Carrier locking and pilot search: VSB
The same parameters that affect carrier locking for QAM signals also affect carrier
locking for VSB signals. For VSB signals, the analyzer must also find the pilot
signal to achieve carrier lock.
When you start a VSB measurement, the analyzer displays SEARCHING FOR
PILOT. This message appears for a fixed amount of time while the analyzer
searches for the pilot signal. The measurement begins when the pilot search ends
(when the SEARCHING FOR PILOT message disappears).
If your VSB signal uses a high-side pilot, you must configure the analyzer to
demodulate a high-side pilot by selecting the Video Demodulation instrument mode
and pressing [
Instrument Mode], [demodulation setup], [more], [freq spectrum mirror]. If you don’t
do this, the analyzer cannot find the pilot signal and cannot lock to your signal.
If the analyzer cannot lock to your signal, it displays CARRIER LOCK?. There are
several conditions that may cause loss of carrier lock, one of which is an
unsuccessful pilot search. If carrier lock is not obtained after several
measurements, the analyzer assumes the pilot search failed and repeats the pilot
search.
CARRIER LOCK? may appear even when the data appears to be locked. If this
occurs, the results may be inaccurate. Normally, this condition clears in a few
measurements as the instrument obtains better estimates of the carrier frequency.
The following paragraphs describe other conditions that may cause loss of carrier
lock. The analyzer always assumes pilot search failed if there are carrier lock
problems, even if the cause is one of the conditions below.
Like QAM signals, the analyzer’s center frequency must be close to the transmitted
carrier frequency to achieve carrier lock for VSB signals. The following formulas
show you how to calculate the ideal center frequency. Use the formula for low-side
pilot if your pilot is on the left (low) side of your spectrum; use high-side pilot if
your pilot is on the right (high) side of your spectrum.
Center Frequency (LOW SIDE PILOT)=
Symbol Rate
4
+(Pilot Frequency)
Center Frequency (HIGH SIDE PILOT)=(Pilot Frequency)−
Symbol Rate
4
Hint The ‘’Using Video Demodulation’’ chapter contains a task that shows you how to
use the analyzer’s frequency counter to measure your pilot frequency.
The result length also affects the analyzer’s carrier locking. For VSB
measurements, the result length should be at least 800 symbols. Smaller result
lengths may cause unreliable carrier locking.
Video Demodulation Concepts (Opt. AYH)
23 - 9
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