Tektronix OM4006D, User Manual

Page: 163 / 205

Appendix E: Equivalent-Time (ET) oscilloscope operation
Oscilloscope sampling rate
Reference magnitude
response
reference phase
≤ 2x the bandwidth of the
OM4000 (BW)
Flat Linear
> 2x the bandwidth of the
OM4000 (BW) 4th order digital (bilinear)
Bessel
fi lter with 3 dB cutoff
at BW + 2 GHz
Linear
Equalization is speci fi c to the sampling rate of the oscilloscope. As an example, if
the bandwidth of the OM4000 is 30 GHz and the sampling rate of the oscilloscope
is 80 Gs/s, the combined effect of the OM4000 receiver front end response and
the digital equalization
fi lter will be that of a 4th order digital Bessel fi lter with
3 dB cutoff at 32 GHz, as shown in the following graph.
The digital fi lter is applied directly to the MATLAB workspace variables
Vblock(1).Values through Vblock(4).Values using a 100 tap FIR fi lter.
Equalization provides a challenge when working in ET mode. Equivalent-Time
captured signals can be equalized to see the corrected waveshape when using
small enough time resolution or time step to emulate a real-time captured
waveform. A number of
fi lters are provided with the most commonly used time
steps which are valid for correction.
The OUI searches through the Program Files\Optametra\OUI4006\ExecFiles
directory and applies the
fi lter it fi nds with the proper time step. If the proper
fi lter is not found, then none is applied and a warning is printed in the Engine
Command Response Window. When the SNR is < 20 dB in ET mode, it is best to
turn off the correction
fi lter using EqFilt = []; in the Engine Command Window.
For more information, or for equalization support of a different scope sampling
rate, contact customer support.
OM4000D Series Coherent Lightwave Signal Analyzer 139