AP01 HD rev.09 UK
38
7.3.7.
AER (Adaptive Equalizer Response) in OFDM
The OFDM 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.
This delay, or echo, is not dangerous whatever its level is, until it is inside the guard
interval. If it exceeds the guard interval instead, it behaves as a real disturbance and it may
compromise reception already when level is just 20 dB lower than the main signal.
The equalizer response is also used to control in case of iso-frequency.
In case of reception from more transmitters, their signal should ideally arrive at the same
time.
Since transmitters are usually located in different places, their signals arrive at different
moments. The maximum difference between these moments must be inside the guard
interval otherwise, as for echoes, reception is compromised.
In the instrument, through the shaft encoder it is possible to position a marker on
reflection; the time delay (DT) and space delay (DS) are displayed, so it is possible to
determine whether reflection is inside (or outside) the guard interval.
As already explained, the instrument measures delays between echoes or carriers; however,
since the propagation speed of a radiofrequency signal in the air is almost steady and equal
to the light speed, it is also possible to calibrate the scale in length.
In order to do it, press
SPECT
, turn the shaft encoder to select the desired scale and press
ENTER
to confirm.
AER analysis for COFDM digital signals
Analysis range
⇒
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
Through marker with resolution 7.33x10
-8
s = 22 m
Guard interval
Automatic display with Marker
