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ENGLISH
TV & SAT Level Meter MTD 120
User Manual
5.4.- QPSK modulation
Before to measure the BER it is necessary to define some parameters concerning the digital signal, which are described next. To see its
present value or to modify it, being in the BER measuring screen, press key ENTER [5], a multiple-choice menu showing the functions relative
to the BER measurement will appear:
1) Code Rate
Also known as Viterbi ratio. It defines the ratio between the number of data bits and actual transmission bits (the difference corresponds to
the control bits for error detection and correction). It permits to choose between 1/2, 2/3, 3/4, 4/5, 5/6, 6/7, 7/8, 8/9 and Auto. If Code
Rate parameter is not known it is possible to assign the Auto option.
2) Symbol Rate
It is possible to choose between the following values: 30000, 27500, 22000, 20000, 19995, 6110, 6000, 5998, 5632, 5062, 4340, 4000
kbauds, or well to define any other value (Other). When selecting the Other option, a screen titled QPSK SYMBOL RATE will appear.
This screen shows present value, to modify it press key SHIFT [31] and enter the new value through the keyboard. The unit accepts any
number with 5 figures between 2000 and 35000 kbauds (fifth figure acts as validation). For example, to select a symbol rate of 8200
kbauds it must be entered: 08200.
3) Spectral Inv.
Finally, if necessary, activate the Spectral inversion. The Auto mode permits to automate this selection except for the following Code
Rate: 4/5, 5/6, 6/7 and 8/9. If the spectral inversion is not correctly selected, reception will not be correct.
4) Attenuator
It permits to select attenuation between 0 and 30 dB. It is advisable to activate the 30 dB attenuator under that measurement conditions
where the signal level is near to the maximum input level (approximately starting from 20 dB under the maximum level) and it is possible
that the tuner becomes saturated. Under no-saturation conditions, when increasing the attenuation value the BER measure must to
maintain or to increase (insufficient signal level) but never to decrease.
Once you have defined the QPSK signal parameters, it will be possible to measure the BER.
When the BER measuring mode is selected, the monitor will show a picture like the following:
Figure 28.- 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 29).
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 and the number of non correctable errors (W.P. - wrong packets-) received after Reed-Solomon for the measuring
time.
Figure 29.- Digital reception system via satellite.
TUNER
QPSK
DEMOD.
DUAL A/D
VITERBI
REED-
SOLOMON
BER before FEC
BER after Viterbi
Wrong packets
I
Q
QPSK
1.0E-7
BER before FEC:
2.0E-3
-6 -5 -4 -3 -2 -1
BER after Viterbi:
2.0E-3
-8 -7 -6 -5 QEF -3 -2
Freq: 1777.0 + 1.2 MHz
W.P. 0 in 00:22:59
MPEG-2
