Conformity Magazine - July 2008 - (Page 40)

where: F = Noise figure power ratio Si = Input signal power Ni = Input noise power So = Output signal power No = Output noise power This expression can be simplified for a spectrum analyzer or EMI receiver. First, the output signal is the input signal times the gain of the instrument. Second, the gain of the instrument is unity because the signal level at the output (indicated on the display) is the same as the level at the input (input connector). With these assumptions, the equation becomes: No F= Equation 3 Ni This means that the determination of the noise figure can be reduced to a simple comparison of the noise level displayed by the instrument to the “true” – not the effective – noise level at the input of the instrument. Equation 3 can be expressed in logarithmic terms as: NF = 10 * log(F) = 10 * log(No) – 10 * log(Ni) Equation 4 In order to determine the noise figure, the noise power has to be measured in some bandwidth, normalize this value to a 1Hz bandwidth and compare it to -174 dBm/Hz. Example: if a noise signal of -110 dBm was measured in a 10 kHz bandwidth, the noise figure would be determined as follows: NF = [measured noise in dBm] – 10 * log = –110 dBm – 10 * log = 24 dB The noise figure of 24 dB noise figure means that a sinusoidal signal must be 24 dB above kTB to be equal to the displayed average noise level on this particular instrument. Thus, the noise figure can be used to determine the DANL for a given bandwidth or to compare the DANL of different instruments. Preamplifiers The noise figure concept aids in the determination of the possible benefit of using a preamplifier in the test system. A 24 dB noise figure, while reasonable for a general purpose spectrum analyzer, is not adequate for radiated emission measurements. However, by placing an appropriate preamplifier in front of the spectrum analyzer or EMI receiver, a system (preamplifier/spectrum analyzer) noise figure can be obtained that is lower than that of the spectrum analyzer itself. To the extent that the noise figure is lowered, the system sensitivity is improved as well. The concept of noise figure was introduced on the basis of sinusoidal input signals. The benefits of a preamplifier can be determined on the same basis. However, a preamplifier also amplifies noise, and this output noise can be higher than the effective input noise of the analyzer. A spectrum analyzer or EMI receiver using log power averaging displays a random noise signal 2.5 dB lower than its actual value. The following derivations will take this fact into consideration by introducing a suitable correction factor. () 10000 1 () RBW 1Hz – kTBB = 1Hz – (–174 dBm) Equation 5 The true noise level at the instrument input, rather than the effective noise level, is to be used because the input signal-tonoise ratio is based on the true noise. As shown above, when the input is terminated in 50 Ω, the kTB noise level at room temperature in a 1 Hz bandwidth is -174 dBm. Figure 3: Preamplifier impact on dynamic range 0 Conformity JULy 2008

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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