R&S FSV-K70, Руководство пользователя

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Brief Description of Vector Signal Analysis
R&S
®
FSV-K70
60 Operating Manual 1176.7578.02 ─ 09
Noise
Figure 2-55: Additive noise
The Figure 2-55 shows a 64QAM signal (only the 1st quadrant is shown) with additive
noise. The symbol decision thresholds are also shown.
The noise signal forms a "cloud" around the ideal symbol point in the constellation dia-
gram. Exceeding the symbol decision boundaries leads to wrong symbol decisions and
increases the bit error rate.
Similar displays are obtained in case of incorrect transmitter filter settings. When an
incorrect filter is selected, crosstalk occurs between neighbouring symbol decision
points instead of the ISI-free points. The effect increases the more the filtering deviates
from actual requirements.
The two effects described cannot be distinguished in the Constellation I/Q diagram but
in statistical and spectral analyses of the error signal.
2.5.2 FSK Modulation
Signal Model
Frequency shift keying (FSK) involves the encoding of information in the frequency of a
transmitted signal. As opposed to other modulation formats such as PSK and QAM,
the FSK process is a non-linear transform of the transmitted data into the transmitted
waveform.
A sequence of symbols {s
i
} are modulated using a "frequency pulse" g(t) to form the
instantaneous frequency of the transmitted complex baseband waveform, denoted by
f
REF
(t) and defined as:
   

   
i
i REF
T i t g s h t f
where f
Symb
=1/T is the symbol rate and h is a scaling factor, termed the modulation
index. The transmitted (or reference) FSK signal is formed by frequency modulation of
the instantaneous frequency:
Signal Model, Estimation and Modulation Errors