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Agilent Fundamentals of RF and Microwave Noise Figure Measurements Application Note 57 1 2 Table of Contents 1 What is Noise Figure 4 Introduction 4 The Importance of Noise in Communication Systems 5 Sources of Noise 6 The Concept of Noise Figure 7 Noise Figure and Noise Temperature 8 2 Noise Characteristics of Two Port Networks 9 The Noise Figure of Multi stage Systems 9 Gain and Mismatch 10 Noise Parameters 10 The Effect of Bandwidth 11 3 The Measurement of Noise Figure 12 Noise Power Linearity 12 Noise Sources 12 The Y Factor Method 13 The Signal Generator Twice Power Method 14 The Direct Noise Measurement Method 14 Corrected Noise Figure and Gain 15 Jitter 15 Frequency Converters 16 Loss 16 LO Noise 16 LO Leakage 16 Unwanted Responses 16 Noise Figure Measuring Instruments 17 Noise Figure Analyzers 17 Spectrum Analyzers 17 Network Analyzers 18 Noise Parameter Test Sets 18 Power Meters and True RMS Voltmeters 18 4 Glossary 19 5 References 29 6 Additional Agilent Resources Literature and Tools 3 31 Chapter 1 What is Noise Figure Introduction Modern receiving systems must often process very weak signals but the noise added by the system components tends to obscure those very weak signals Sensitivity bit error ratio BER and noise figure are system parameters that characterize the ability to process low level signals Of these parameters noise figure is unique in that it is suitable not only for characterizing the entire system but also the system components such as the pre amplifier mixer and IF amplifier that make up the system By controlling the noise figure and gain of system components the designer directly controls the noise figure of the overall system Once the noise figure is known system sensitivity can be easily estimated from system bandwidth Noise figure is often the key parameter that differentiates one system from another one amplifier from another and one transistor from another Such widespread application of noise figure specifications implies that highly repeatable and accurate measurements between suppliers and their customers are very important The reason for measuring noise properties of networks is to minimize the problem of noise generated in receiving systems One approach to overcome noise is to make the weak signal stronger This can be accomplished by raising the signal power transmitted in the direction of the receiver or by increasing the amount of power the receiving antenna intercepts for example by increasing the aperture of the receiving antenna Raising antenna gain which usually means a larger antenna and raising the transmitter power are eventually limited by government regulations engineering considerations or economics The other approach is to minimize the noise generated within receiver components Noise measurements are key to assuring that the added noise is minimal Once noise joins the signals receiver components can no longer distinguish noise in the signal frequency band from legitimate signal fluctuations The signal and noise get processed together Subsequent raising of the signal level with gain for example will raise the noise level an equal amount 4 This application note is part of a series about noise measurement Much of what is discussed is either material that is common to most noise figure measurements or background material It should prove useful as a primer on noise figure measurements The need for highly repeatable accurate and meaningful measurements of noise without the complexity of manual measurements and calculations has lead to the development of noise figure measurement instruments with simple user interfaces Using these instruments does not require an extensive background in noise theory A little noise background may prove helpful however in building confidence and understanding a more complete picture of noise in RF and microwave systems Other literature to consider for additional information on noise figure measurements is indicated throughout this note Numbers appearing throughout this document in square brackets correspond to the same numerical listing in the References section Related Agilent Technologies literature and web resources appear later in this application note NFA simplifies noise figure measurements The Importance of Noise in Communication Systems The signal to noise S N ratio at the output of receiving systems is a very important criterion in communication systems Identifying or listening to radio signals in the presence of noise is a commonly experienced difficulty The ability to interpret the audio information however is difficult to quantify because it depends on such human factors as language familiarity fatigue training experience and the nature of the message Noise figure and sensitivity are measurable and objective figures of merit Noise figure and sensitivity are closely related see Sensitivity in the glossary For digital communication systems a quantitative reliability measure is often stated in terms of bit error ratio BER or the probability P e that any received bit is in error BER is related to noise figure in a non linear way As the S N ratio decreases gradually for example the BER increases suddenly near the noise level where l s and 0 s become confused Noise figure shows the health of the system but BER shows whether the system is dead or alive Figure 1 1 which shows the probability of error vs carrier to noise ratio for several types of digital modulation indicates that BER changes by several orders of magnitude for only a few dB change in signal to noise ratio The output signal to noise ratio depends on two things the input signal to noise ratio and the noise figure In terrestrial systems the input signal to noise ratio is a function of the transmitted power transmitter antenna gain atmospheric transmission coefficient atmospheric temperature receiver antenna gain and receiver noise figure Lowering the receiver noise figure has the same effect on the output signal to noise ratio as improving any one of the other quantities In satellite systems noise figure may be particularly important Consider the example of lowering a direct broadcast satellite DBS receiver s noise figure from 2 dB to 1dB by improving the LNA low noise amplifier in the receiver This can have nearly the same effect on the signal to noise ratio as doubling the transmitter power Doubling the satellite transmitter power if allowed can be very costly compared to the small cost of improving the LNA 10 7 DBS receiver 8 APK OPR


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