AvMesh
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RF Communications Systems
Installation & Service Manual
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17
3.7.1.3 Real time Spectrum Analyzer
The Real Time Spectrum Analyzer is a variation of the FFT Spectrum Analyzer.
However, unlike the FFT Spectrum Analyzer, the real time spectrum analyzer has a very
fast digital signal processor that is capable of capturing and analyzing all of the signals
within a particular bandwidth very quickly, virtually in real time. As a result, the Real
time spectrum analyzer is also able to capture signals that have an intermittent nature
(transients) that would otherwise be difficult to capture using other spectrum analyzers.
3.7.1.4 USB Spectrum Analyzer
USB Spectrum Analyzers use the computer to which they are connected to carry out the
data processing, thereby saving a significant amount of the cost associated with a more
traditional bench top instrument. Real time analysis is not possible as the computer system
that the USB spectrum analyzer is connected to usually runs on a PC Application not
capable of real time operation.
An understanding of Decibels is essential to be able to comprehend signal measurements.
The Bel is used as a way of comparing signal strength, however, its large values make it
difficult to use for precise measurements and calculations. The decibel (dB) is a smaller
unit as it is one tenth of a Bel and therefore provides more precise measurements and
calculations. The decibel (dB) is a ratio used for comparing two values and is used primarily
as a comparison of power level (although the ratio is applicable for other values such as
voltage and intensity as well). By using a logarithmic scale, the decibel is able to compare
quantities that may have a significant difference between them.
The comparison of power levels is computed using the following formula:
Where the log function is of base 10, ‘X’ is the number of decibels, ‘P
out
’ is the output power
level and ‘Pin’ is the input power level, also known as the reference power level. The value
of P
out
/P
in
is known as the ‘Power ratio’. The factor ‘10’ has been included in the formula to
accommodate for the conversion from Bels to Decibels (there are 10 Decibels in one Bel).
If the output power is larger than the input power, the power ratio is more than 1 and the
resultant decibel value (X) becomes positive. This is known as a ‘gain’ or ‘amplification’
which results in an increase of power (signal strength).
If the output power is less than the input power, the power ratio is less than 1 and the
resultant decibel value (X) becomes negative. This is known as a ‘loss’ or ‘attenuation’ and
results in a loss of power (signal strength). A loss (Attenuation) is therefore expressed as a
negative gain.
The table below shows how the decibel value can be used as a comparison between the
output and input power values. For example, a decibel value of 3dB (gain) indicates that the
output power is twice the input power whereas a decibel value of -3dB (loss) indicates that
the output power is half the input power. Because of the logarithmic scale, even a small
change in dB value has a significant impact on the actual power (signal strength).
3.7.2 The Decibel formula
