Promax PROLINK-4C, Manual

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USER’S MANUAL. PROLINK-4C
Page 48 02/2003
Figure 17.- BER measuring screen for QPSK modulated signals.
Two BER measurements are shown:
1) BER before FEC (Forward Error Corrections)
2) BER after Viterbi
In a digital reception system for satellite signals, after the QPSK decoder two
different correction methods are applied (see figure 18). Obviously, each time we apply
an error corrector to a digital signal, the error rate changes, therefore if we measure in a
digital satellite television system, for example, the error rate at the output of the QPSK
demodulator, at the output of the Viterbi decoder, and at the output of the Reed-
Solomon decoder, we obtain nothing more than different error rates. This is the reason
because the BER measurement is provided before FEC, after Viterbi.
Figure 18.- Digital reception system via satellite.
The BER measurement is provided in scientific notation (i.e. 2.0 E-3 means
2.0x10
-3
, that is to say two incorrect bits of every 1,000) and through an analogue bar
(as its length is smaller the signal quality will be better). The analogue representation is
done on a logarithmic scale (not linear).
With the aim to have a reference about the signal quality, it is considered that a
system has a good quality when it decodes less than one non-correctable error for
every transmission hour. This border is known as QEF (Quasi-Error-Free) and it
corresponds approximately to a BER after Viterbi of 2.0E-4 BER (2.0x10
-4
). This value
is marked on the measurement bar of the BER after Viterbi and therefore, BER for
acceptable signals must be at the left side of this mark.
Next it is shown the tuning frequency and the frequency deviation in MHz between
the tuned frequency and the one which optimizes the BER (i.e. Freq: 1777.0 + 1.2 MHz).