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7.4.5.
Use of module
Set the frequency of QAM digital channel.
To activate the OFDM module, press the DIGI key.
If also the OFDM module is installed on the instrument, a menu is displayed to choose
between the two systems: in this case, select QAM through the shaft encoder and press
ENTER
.
"MODULATION"
and
"SPECTRUM"
are automatically-set parameters.
The "
SYMBOL RATE"
must be set by the user as follows:
•
Press
MENU
key; the cursor appears next to “SYMBOL RATE” on the screen.
•
through the shaft encoder, select the parameter to be modified moving the cursor to the
corresponding item.
•
Press
ENTER
again.
•
Select the desired value through the shaft encoder.
•
Press
ENTER
again to confirm the new setting.
In case of program storage, settings of Symbol Rate, Modulation and Spectrum will be
stored, too, together with the other usual values such as frequency, etc. pressing keys
2nd
+
STORE
.
Pressing the
TV
key, instead, you activate the MPEG4/MPEG2 decoder to display free-to-air
digital programs (see the related section).
To activate the RESPONSE-TO-PULSE (AER) function, press
MENU
. Then select the
corresponding box and press
ENTER
to confirm your selection.
To exit the QAM function, press
SPECT
.
7.4.6.
AER (Adaptive Equalizer Response) in QAM
The QAM transmission, as per all the other digital signals, is a sequence of symbols. In the
RESPONSE function of the equalizer the screen displays the symbol-reception trend over the
time. It can be noticed that the symbol arrives concentrated in a short time span, so it
usually looks like a single and well-defined peak.
In case the signal arrives not only directly but also from another path, for example from a
reflection, there will be another peak which is as much more delayed as the covered
distance is longer.
The AER function in QAM enables to display possible reflections or excessive gains of the
equalizer due to an anomaly on the RF cable and enables also to calculate the point of
installation where the anomaly is.
Turning the shaft encoder, move the marker onto the reflection: the instrument indicates, in
µsec, the time distance to this fault (
t
).
To calculate the distance between the signal-application point and the fault point, the
dielectric constant of the insulating material placed between the core and shielded braiding
should be known. For instance:
2.29 for cable with “PE” compact polyethylene insulation
1.59 for cables with “PEE” foamed polyethylene insulation.
Propagation speed
v
of electromagnetic waves in the cable is calculated taking into account
the light speed, which is 300000 km/s:
v
= 300000/
√
2.29 = 198000 km/s for PE
cables
√
