the frequency range where |Glink(m)| is constant. Furthermore, the phase response must
be linear to ensure there is no phase distortion.
As discussed in Section 3.2.1, the calibration waveform is a 100 Hz square wave
with a peak-to-peak-voltage of either 1 V or 0.1 V in 50 Q. The waveform generator
circuit was designed by George Schnetzer and is very immune to variations in time and
temperature. Therefore, the value of source measured in the lab is an accurate measure
of what source will be when the signal generator is used in the field. In the frequency
domain, the square wave consists of the fundamental frequency, 100 Hz, along with an
infinite number of discrete harmonic frequencies, all of which are odd multiples of the
fundamental. The magnitude of each harmonic is inversely proportional to the order
of the harmonic, meaning, for example, that the magnitude of the third harmonic is
one third that of the fundamental. Although the bandwidth of an ideal square wave is
infinite, it can be considered finite relative to the bandwidths of the fiber-optic links
used in the MSE. For example, the magnitude response of the Opticomm MMV-120C
link is flat out to at least 20 MHz. A 100 Hz square wave can be assumed to have zero
frequency content at 20 MHz. Therefore, if |Glink(m) I is constant up to 20 MHz and the
link has a linear phase response, then the 100 Hz square wave calibration waveform can
be used to estimate the gain of the link.
The same argument can be applied to the Nicolet Isobe 3000 and Meret fiber-optic
links, which have -3 dB bandwidths of about 15 and 20 MHz, respectively. However,
as discussed in Section 3.4.3, AC coupled low-pass filters are used at the output of
the fiber-optic receivers, which limit the low-frequency response of the Meret links to
about 1.5 Hz. This does introduce some distortion into the received calibration signal.
It is important to know the time delays associated with each fiber-optic link so that
all waveforms can be properly aligned when performing data analysis. Furthermore,
it is desirable to know the time delays to within one sample (digitization) point. If
each of the fiber-optic links is of the same type and uses the same length of fiber, the