Cable TV Measurements (Option K20) R&S FSL
1300.2519.12 2.68 E-11
Fig. 2-48 Raised cosine filtered dirac signals I
RC
(t) and Q
RC
(t)
Let us get back to the ideal transmitter from Fig. 2-44. The next task is to modulate the base band
signals
RC
(t) and Q
RRC
(t) onto a carrier. The carrier frequency f
CF
stands for the center of a given TV
channel. Fig. 2-49 shows the modulated signal IQ
RF
(t) with a carrier frequency f
CF
=4*symbol_rate.
Please note that in real system the carrier frequency is much higher than here in our example.
Fig. 2-49 QAM modulated RF signal IQ
RF
(t)
In a real cable TV transmission system however, the receiver would encounter a much worse situation.
The measurement signal, i.e. the signal fed into the R&S FSL's RF input, suffers from distortion. Some
of it is caused by a non–ideal transmitter, some originates from the TV cable and last but not least there
is thermal noise in every transmission system. Very often it is even not possible to find out from which
component the distortion comes from. Luckily this can be found out by driving measurements starting at
the transmitters location and continuing at different test points in the cable TV network up to the plug at
the subscriber's home.
The objective of the digital TV measurements offered by the Cable TV Measurements option is to
analyze and separate different sources of distortion and erroneous parameters.
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