Transhybrid Loss - Nortel Meridian 1 Succession 1000 Manual

Transhybrid loss

Impedance mismatches between hybrid compromise networks and 2-wire
terminations (line or trunk) can result in instability and listener echo
degradations in the 4-wire switching path of a digital PBX. The echo return
loss requirements presented in Tables 58 and 59 do not adequately address
this problem. Thus, for digital PBXs, requirements are placed on the return
loss at the hybrid between the 2-wire interface and the 4-wire switching path.
This requirement is called transhybrid loss.
Two-wire ports with external facilities present a distribution of impedances
to the PBX interface. To effect a good match with this distribution and to
achieve the transhybrid loss specifications shown in Table 60, a
three-element compromise impedance network is used in 2-wire analog trunk
ports to balance the impedance of the trunk (see Figure 1).
Figure 1
Compromise impedance network
Trans-hybrid loss is measured from a balanced 4-wire port (with transmit and
receive legs at equal level) to the 2-wire port. The 2-wire port is terminated
in a compromise impedance network that consists of 600 ohms (for stations)
or the network in Figure 1 (for 2-wire trunks). Table 60 gives the minimum
trans-hybrid loss over the indicated frequency ranges for input signals at the
Transmission characteristics—µ-Law
350 ohm
Page 173 of 236
0.21 µF
1000 ohm
553-2082
Transmission Parameters