ADV3205 Datasheet, PDF (18/20 Pages) Analog Devices – 60 MHz, G = +2, 16 × 16

ADV3205
Data Sheet
Areas of Crosstalk
A practical ADV3205 circuit must be mounted to some sort of
circuit board to connect it to power supplies and measurement
equipment. This, however, raises the issue that the crosstalk of a
system is a combination of the intrinsic crosstalk of the devices
in addition to the circuit board to which they are mounted. It is
important to try to separate these two areas when attempting to
minimize the effect of crosstalk.
In addition, crosstalk can occur among the inputs to a cross-
point and among the outputs. It can also occur from input to
output. Techniques are presented in the following sections for
diagnosing which part of a system is contributing to crosstalk,
as well as minimizing crosstalk.
Measuring Crosstalk
Crosstalk is measured by applying a signal to one or more channels
and measuring the relative strength of that signal on a desired
selected channel. The measurement is usually expressed as dB
down from the magnitude of the test signal. The crosstalk is
expressed by
|XT| = 20log10(Asel(s)/Atest(s))
where:
s = jw, the Laplace transform variable.
Asel(s) is the amplitude of the crosstalk induced signal in the
selected channel.
Atest(s) is the amplitude of the test signal.
It can be seen that crosstalk is a function of frequency, but not a
function of the magnitude of the test signal (to the first order).
In addition, the crosstalk signal has a phase relative to the test
signal associated with it.
A network analyzer is most commonly used to measure crosstalk
over a frequency range of interest. It can provide both magnitude
and phase information about the crosstalk signal.
As a crosspoint system or device grows larger, the number of
theoretical crosstalk combinations and permutations can become
extremely large. For example, in the case of the 16 × 16 matrix
of the ADV3205, note the number of crosstalk terms that can be
considered for a single channel, such as the IN00 input. IN00 is
programmed to connect to one of the ADV3205 outputs where
the measurement can be made.
First, the crosstalk terms associated with driving a test signal into
each of the other 15 inputs can be measured one at a time, while
applying no signal to IN00. Then, the crosstalk terms associated
with driving a parallel test signal into all 15 other inputs can be
measured two at a time in all possible combinations, then three at
a time, and so on, until finally, there is only one way to drive a test
signal into all 15 other inputs in parallel.
Each of these cases is legitimately different from the others and
may yield a unique value, depending on the resolution of the
measurement system, but it is hardly practical to measure all
these terms and then specify them. In addition, this describes
the crosstalk matrix for just one input channel. A similar crosstalk
matrix can be proposed for every other input. In addition, if the
possible combinations and permutations for connecting inputs
to the other outputs (not used for measurement) are taken into
consideration, the numbers grow impractically large. If a larger
crosspoint array of multiple ADV3205 devices is constructed, the
numbers grow larger still.
Clearly, some subset of all these cases must be selected to be used as
a guide for a practical measure of crosstalk. One common method
is to measure all hostile crosstalk; this means that the crosstalk to
the selected channel is measured while all other system channels
are driven in parallel. In general, this yields the worst crosstalk
number, but this is not always the case, due to the vector nature
of the crosstalk signal.
Other useful crosstalk measurements are those that are created by
one nearest neighbor or by the two nearest neighbors on either
side. These crosstalk measurements are generally higher than those
of more distant channels, so they can serve as a worst-case measure
for any other 1-channel or 2-channel crosstalk measurements.
Input and Output Crosstalk
The flexible programming capability of the ADV3205 can be
used to diagnose whether crosstalk is occurring more on the
input side or the output side. Some examples are illustrative. A
given input channel (IN07 in the middle for this example) can
be programmed to drive OUT07 (also in the middle). The input
to IN07 is just terminated to ground (via 50 Ω or 75 Ω) and no
signal is applied.
All the other inputs are driven in parallel with the same test signal
(provided by a distribution amplifier), with all other outputs
except OUT07 disabled. Because grounded IN07 is programmed
to drive OUT07, no signal should be present. Any signal that is
present can be attributed to the other 15 hostile input signals
because no other outputs are driven (they are all disabled).
Thus, this method measures the all-hostile input contribution
to crosstalk into IN07. Of course, the method can be used for
other input channels and combinations of hostile inputs.
For output crosstalk measurement, a single input channel is
driven (IN00, for example) and all outputs other than a given
output (IN07 in the middle) are programmed to connect to
IN00. OUT07 is programmed to connect to IN15 (far away
from IN00), which is terminated to ground. Therefore, OUT07
should not have a signal present because it is listening to a quiet
input. Any signal measured at OUT07 can be attributed to the
output crosstalk of the other 16 hostile outputs. Again, this method
can be modified to measure the other channels and the other
crosspoint matrix combinations.
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