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![]() Figure 1 shows a simplified system model for a superheterodyne receiver, including source noise. Using a receiver with matched-impedance front end (the usual goal) allows one to speak of the available noise power from the source, and presented to the receiver input. To allow the design and comparison or receivers independent of the antenna and environment a simplification is made where the equivalent noise temperature is set to 290 K. The term T becomes quite complex as it is coupled to the environmental mechanisms discussed previously, and the antenna’s parameters at that moment in time. V ns = √(4kTB)where k is Boltzman’s constant (1.38 x 10 -23 ) and T is the “equivalent noise temperature”, Reference 2. For wireless receivers, the principle source of natural radiation is the ground, atmosphere, and the sun. Although extremely weak this radiation may be detected by a sensitive receiver. Because of thermal agitation of charged particles, nearly everything around us radiates radio waves. It is useful to first recognize there will always be some minimum amount of noise to contend with, due to the natural environment and is independent of the receiver itself. This usually requires many complex tradeoffs in order to optimize to a particular requirement. Instead, the task is to design systems that will come as close to the ideal as possible. Fortunately for circuit designers who like challenges, this is not the case. In an ideal world circuits would consume no power, create no noise and system dynamic range would be infinite. The many ways to specify, measure, and characterize receiver noise performance can sometimes lead to confusion and misinterpretation. Commonly used figures-of-merit include noise figure (NF), sensitivity, minimum discernable signal (MDS), carrier-to-noise ratio (CNR), error vector magnitude (EVM), signal-to-noise ratio (SNR) and bit-error rate (BER). Noise in radio receivers is of great interest to anyone involved in wireless product design. ![]()
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