Tektronix OM4006D, Руководство пользователя

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Core Processing function reference
1. The oscilloscope waveforms are adjusted for the known relative delays
ChDelay between the four scope channels. The delay-adjusted values are
integrated into a dual polarization representation (a Jones vector) vs. time,
given the known relative phase and polarizations of the optical hybrid
pHyb. Often there is no content at the symbol clock frequency in this dual
polarization signal.
2. A nonlinear function is applied to the signal to generate some clock content.
The nonlinear function is de
fi ned in the input variable NonlinFunc.
3. Teh application sweeps the frequency within the given window, FreqWindow,
to search for a spike at the clock frequency.
Nonlinear function The nonlinear function is set by the user via the input parameter NonlinFunc.
This parameter is a string which should contain a MATLAB function evaluating
variable Y in terms of variable X. If NonlinFunc is an empty string then the
function defaults to
Y = abs(X(1,:)).^2+abs(X(2,:)).^2
The same function is used if NonlinFunc cannot be evaluated, or if it does not
produce a valid Y, and an alert is generated.
The default function works in most cases with a non-return to zero (NRZ) signal.
If the signal is a return-to-zero (RZ) signal then it may be better to use
Y = sqrt(abs(X(1,:)).^2+abs(X(2,:)).^2)
If the edges of the signal have excessive ringing then that may cause the clock
phase reported by EstimateClock to be offset compared to the true center of the
symbol. In that case a limiting function can be applied to reduce the impact of
the ringing, such as this one
Y = min(abs(X(1,:)).^2+abs(X(2,:)).^2, 0.2*mean(abs(X(1,:)).^2+abs(X(2,:)).^2))
Frequency window
The symbol clock frequency output lies between FreqWindow.Low and
FreqWindow.High. If FreqWindow.Low = FreqWindow.High then the frequency
search step is omitted, and the clock phase only is calculated at the given
frequency. Also the function takes less time to execute.
Block processing
The three step process outlined above is followed for the fi rst block to be
processed. For subsequent blocks the clock phase is calculated using a fi xed clock
frequency, which is the same clock frequency as determined by the previous
block. The phase of the new block is used together with the phases of earlier
blocks to calculate an average clock frequency and corresponding average
clock phase, which are reported in output parameter Clock. The reported clock
frequency is forced to lie within FreqWindow.Low...FreqWindow.High. Thus, the
clock frequency and phase reported by the
fi nal block are averages of the whole
measurement record, and are more accurate than those reported by earlier blocks.
98 OM4000D Series Coherent Lightwave Signal Analyzer