ADV3201ASWZ datasheet(33/36 Pages) AD

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ADV3200/ADV3201

Rev. 0 | Page 33 of 3

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 decibels below the magnitude of the test signal. The crosstalk

is expressed by

⎟

⎠

⎞

⎜

⎝

⎛

)

(

)

(

log

TEST

SEL

(4)

where:

s = jω (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 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 cross-

talk 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 32 × 32 matrix

of the ADV3200/ADV3201, note the number of crosstalk terms

that can be considered for a single channel, for example, the IN00

input. IN00 is programmed to connect to one of the ADV3200/

ADV3201 outputs where the measurement can be made.

First, the crosstalk terms associated with driving a test signal

into each of the other 31 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 31 other

inputs can be measured two at a time in all possible combina-

tions, then three at a time, and so on until, finally, there is only

one way to drive a test signal into all 31 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 cross-

talk 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 quickly grow to astro-

nomical proportions. If a larger crosspoint array of multiple

ADV3200/ADV3201 devices is constructed, the numbers grow

larger still.

Obviously, some subset of all these cases must be selected as a

guide for a practical measurement 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 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; therefore, they can serve as a worst-

case measure for any other one-channel or two-channel crosstalk

measurements.

Input and Output Crosstalk

Capacitive coupling is voltage-driven (dV/dt) but is generally a

constant ratio. Capacitive crosstalk is proportional to input or

output voltage, but this ratio is not reduced by simply reducing

signal swings. Attenuation factors must be changed by changing

impedances (lowering mutual capacitance), or destructive

canceling must be utilized by summing equal and out of phase

components. For high input impedance devices such as the

ADV3200/ADV3201, capacitances generally dominate input-

generated crosstalk.

Inductive coupling is proportional to current (dI/dt) and often

scales as a constant ratio with signal voltage, but it also shows a

dependence on impedances (load current). Inductive coupling

can also be reduced by constructive canceling of equal and out

of phase fields. In the case of driving low impedance video

loads, output inductances contribute highly to output crosstalk.

The flexible programming capability of the ADV3200/ADV3201

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 pair (IN07 in the middle for this example) can be

programmed to drive OUT07 (also in the middle). The inputs

to IN07 are terminated to ground (via 50 Ω or 75 Ω resistors)

and no signal is applied.

All the other inputs are driven in parallel with the same test signal

(practically provided by a distribution amplifier), with all other

outputs except OUT07 disabled. Because the grounded IN07

input 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 all the

hostile input contribution to crosstalk into IN07. Of course, this

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. Thus 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 15 hostile outputs. Again, this

method can be modified to measure other channels and other

crosspoint matrix combinations.

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