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In
broadband
communications
systems such as CATV equipment,
system performance may critically
rely on gain or attenuation flatness.
In particular, CATV systems are often
plagued by issues resulting from the
frequency-dependent
attenuation
of very long cables (increasing with
frequency) as well as the negative
gain slope of certain amplifiers. This
negative gain slope exhibited by
CATV system components can cause
a variety of headaches for system
designers.
For example, suppose the system’s
operating bandwidth is split into a
number of channels. Channels at
lower frequencies are subject to much
less attenuation than those at higher
frequencies, so the “louder” low-
frequency channels may saturate an
amplifier exhibiting sufficient gain to
amplify the “quieter” high-frequency
channels. Obviously an amplifier
driven to saturation in a spectrally rich
multi-channel system would result in
a whole mess of impossible-to-filter-
out intermodulation products, so this
situation is highly undesirable.
Out-of-band spurious spectral content
(e.g. intermodulation products,
harmonics) from the louder lower-
frequency signals may also degrade
the quality of the quieter, higher-
frequency signals (be it in terms of
MER, SNR, CNR, or any other Figure-
Of-Merit flavors) – and so on.
Because both the amplifiers and
the cables exhibit a negative gain/
attenuation slope, in many cases a
designer may wish to flatten this slope
at the expense of overall gain. This
can be done with a device that has a
positive attenuation slope (over the
desired operating frequency range),
called an equalizer. Equalizers may be
realized, for example, as resistive pi or
tee networks with the series resistive
elements “bypassed” by parallel
capacitors and the shunt resistive
elements “bypassed” by parallel
inductors, effectively forming a lossy
low-order high-pass structure. Figure
1 illustrates this concept.
Passive equalizers are relatively
simple to design, and inexpensive to
implement, but they lack versatility
due to passive component values
being fixed.
Any designer worth his salt will know
Compensating Frequency-Dependent Cable
Loss in CATV Systems with Mini-Circuits
Voltage-Variable Equalizers
Ben Kahtan, Mini-Circuits