AP301-FULL-HD_rev.09_UK
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While the OFDM card is function, the tuning frequency can be changed, like for UHF band in
analogue. It is necessary to keep in mind, however, that the instrument’s response time is
longer in digital mode. When storing a program, by pressing the
2nd
and
STORE
buttons,
the digital parameter settings are stored as well, along with the other usual values like
frequency, etc.
Press
SPECT
to exit the OFDM function.
7.3.7.
AER (Adaptive Equalizer Response) in OFDM
Like that of all other digital signals, OFDM transmission takes place as a succession of
symbols. In the equalizer's RESPONSE function, the screen displays the progress of a
symbol's reception over time. It may be noted that the symbol arrives concentrated in a
narrow timeframe; it therefore normally appears as a single, well-defined peak.
If the signal should arrive both directly as well as from another path, due to a reflection for
example, another peak will be present, which will be delayed based on the longer distance
to be travelled.
There is no danger as long as this delay, or echo, regardless of its level, is contained within
the guard time. If, on the other hand, it arrives beyond the guard time, it will act as an
actual disturbance and can seriously compromise reception even at a level of 20 dB below
that of the main signal.
The equalizer response can also be used as a control in the case of isofrequency
transmissions.
If various transmitters are being received simultaneously, ideally their signals should arrive
at the same instant.
As the transmitters will normally be positioned in various locations, their signals will also
arrive at different times. The maximum difference between these times must fall within the
guard time. Otherwise reception could be compromised, like in the case of the echos.
A marker can be placed upon the reflection within the instrument using the Shaft Encoder;
in this manner, a delay time (DT) and delay space (DS) will be shown, which can be used to
determine whether the reflection falls within (or outside) the guard time.
As has been shown, the instrument is capable of measuring the delays between echos and
carriers, but since a radiofrequency signal’s propagation over the air is nearly constant and
equal to the speed of light, the scale can also be calibrated in length.
In order to do this, press the
SPECT
button, turn the SHAFT ENCODER to select the desired
scale and press
ENTER
to confirm.
AER analyses for COFDM digital signals
Range of analysis
⇒
0 - 4.65x10
-6
s = 0 - 1.4 km.
⇒
0 - 9.29x10
-6
s = 0 - 2.8 km.
⇒
0 - 1.86x10
-5
s = 0 - 5.6 km.
⇒
0 - 3.72x10
-5
s = 0 - 11.2 km.
⇒
0 - 7.43x10
-5
s = 0 - 22.5 km.
⇒
0 - 1.49x10
-4
s = 0 - 45.1 km.
⇒
0 - 2.23x10
-4
s = 0 - 67.6 km.
Indication
By means of markers with resolutions of 7.33x10
-8
s = 22 m
Guard Time
Automatic display with Markers.
