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Publication Number: N/A
Date: November 1996
For a multistage amplifier that has stage power gains G1, G3, G3,..., (linear form) and stage noise figures F1, F3, F3,..., (linear form) respectively, the overall noise figure is
(Stremler, 1979). The measurement system for the Coast Guard DGPS field strength seen in Figure 3, has a single stage gain of 22.4 dB (Mini-Circuits amplifier) with a typical noise figure of 2.9 dB. Cable losses and the insertion loss of the low pass filter are negligible. The noise figure of the spectrum analyzer is 31 dB. From Equation (C1), the overall noise figure of the system is 9.6 dB.
The bandwidth and overall noise figure of the system determine the smallest signal that can be detected and is computed from the following equation:
where NP is the noise power in dBm, -174 is the thermal noise in a 1 Hz bandwidth at room temperature (300 K), B is the bandwidth of the system in Hz, and FdB is the overall noise figure of the system in dB. The quantity NP is the minimum signal power that can be detected at the output of the antenna. In this case, the bandwidth of the system is 300 Hz. Therefore, using Equation (B2), NP is calculated to be -139.6 dBm, or expressed in watts,
Knowing NP, one can calculate the sensitivity of the system at the input to the antenna using Equations (A1) - (A5). Assuming a typical carrier frequency of 300 kHz and the corresponding antenna correction factor of 40.5 dB, one finds that the minimum power density at the input to the antenna must be 16.18e-15 watts/m2 in order to be detected. This corresponds to a field strength of 7.85 dB µv/m.
Stremler, F.G. (1979), Introduction to Communication Systems, (Addison-Wesley Publishing Company; Reading, Massachusetts), pp. 173-174.
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