Conformity Magazine - July 2008 - (Page 42)

of the above criteria: the sum of the preamplifier gain and noise figure must be at least 10 dB less than the noise figure of the spectrum analyzer. In this case, the displayed noise floor will not change noticeably when the preamplifier is connected. Although the dynamic range is shifted down in amplitude by the gain of the preamplifier, the same overall dynamic range is preserved. Ultimately, the selection of a preamplifier depends on the measurement needs. If the best possible sensitivity is required and a reduction in dynamic range is not a concern, then a high-gain, low-noise-figure preamplifier would be the correct choice. In this case, the system noise figure would be identical with the noise figure of the preamplifier, less 2.5 dB. However, this is not suitable for EMI measurements. For EMI measurements, a lower gain preamplifier is the proper choice. Input Attenuation and Noise Figure The input attenuator of the spectrum analyzer or EMI receiver can be used to impact the noise figure (or reduce the gain of the preamplifier). For example, if a slightly better sensitivity is required but no reduction in dynamic range is required, the above preamplifier can be used with 30 dB of RF input attenuation on the spectrum analyzer or EMI receiver. This attenuation increases the noise figure of the instrument from 24 to 54 dB. Now the sum of the gain and noise figure of the preamplifier (36 dB + 8 dB) is 10 dB less than the noise figure of the analyzer, and the second criterion above is met. The noise figure of the system is now: NFsys = NFinst - Gpre = 54 dB – 36 dB = 18 dB This represents a 6 dB improvement over the instrument noise figure itself with 0 dB input attenuation. The sensitivity was improved by 6 dB, but the dynamic range is not reduced. Equation 9 General Selection Guidelines for Preamplifiers Many preamplifiers will not meet either of the two stated criteria in Equation 6 and Equation 7. Figure 4 allows the determination of the system noise floor from the noise figure and gain of the preamplifier and the noise figure of the instrument used in a test system. This graph is to be used in the following manner: 1. 2. Calculate the value of NFpre – Gpre - NFinst If this value is less than zero, the corresponding value has to be found on the dashed curve and the system noise figure is found on the left ordinate in terms of “dB above NFinst - Gpre” If this value is positive, the corresponding value has to be found on the solid curve and the system noise figure is found on the right ordinate in terms of “dB above NFpre” 3. Summary In the frequency range above 1 GHz, the system noise floor is relatively high, and often it is rather difficult to achieve a suitable system noise floor that is 6 dB below the applicable limit. In this frequency range, a resolution wide bandwidth (i.e., 1 MHz impulse bandwidth) is required by the standards for commercial EMI testing, which results in a high DANL. Furthermore, the limits and the test distance are specified in these EMI standards, which also influence the necessary system noise floor. For these reasons, preamplifiers are often used in emissions measurement systems. However, when selecting preamplifiers, the focus is mostly on the improvement of the system sensitivity. The impact of a preamplifier on the system dynamic range is to be considered as well, since larger signals, that are close to the limit, are of particular interest during a compliance test and must be correctly measured. This article offers a practical way to determine if a given preamplifier is suitable for EMI measurements. Conversely, the required noise figure and gain specifications a preamplifier must have to be suitable for use in an EMI measurement system can be determined. Werner Schaefer is a technical leader and quality manager with Cisco Systems, and can be reached at wsemc@cisco.com. The author would like to thank Mrs. Tori Barling for proof reading this manuscript. References 1. 2. Hewlett-Packard Application Note 150-4, “Spectrum Analysis – Noise Measurements”, April 1974. Agilent Technologies Application Note AN1315, “Optimizing RF and Microwave Spectrum Analyzer Dynamic Range”, 1999. Figure 4: System noise figure for sinusoidal signals 2 Conformity JULy 2008 FAST Link www.conformity.com/1950 http://www.conformity.com/1950

Table of Contents Feed for the Digital Edition of Conformity Magazine - July 2008

Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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