Wavetek 147 Specifikace stáhnout pdf (Strana 179)

Digital Analysis (QAM View OPT4): Digital Summary Mode

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of bits in the data transmission. This ratio is expressed in scientific notation,

where 1e-3 equals one error in every 1000 bits transmitted. One error in

every 1,000,000 bits transmitted is expressed as 1e-6. A more negative

exponent value indicates a lower error rate.

BER is another good indicator of overall system performance. The SDA has

Pass/Fail BER default thresholds of 1e-8 before Forward Error Correction

and 1e-9 after Forward Error Correction.

Forward Error Correction (FEC)

Forward Error Correction adds redundant information to the data stream to

decrease the number of bit errors introduced by the transmission channel.

The BER before FEC is the sum of all bit errors (correctable and

uncorrectable). The BER after FEC indicates the number of uncorrectable

errors only -- that is, those errors that FEC was unable to correct and that

were passed on to the decoding circuitry. The difference between pre- and

post-FEC, therefore, will indicate how hard the FEC function is working and

how close the system is to failure.

Modulation Format

The SDA analyzes two Quadrature Amplitude Modulation formats, 64 QAM

and 256 QAM. Changes to the modulation format should be made only when

it is certain that the modulation of the digital signal is not compatible with the

current setting. Edits to the modulation format may be saved to the Channel

Plan (see Fig. 9-6).

Symbol Rate

The Symbol Rate corresponds to the size and shape (bandwidth) of the

digital signal. This rate is expressed in “millions of symbols per second”

(Msym/s) and may be edited in the Digital Summary Mode. The default for

64 QAM is 5.057. The default for 256 QAM is 5.360. Edits to the default

symbol rate should be made only when more accurate data is obtained from

equipment specifications or further analysis of the signal. Edits to the symbol

rate may be saved to the Channel Plan (see Fig. 9-7).

DETAIL Display

For additional information, the digital DETAIL display (see Fig. 9-4) shows

the channel and modulation format, the symbol rate, the carrier offset (in

KHz), the equalizer stress, and a Pass/Fail value for each of these

measurements based on default settings or user-defined limits.

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Wavetek 147 Specifikace Strana 179