R&S FSL Cable TV Measurements (Option K20)
1300.2519.12 2.67 E-11
Fig. 2-47 Root raised cosine filtered dirac signals I
RRC
(t) and Q
RRC
(t)
We notice that the symbol instants (highlighted with squares) are not on the horizontal lines (possible
symbol levels) anymore. This is due to the inter–symbol interference introduced by the root raised
cosine TX filter. Contrary to the signal filtered with two root raised cosine filters (TX and RX) the one
filtered with a single root raised cosine filter (TX) does not satisfy the condition for zero inter–symbol
interference. Fig. 2-48 shows the signal obtained by filtering the dirac signals with two root raised
cosine filters. The convolution of two root raised cosine filters is also called raised cosine filter and it
results in an overall inter–symbol interference free system. We observe that all symbol instants
(highlighted with squares) of I
RC
(t) and Q
RC
(t) have exactly the same levels as the signals I
dirac
(t) and
Q
dirac
(t) in Fig. 2-45. Please note that this is only true for the signals I
RC
(t) and Q
RC
(t) if there is no noise
present in the receiver. In the case of noise points will turn into clouds, that is what can be seen in the
Constellation Diagram measurement (modulation analysis).
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