Tektronix OM4006D, User Manual

Page: 62 / 205

Operating basics
Front end fi ltering.
The signal may be fi ltered according to the settings of the
Front end fi lter group. The fi lter is a bandpass fi lter in the optical domain, which
is equivalent to a lowpass fi lter acting on the electrical input signals to the
oscilloscope (assuming that the center frequency is zero). The cutoff frequencies
speci
fi ed are those corresponding to a lowpass fi lter. The width of the bandpass
(optical domain) fi lter is twice the speci fi ed lowpass cutoff frequency.
When Auto-center is checked, the fi lter is tuned to the exact center frequency of
the signal. Otherwise the fi lter is centered at the frequency speci fi ed in the Phase
group under Analysis Parameters.
There are several fi lter types available, which fall into three fi lter categories:
Fixed fi lters: Bessel (also known as Bessel-Thomson), Butterworth, square
root raised cosine, raised cosine
User-speci fi ed fi lter
Adaptive fi lters: Matched fi lter, Nyquist fi lter
The fi xed fi lter types have their cutoff frequency (either 3 dB point for Bessel,
Butterworth and square root raised cosine; or 6 dB point for raised cosine)
speci
fi ed by the relevant control. The steepness of the fi lter is set by the order in
the case of Bessel and Butterworth, and by the roll-off factor in the case of the
square root raised cosine and raised cosine
fi lters.
When the User-speci fi ed fi lter is selected as the fi lter type, core processing applies
an FIR
fi lter de fi ned in a variable UserFilter. If the variable does not exist, or if it
is not valid, then core processing continues without applying a fi lter, and an Alert
is issued in the Alerts window to that effect.
UserFilter should have three fi elds: .t0, .dt and .Values. The .Values fi eld should
be a row vector of complex numbers, corresponding to the FIR coef fi cients.
The time grid (speci fi ed by UserFilter.dt) does not have to be the same as the
oscilloscope sample time interval, or be synchronous with the symbol rate. Core
processing resamples the UserFilter time grid to the input signal time grid before
it is applied. Core processing also tunes the UserFilter in frequency to the center
frequency speci
fi ed in the Phase group of Analysis Parameters, and tunes it to the
exact center frequency of the signal if Auto-center is checked. Therefore, the FIR
coef
fi cients in UserFilter should be de fi ned so that it is centered at zero frequency.
The matched and Nyquist fi lter types are not fi xed, but are de fi ned based on the
signal. The matched fi lter type, as its name implies, is the matched fi lter having
FIR coef fi cients equal to the time inversion of the signal’s impulse response. The
matched fi lter is the best possible fi lter in terms of the height of an isolated pulse
compared to the noise standard deviation. The matched fi lter may suffer from
intersymbol interference (ISI). In general, a Nyquist fi lter is a fi lter chosen for a
speci fi c signal to have the property that there is no intersymbol interference.
38 OM4000D Series Coherent Lightwave Signal Analyzer