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AM5(e) Classic Instruction Manual
5.3
LINE TERMINATION IMPEDANCES
For correct level measurements and to ensure that the tested equipment operates
properly, the correct terminating impedance must be selected. The AM5(e) Classic can
terminate TX/2W lines with 135, 150, 600, 900, or 1200 Ω impedance (see 5.2.6).
RX lines may be similarly terminated. In addition for a 4-Wire line, the Bridging position
provides a high-impedance (greater than 50,000 Ω). Bridging is useful where
measurements are made on a circuit between two interconnected pieces of equipment
which are already properly terminated to one another (see 5.2.8).
The terminations provided by the AM5(e) Classic are resistive for AC and open-circuit
for DC. However, for some types of 2-Wire lines, a series capacitance may be inserted
into the circuit through the LINE 2W 2µf selection.
For TX/2W or RX circuits which require a DC path between tip and ring (A and B), the
OFF HOOK selection provides the electronic equivalent of a loading coil. OFF HOOK
provides a DC path of 200Ω or less for DC, while maintaining the AC impedance that
has been selected.
The general guidelines for terminating lines correctly are:
1.
For 4-Wire circuits
•
Select proper impedance for incoming (RX) line unless the line is already
terminated and the AM5(e) Classic is being used to monitor the line. If
monitoring, set BRDGing and then set the impedance to correspond to the
impedance of the circuit being monitored.
NOTE: Impedance must be set to obtain proper level measurements. This is
necessary because RX level calculation is based on the RX impedance setting. For
more information refer to 5.2.10
•
Select proper impedance for outgoing (TX) line.
•
If a DC path is required, select OFF HOOK.
2.
For 2-Wire circuits
•
Select proper impedance for the TX/2W line. To obtain proper level
measurement calculations, set the RX impedance to match since level
measurements are based on the RX impedance settings (see NOTE above).
•
If a low-value series capacitance is required, select LINE 2W 2.16µf.
•
If a DC path is required, select OFF HOOK.
NOTE: An external instrument connected to the DIAL terminals should also be
set to match the impedance of the TX/2W line that is connected to the TX/2W jack.
No internal AM5(e) Classic circuitry is connected when the DIAL is selected.
5.4
TEST CONFIGURATIONS
There are three basic AM5(e) Classic configurations for testing 2-Wire and 4-Wire
circuits.
November 19, 1997
(18-0047)
Page 5-7
LINE FUNCTIONS
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