For QAM signals, the analyzer’s center frequency must be close to the transmitted
carrier frequency to achieve carrier lock. The required proximity of the center
frequency to the carrier frequency varies depending on the signal type, symbol rate,
and system noise. If symbol locking appears poor, you may achieve better carrier
locking by observing the “Freq Err” value in the symbol table of any successfully
locked measurement and adding that amount to the center frequency.
To obtain reliable carrier locking, the difference between the analyzer’s center
frequency and carrier frequency should be within 3% of the symbol rate for 16 and
32 QAM. For 64 and 256 QAM, the difference should be within 1% and 0.2%:
16 or 32 QAM/DVB QAM:
(center frequency)−(carrier frequency)≤±.03 (symbol rate)
64 QAM/DVB QAM:
(center frequency)−(carrier frequency)≤±.01 (symbol rate)
256 QAM/DVB QAM:
(center frequency)−(carrier frequency)≤±.002 (symbol rate)
For QAM signals, the result length also affects the analyzer’s ability to achieve
reliable carrier lock, as shown in the following table. Result lengths less than those
shown may result in unreliable carrier lock. Optimal carrier locking occurs when
the result length is 1000 or greater.
If the modulation type is:
The minimum result length
for reliable carrier lock is:
16 QAM or 16 DVB QAM 50 symbols
32 QAM or 32 DVB QAM 75 symbols
64 QAM or 64 DVB QAM 150 symbols
256 QAM 400 symbols
The following may also affect carrier locking:
l
A frequency span that is too narrow.
l
An incorrect range setting.
l
Using the wrong measured or reference filter.
Video Demodulation Concepts (Opt. AYH)
23 - 8
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