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NTEGRATED
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LITELINK III Evaluation Board Users Guide
6
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UG-CPC5622-EVAL-600R - Rev A
polarity. This functionality is provided by the compact
CPC5712U Voltage Monitor that uses high value
resistors to bridge the barrier and only five small
resistors to set the voltage detectors switching point
thresholds and hystereses.
Three voltage detector outputs provide the information
necessary to determine the loop’s status. They are: 1)
LOOP; 2) LIU*; and 3) POLARITY. The loop status
can be determined by examining the state of the
detectors.
For example, equipment such as satellite set top
boxes and security systems that automatically dial out
need to know if the line is attached to the network
(loop battery present), and if the line is available for
use (no off-hook by another device or a phone). To
make this determination, the logical levels of the
LOOP and LIU* signals need to be considered.
Because LOOP = 0 indicates there is no battery feed
from the network, the line is not available for service
and the LIU* output has no meaning. With LOOP = 1,
battery feed from the network is present and the LIU*
output is used to determine the availability of the line.
With LIU* = 0, another device on the line is off-hook
and again, the line is not available. Only when
LOOP = 1 AND LIU* = 1 is the line available.
3.6 PCB Capacitor and Noise Cancellation Synopsis
This evaluation board addresses a noise issue
reported by customers using ungrounded two-prong
AC switching power supplies. Ungrounded power
supplies generate a common mode voltage on the low
voltage side of the device that has frequency
components consisting of the power line fundamental,
harmonics of the power line frequency, and power
supply switching frequencies with harmonics.
A low cost solution is presented that uses the small
capacitance from copper shapes on the printed circuit
board to insert an inverted phase noise current into a
summing amplifier to resolve the transmit path noise
issue and manipulation of existing feedback loops to
solve the receive path noise issue.
Above the LITELINK symbol on the schematic shown
in
Figure 3 on Page 8
is a capacitor labeled C_PCB.
This very small capacitor of approximately 60 - 85 fF is
constructed from PCB copper shapes to couple the
common mode noise from the low voltage side into
LITELINK’s transmit path at the NTS node on pin 26.
NTS is an inverting input of an amplifier whose output
is located at node NTF on pin 26. Using this amplifier’s
input as a summing node for the noise, the common
mode conversion in the transmit path is greatly
reduced. This performance improvement is easily
verified by the Longitudinal Balance measurement.
Enhancing the common mode rejection in the transmit
path greatly reduces the noise power output onto tip
and ring. This provides the mechanism to improve the
common mode noise conversion in the receive path.
In practice, normal full duplex voice transmission onto
tip and ring utilizes a cancellation circuit, commonly
referred to as a “transhybrid” circuit, to reduce the
power level of the transmit signal being returned to the
talker via the receive path. This returned signal is
ofttimes referred to as the reflected signal. Transhybrid
cancellation circuits are generally implemented using
the summing node of an amplifier to null out the
reflected signal. Perfect cancellation occurs when the
currents of the reflected signal in the receive path and
the transmit signal from the transmit path into the
summing node have equal magnitude and are 180
apart. In this type of circuit configuration when one
signal into the summing node is missing, the other
signal is passed through the amplifier and continues
through the receive path.
The noise cancellation circuit for the receive path
takes advantage of this behavior. With very little of the
transmit noise remaining in the signal output onto tip
and ring, the noise generated in the transmit path is
passed through the transhybrid cancellation circuit
and continues through the receive path. When this
noise signal combines with the common mode
conversion noise generated by the receive path, the
two noise signals effectively cancel each other out
resulting in a clean receive transmission path. This is
due to the noise signals being out of phase with each
other and the original transmit noise signal being
properly scaled to match the magnitude of the
generated receive path noise.
3.7 Stuffing Options
CPC5622-EVAL-600R Evaluation Boards can be used
to evaluate LITELINK III circuits connected to virtually
any type of host equipment having an analog interface
with many types of telephone networks. This flexibility
will often require changes to the components in the
circuit.
