Conformity Magazine - July 2008 - (Page 38)

connector of the instrument and the first gain stage; all of these components generate noise. However, this internally generated noise is at or near the absolute minimum of 174 dBm/Hz. These elements do not significantly affect the noise level input to the first gain stage. While the input attenuator, mixer, and other circuit elements between the input connector and first gain stage have little effect on the actual system noise, they do have an effect on the ability of an instrument to display low-level signals because they attenuate the input signal. That is, they reduce the signalto-noise ratio and so degrade sensitivity. The DANL can be determined by noting the noise level indicated on the display when the instrument input is terminated with a 50 Ω load. This level is the noise floor of the instrument. Signals below this level are masked by the noise and cannot be discerned on the screen. However, the DANL is not the actual noise level at the input of the instrument, but rather the effective noise level. The display of the instrument is calibrated to reflect the level of a signal at its input, so the displayed noise floor represents a fictitious, or effective noise floor at the input. The effective input noise floor includes the losses caused by the input attenuator, mixer conversion loss, and other circuit elements prior to the first gain stage. The conversion loss of the mixers cannot be avoided, but the RF input attenuator setting can be changed. This provides control over the input signal power to the first mixer and thus has an effect on the displayed signalto-noise floor ratio. The lowest DANL can be achieved by selecting zero RF attenuation. Input Attenuation and Resolution Bandwidth In modern spectrum analyzers and EMI receivers, an internal microprocessor changes the IF gain to offset changes in the input attenuator. Thus, signals present at the input of the instrument remains stationary on the display as the input attenuator setting is changed, while the displayed noise moves up and down on the screen. In this case, the reference level remains unchanged. This is shown in Figure 1. As the attenuation increases from 10 to 30 dB in 10 dB steps, the displayed noise rises while the -10 dBm signal remains constant. The best signal-to-noise ratio can be achieved by selecting minimum input attenuation. The selected resolution bandwidth also affects signal-to-noise ratio, or sensitivity. The noise generated in the analyzer is random and has a constant amplitude over a wide frequency range. Since the resolution bandwidth filters are located after the first gain stage, the total noise power that passes through the filters is determined by the width of the filters. This noise signal is detected and ultimately reaches the display. The random nature of the noise signal causes the displayed level to vary in accordance with the following: ∆Noise = 10 * log () BW1 BW2 Equation 1 where: BW1 = starting resolution bandwith BW2 = ending resolution bandwidth If the resolution bandwidth is changed by a factor of 10, the displayed noise level changes by 10 dB, as shown in Figure 2. For continuous wave (CW) signals the best signal-to-noise ratio, or best sensitivity, is achieved when using the minimum resolution bandwidth available in the instrument. Noise Figure Many instrument manufacturers specify the performance of receivers in terms of noise figure, rather than sensitivity. The following evaluation of the noise figure will be based on a sinusoidal signal as the input signal to the instrument. Noise figure can be defined as the degradation of signal-tonoise ratio as a signal passes through any device, including a spectrum analyzer or EMI receiver. Noise figure can be expressed as the following ratio: Figure 1: Signal to noise ratio as a function of input attenuation Figure 2: Displayed noise level changes per bandwidth change 38 Conformity JULy 2008 F= Si/Ni So/No Equation 2

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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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