Reference
For electrical bandwidths the reference of a system is commonly the response of
the system to a sinusoidal frequency at or near DC. The point at which the system
response (power) is at one half would therefore be:
In terms of frequency, voltage, and resistance the bandwidth is expressed as:
where V(f) is the RMS of the voltage swing response at the bandwidth frequency,
and V(DC) is the RMS voltage swing response at a frequency approaching DC.
Further math yields V(f) = 0.707 V(DC).
The expression is simpli
fi
ed by canceling the R and moving the squared term
inside the log expression to a multiple outside the log expression:
In the DSA8300, DSA8200, CSA8000 and TDS8000 Series instruments, the
vertical units displayed for an optical module are not in volts, but in watts, which
are units of power. The optical-to-electrical converter inside the module outputs
a voltage the amplitude of which is linearly dependent on the incoming optical
power; in this condition the voltage applied at the electrical sampler already
represents optical power in its linear form (as opposed to having to square the
voltage and divide by R).
For the optical sampling modules, the bandwidth where the displayed optical
power is one half that approaching DC is:
The V(f) is the frequency at which the vertical swing is one half (0.5) the V(DC)
(not 0.707). The optical bandwidth therefore corresponds to the electrical
bandwidth of -6 dB.
During impulse testing of optical modules, the resulting impulse waveform is
converted to a frequency by Fourier transform and the bandwidth is de
fi
ned
as –3 dB = 10 log(vertical swing at frequency/vertical swing at DC). During
reference receiver curve calculation, however, the de
fi
nition is changed to match
the industry standard de
fi
nition which assumes electrical bandwidths are –3 dB =
20 log(vertical swing at frequency/vertical swing at DC).
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80C12B Optical Sampling Module User Manual