Conformity Magazine - July 2008 - (Page 41)

The benefit from a preamplifier can be determined pragmatically by looking at two extreme cases: 1. If the noise power out of the preamplifier (in a bandwidth equal to that of the spectrum analyzer) is at least 15 dB higher than the DANL of the spectrum analyzer, then the noise figure of the system is approximately that of the preamplifier less 2.5 dB. The presence of this situation can be determined by simply connecting the preamplifier to the instrument and observing the change in displayed noise. If the noise increases by 15 dB or more, this situation is present. If the noise power out of the preamplifier (in the same bandwidth as that of the spectrum analyzer) is 10 dB or more lower than the displayed average noise level on the analyzer, then the noise figure of the system is that of the spectrum analyzer, less the gain of the preamplifier. This can be determined by connecting the preamplifier to the instrument and again observing the change in displayed noise. If the displayed noise does not change, this situation is present. on the dynamic range of the test system is required, this preamplifier is not the right choice. Figure 3 illustrates this scenario. A spectrum analyzer with a noise figure of 24 dB will have an average displayed noise level of -110 dBm in a 10 kHz resolution bandwidth. If the 1 dB compression point for that analyzer is 0 dBm, the dynamic range is 110 dB. When a preamplifier is used, the maximum input to the system the input signal amplitudes must be kept below a level that is determined by the gain of the preamplifier (i.e., 36 dB) in order to avoid input overload. In this example, this means that the input signals cannot exceed -36 dBm when using the preamplifier. Furthermore, when using the preamplifier, the displayed average noise level will rise by about 17.5 dB because the noise power out of the preamplifier is that much higher than the noise floor of the analyzer, even after accounting for the correction factor of 2.5 dB. This increased noise level determines the dynamic range of the system. With the preamplifier in place, the dynamic range is 92.5 dB, 17.5 dB less than without the preamplifier. The reduction in dynamic range equals the change in the displayed noise when the preamplifier is used. A preamplifier that provides better sensitivity without adversely impacting the dynamic range, must meet the second 2. The two cases can be stated mathematically as follows: If NFpre + Gpre ≥ NFinst + 15 dB, then NFsys = NFpre – 2.5 dB Equation 6 and If NFpre + Gpre ≤ NFinst – 10 dB, then NFsys = NFinst – Gpre Equation 7 Using these expressions, the impact of a preamplifier on the sensitivity can be determined. Assume that the test instrument has a noise figure of 24 dB, and the preamplifier has a gain of 36 dB and a noise figure of 8 dB. The gain plus noise figure of the preamplifier is to be compared to the noise figure of the spectrum analyzer. The gain plus noise figure of the preamplifier is 44 dB, more than 15 dB higher than the noise figure of the spectrum analyzer, so the noise figure of the preamplifier/instrument combination is that of the preamplifier, less 2. 5 dB, or 5.5 dB. In a 10 kHz resolution bandwidth, the preamplifier/instrument system has a sensitivity of: Sensitivity = kTBB=1 + 10log Finger Stock Shielding All Shapes, All Sizes Beryllium copper and stainless steel Maximum shielding attenuation Excellent durability Gap range: .02" to .33" Snap-on, Clip-on, Stick-on, Hook-on and Track mounting DiamondBack (dB) textured surface for high frequencies Wide range of plating options Multiple thicknesses to meet compression requirements Single finger to 35 feet long Immediate delivery Free Samples at www.tech-etch.com/shield () RBW 1 + NFsys Equation 8 = –174 + 40 + 5.5 = – 128.5 dBm This is an improvement of 18.5 dB over the noise floor of -110 dBm without the preamplifier. However, there is a possible disadvantage to using a preamplifier, depending upon the signals to be measured. If the best possible sensitivity is required but no adverse impact ISO 9001:2000 REGISTERED TECH-ETCH, INC., 45 Aldrin Road, Plymouth, MA 02360 JULy 2008 Conformity 1 http://www.tech-etch.com/shield http://www.tech-etch.com/shield

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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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