is not as large as would be obtained with the narrowest permitted bandwidth of near 120 kHz. However, it's technically feasible and the price, though high, is within reach of some users.

A somewhat different compromise was chosen for the Tektronix 2706 Stepping RF Preselector discussed in this application note. The 2706 consists of eight calibrated stepped bandpass filters and a full band by-pass mode. These filters, as well as associated switches, are packaged together for manual or automated operation via GPIB commands. Tektronix EMC120 EMI Test Software incorporates commands that integrate the preselector as part of automated EMI measurements. The operating bandwidth is wider than that for the previously discussed sweeping preselector configuration. Rather than 50 MHz, the bandwidth is 250 MHz for a 400 MHz signal. This implies a somewhat greater probability of spectrum analyzer overload and less measurement dynamic range for broadband signals in accordance with the bandwidth relationship of:

20Log(250/50) = 14 dB

On the positive side, the cost is about one fifth that of a 10% bandwidth preselector. Thus, the Tektronix 2706 Stepping RF Preselector is based on an 80/20 compromise. Eighty percent of the job is done for 20% of the cost. By combining this preselector with the automated compression test resident within the EMC120 EMI Test software, nearly 100% of the job of making quality pre-qualification measurements is accomplished for about 20% of the cost.

The 2706 Stepping RF Preselector

The Tektronix 2706 Stepping RF Preselector provides the following switch-selectable passband characteristics:

• 9 kHz to 150 kHz
• 150 kHz to 3 MHz
• 3 MHz to 30 MHz
• 30 MHz to 125 MHz
• 125 MHz to 250 MHz
• 250 MHz to 500 MHz
• 500 MHz to 1000 MHz
• 1000 MHz highpass
• Bypass mode for 9 kHz to 1800 MHz

Figures 1, 2, and 3 show the bandpass characteristics of several 2706 signal transmission bands.

Figure 1. Preselector passband characteristics for 9 to 150 kHz and 150 kHz to 3 MHz bands.

Figure 2. Preselector characteristics for 0.150 to 3 MHz and 3 to 30 MHz bands.

Figure 3. Preselector characteristics for 30 to 150 MHz band.

The bandpass transmission start and stop frequencies were chosen on the basis of CISPR band requirements, such as 150 kHz to 30 MHz, and on EMI signal intensity expectation at various frequencies. It's well established, for instance, that broadband EMI signals are usually of highest intensity at lower frequency. Thus, while it's theoretically possible to postulate an EMI signal that will overload the spectrum analyzer using the 2706, it's not highly likely given the usual spectral distribution of such signals in combination with the 2706 bandpass settings. Such signals will be identified via the automated signal overload software provided by Tektronix so as to prevent erroneous measurements. Also, choice of preselector bands to complement CISPR-designated bands optimizes EMI measurement in accordance with regulatory requirements. Furthermore, the 2706 is very versatile in how it can be used. While this application note illustrates applications using the Tektronix 2712 Spectrum Analyzer, the 2706 can also be used with other Tektronix spectrum analyzers.

Measurement Illustrations

Figure 4 shows a broadband pulsed EMI signal spectrum with and without the 2706 Preselector. As is usual, the pulsed EMI spectrum shows greatest intensity in the lower frequency range. Removing the high-level interference below 3 MHz significantly improves the spectrum analyzer overload and dynamic range and permits accurate measurement of low-level higher frequency components.

Figure 4. Using the preselector to remove lower frequency components.

Naturally, regulatory requirements also call for measurements at lower frequencies. Here the 2706 Preselector is used to restrict the transmission bandwidth in accordance with specifications which significantly reduces the signal input level to the spectrum analyzer and this increases the measurement dynamic range. Figure 5 shows the CISPR band A EMI results for the same pulsed EMI signal as discussed for Figure 4. The total EMI signal spectral intensity extends over approximately 2000 kHz (the lobe width), while the 2706 Preselector band restricts the transmission bandwidth to only 150 kHz. This is an input signal drive level reduction of:

20Log(2000/150) = 22.5 dB

Figure 5. Using preselector to test results for CISPR band A, 9 to 150 kHz broadband EMI components.

Figure 6 shows a crowded open-area test site (upper trace) as intercepted by a test antenna. The many signals are confusing to observe, and these can inter-modulate or overload the measuring spectrum analyzer. The lower trace shows the result when using the 2706 Preselector set for the 3 MHz to 30 MHz band. Most of the interference has been eliminated and the measurement and analysis of what remains is much simpler.

Figure 6. Using preselector with open site measurements.

The final illustration shows how the 2706 preselector will eliminate isolated large interfering signals. The upper trace of Figure 7 includes a large narrowband signal at 98 MHz (center screen). Viewing the same input through the preselector eliminates the 98 MHz interference.

Figure 7. Using preselector to reduce level of large narrowband signal.

Conclusion

The purpose of a preselector in EMI measurements is to eliminate display spurious responses and prevent spectrum analyzer gain compression due to input signal overdrive. This is accomplished by restricting the input signal frequency range by use of a narrowband input filter (the preselector). An ideal pre-selector will have a very nar row bandwidth. Such preselectors are not practical due to complexity and cost. A practical swept filter preselector will provide an input bandwidth of about 10% of the input frequency, i.e. about 50 MHz at 500 MHz input. Such a preselector does much less than an ideal unit of less than 1 MHz bandwidth. However, it does provide sufficient performance for virtually all practical situations. The 2706 is a "stepping" rather than "swept" filter unit. It provides less input signal protection than the swept variety due to a wider filter bandwidth. However, it costs much less.

The 2706 Stepping RF Preselector provided by Tektronix performs as follows for EMI applications.

• It eliminates high-level low-end (near DC) EMI typical of most clock pulses. This permits the measurement of lower level higher frequency signals that would otherwise be difficult or impossible.
  
  
• It restricts the input frequency range to a sufficiently narrow band near DC so that accurate measurements, in this critical EMI area, can be performed even in the presence of high-level or wide-band higher frequency signals.
  
  
• It eliminates large amplitude narrowband interference, such as from a near-by broadcast station, to make open-site procedures easier and more accurate. However, the stop-band is quite wide. Therefore, no measurements will be possible within tens, or even hundreds, of MHz around the frequency of the signal to be eliminated.
  
  
• It reduces the total input level to which the spectrum analyzer is subjected in the presence of EMI of significant occupied bandwidth, such as 1000 MHz. However, the upper end at 500 to 1000 MHz is not completely protected from excessive input levels. Such an overload condition will be detected by Tektronix provided software such as the EMC120 EMI Test Software.

Tektronix Measurement products are manufactured in ISO registered facilities.