GPS measurement system

　　The main product for testing GPS receivers is a set of RF vector signal generators that can simulate GPS signals. In this application description, the reader will know how to use NI PXI-5671 and NI PXI-5672 RF vector signal generators to achieve measurement purposes. This product can also be used with the NI GPS tool set to simulate 1 to 12 groups of GPS satellites.

　　The complete GPS measurement system should also include a variety of different accessories to achieve the best efficiency. For example, the external fixed attenuator can improve the power accuracy and noise floor efficiency. In addition, depending on whether the receiver supports the DC bias (Bias) of its direct input port, some receivers may also need a DC blocker

　　When testing a GPS receiver, an external RF attenuation of up to 60 dB is often used (left blank, Padding). The fixed attenuator can provide at least two advantages of the measurement system. First, the fixed attenuator can ensure that the noise level excited by the test is lower than the thermal noise floor of - 174 dBm/Hz. Secondly, the fixed attenuator can also improve the power accuracy because the signal alignment can be calibrated through a high-precision RF power meter. Although only 20 dB attenuation is required to meet the requirements of noise layer, if 60~70 dB attenuation is used, higher power accuracy and noise layer efficiency can be achieved. RF power calibration will be discussed later, while Figure 2 first illustrates the impact of attenuation on noise layer efficiency.

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