To determine the center frequency for a VSB signal
Choosing the correct center frequency is important for all digital video
demodulation measurements. This task shows you how to determine the
correct center frequency for VSB measurements. To learn how to
determine the correct center frequency for QAM measurements, see the
‘’Video Demodulation Concepts’’ chapter. Note that you cannot use the
time-capture signal from the signals disk to perform this task. This task
uses the analyzer’s frequency counter, which cannot be used on time
capture data.
1. Initialize the analyzer:
Press [
Instrument Mode
], [
receiver
], then press:
89410A: [
input section (0-10 MHz)
].
89441A: [
RF section (0-10 MHz)
].
Press [
Preset
].
Press [
Instrument Mode
], [
vector
].
2. Increase the display resolution:
Press [
ResBw/Window
].
Press [
num freq pts] and set to 1601.
Press [
rbw mode arb].
Press [
rbw] and press the down-arrow key to select the smallest resolution
bandwidth.
3. Position the marker on the pilot:
Press [
Shift
], [
Marker
].
4. Turn on the frequency counter:
Press [
Marker Function
], [freq counter on].
5. Compute the ideal center frequency:
Center Frequency (LOW SIDE PILOT)=
Symbol Rate
4
+(Pilot Frequency)
Center Frequency (HIGH SIDE PILOT)=(Pilot Frequency)−
Symbol Rate
4
In this example, the symbol rate is 10.762 MHz, the frequency counter
shows the pilot frequency at 3.308894876 MHz, and the pilot is on the low
side of the spectrum. Using the formula for low-side pilot, the ideal
center frequency is 5.999394876 MHz. A center frequency of 6 MHz is
close enough to ensure carrier lock.
Using Video Demodulation
(Opt. AYH)
9 - 4
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