Because the LNA in series can provide a 60 dB gain, users can greatly
increase the power of the front end of the vector signal analyzer. In our
measurement operation, a gain of 60 dB is sufficient to increase the peak power
from - 116 dBm to - 56 dBm. With a 60 dB gain (and a 1.5 dB noise figure), the
noise power of the signal will be – 112 dBm/Hz (- 174+gain+F). Therefore, the
SNR can be captured up to 56.5 dB (- 56 dBm+112.5 dBm), which is also lower than
the actual dynamic range of the instrument. It can be seen that if there is a
dynamic range of 80 dB, the VSA will be able to record the maximum SNR without
the noise impact of wireless signals.
When any wireless signal is to be recorded, the reference level can be set
at least 5 dB higher than the normal peak power to cope with any abnormal signal
strength. In some cases, although this step will reduce the effective dynamic
range of the VSA, the GPS signal will not be affected. Since the maximum ideal
SNR of the GPS signal input to the antenna is 58 dB (- 116+174), it is
meaningless for VSA to record a dynamic range exceeding 58 dB. Therefore, we can
even "discard" the dynamic range of the instrument to more than 10 dB without
affecting the quality of the recorded signal (in this bandwidth, PXI-5661 will
provide a dynamic range better than 75 dB).
Since it is necessary to set the appropriate reference level, it is equally
important to set the RF front end of the recording device appropriately. As
previously mentioned, to obtain the best RF recording data, it is recommended to
use an active GPS antenna. Since the active antenna has a built-in LNA that
provides a gain of up to 30 dB with a low noise figure, DC bias can also be
supplied. A variety of bias modes will be described below.
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