UM_1966B_024 73M1966B Evaluation Kit User Manual
Rev. 1.0 9
The circuit setup shown in Figure 3 is required when using a PCM test set. It allows more flexibility and
assures the power supply and current limit resistor will not affect the impedance selected in the PCM test
set. The inductors isolate the power supply and have a high impedance at audio frequencies. The large
coupling capacitors isolate the DC feed from the channel tester and have a very low impedance at audio
frequencies to prevent measurement errors.
Termination/current limit
Figure 4: Simple DC Feed Circuit
Error! Reference source not found. illustrates how to use a series resistor DC feed. The series resistor
does not work when using a channel tester like the Wandel & Goltermann because the current limiting
resistor affects the termination impedance set inside the test set (see below). The effect is to cause
erroneous results. For a 600 Ohm termination, the current limit resistor can be set to 600 Ohms and the
DC voltage can then be adjusted to get the desired current. This arrangement does not take into
consideration the power supply source resistance, however.
If the setting for the receive termination is set to a high impedance in the PCM-4, some transmit tests can
be performed, but be sure the PCM-4 is AC coupled to the set-up! The PCM-4 does not allow DC input
voltages. For receive tests, the current limit resistance will appear to the PCM-4 as an additional load in
parallel with the impedance of the 73M1966B termination impedance, so the measurements will not be
accurate.
2.3.1 Considerations When Connecting to the Telephone Network
Special care must be taken when using equipment connected to the telephone network RJ-11 socket.
Equipment must not be connected to the ground of the main socket when using commercially available
line simulators since these are intended to have a floating connection relative to power ground. Also, if
one oscilloscope probe is connected between TIP, RING or other line-side points, the ground reference
must also be connected to the network, not to ground. No other probe can be connected to the host side
of the board to monitor other signals using the host ground as reference unless isolated grounds are
supported for each channel. Oscilloscopes with isolated channels can be used. Separate ungrounded
oscilloscopes (using a “cheater plug” on the power connector) can be used to monitor the line side only,
but this can lead to additional noise when making measurements. If you use a non-commercial network
simulator, some of these problems can be avoided, but care must still be taken with the connections.
2.4 Graphical User Interface
The 73M1966B comes with a Graphical User Interface (GUI) that allows access to various components
on the board through a USB connection from a host PC computer running Windows XP. The GUI is used
to configure the 73M1966B Demo Board, control the registers and read the status conditions of the board.
Install the GUI following the instructions in the 73M1866B/73M1966B GUI User Guide.
