Chapter 6 - TV Measuring Range
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Impulse response
6.2.2.4.9
As with DVB-T, DVB-T2 is intended for operation in a single frequency network. This means
several transmitters transmit on the same frequency.
The transmitters involved must operate synchronously on the same frequency.
The maximum transmitter distance depends on the Guard Interval used.
At the receiving location, the signals from individual transmitters superimpose on each other to form
a sum signal.
The result can be constructive or destructive depending on the transit time difference and the
received field strength. The impulse response graphically represents attenuation and transit time
difference of the individual signals.
In order to calculate the channel impulse response, the DVB-T2 receiver requires information on
the channel transmission function. The demodulator obtains this information by evaluating the pilot
carrier in the OFDM signal.
The measuring receiver must receive a DVB-T2 signal in order to measure the impulse response.
The instrument should be tuned to an appropriate channel to do this.
The instrument displays the impulse response on the screen when the
IMPULSERES
menu item is
selected. A menu for additional settings will appear at the same time.
You can “freeze” the picture using
FREEZE
. You can expand the impulse response in the
horizontal direction using
ZOOM
. You can define the unit of the x-axis with
µs
or
km
. Time and
length are related via the speed of light, c:=3•10
8
m/s. You can end the display of the impulse
response via the menu item
BACK
.
Figure 6-20 DVB-T2 Impulse response
The printed example shows an impulse response with a primary impulse (left picture edge) and
several secondary impulses at a distance of approximately 16 km from the primary impulse.
You can move the cursor (small triangle) left and right using the
←/→
keys. At the top right edge of
the picture, the transit time difference and attenuation in relation to the primary impulse is displayed
at the cursor position.
Peak-Search Function
While the impulse response is built up, the instrument determines the four highest secondary
impulses apart from the main impulse. If there are echoes, the cursor moves to the highest
secondary impulse after the second cycle. By pressing the keys ↑ and ↓ the cursor may be moved
to further echoes cyclically one after the other. The distance and/or the delay as compared to the
main impulse may be taking taken from the readings in the header of the diagram.
