TELE-audiovision - The World’s Largest Digital TV Trade Magazine - page 40

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TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine
— 01-02/2014
Testing
10. Time for to put the Catline TVB-02 under a stress test.
In order to take amplification systems to the limit we use
a cable reel with 100 m of class A coaxial cable. This will
attenuate the signal considerably and allows to test for
example HF amplifiers. It would be quite unusual for a
home or business building to require such a long cable,
but we do like to be on the safe side. We started by testing
a DVB-T2 signal without any amplification. The signal
power was -30dBm at the source, but at the end of our
100m long cable no suitable signal could be found.
11. Hence we attached a simple amplifier to increase the
signal prior to feeding it into the cable reel. This time we
could get the signal at the other end of the cable.
12. The spectrum shows the DVB-T2 transponder at
754MHz with -39.8dBm. This alone is not sufficient to
evaluate the signal quality.
13. The measurements, however, indicate that there is a
lock and the LBER value (Bit Error Ratio after the error
correction) of under 1.0E-8 is pretty good. It means that
you will get less than one erroneous bit in every ten
million bits. The CBER (BER before error correction)
however, is just acceptable at 1.1E-04, it could definitely
be better. The signal power is -30.8dBm, so after the
amplification and the natural attenuation due to the cable
itself we are just 0.8dBm worse than without the 100m
long cable.
14. The constellation diagram of the DVB-T2 signal shows
a near to perfect pattern. Notice that the tilting of the
DVB-T2 constellation is done at the DVB-T2 modulator and
has nothing to do with the tilting obtained by a Barscan.
This is done because at this high modulation schemes a
tilted constellation is more robust and efficient than the
non-tilted modulations of QSPK, QAM or COFDM.
15. Nothing beats the visualisation of the TV live image to
really make sure that the signal is received in full quality.
No problems here.
16. Just to make sure we repeated the test, but this time
with a DVB-C signal. This is a less problematic signal. The
spectrum looks all right.
17. After 100m the signal has a power of -37.9 dBM. Notice
the PRE-BER and POST-BER: absolute dream values.
As a side note: it pays off using quality class “A” cable.
The golden rule is: never try to save money by using low
quality cable to save a bit of money – you will later on
spend much more to compensate for that.
18. Perfect constellation diagram of the DVB-C signal.
This concludes that we can indeed distribute the HF
signal over a 100m long coaxial cable, provided that we
use an amplifier.
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