Preliminary Information
Functional Description
The MH88524 is a Dual 2-4 Wire Circuit used to
interface between ground reference 2-Wire circuitry
and ground referenced 4-Wire circuitry. The device
can accommodate two full duplex audio links.
MH88524
Figure 6, shows, illustrates a typical connection
between an Interface Circuit (MH88631) and the
MH88524. Note how the return loss occurs when
JUNi is terminated with 754Ω.
Figure 8 illustrates a typical connection between two
interface circuits (MH88631), through an MH88524
and two crosspoint switches. Optimum return loss
occurs when JUNi is terminated with 754Ω. Since the
JUNi input/output impedance is 604Ω, the MH88510
JUNC input/output impedance is 604Ω, and the
crosspoint switches resistance are 75W + 75Ω, this
configuration gives optimum return loss.
Hybrid
The 2-4 Wire hybrid circuit separates the ground
reference full duplex signal at JUNi (where i=1 or 2)
of the switched line into receive and transmit ground
referenced signals the RXi (Receive) and TXi
(Transmit). The hybrid also prevents the input signal
at RXi from appearing at TXi. The degree to which
the hybrid minimises the contribution to the RXi
signal at the TXi output is specified as transhybrid
loss. For maximising transhybrid loss, see the
Transhybrid Loss section.
The 4-Wire side can be interfaces to a COIC such as
the MH88631 for use in analog voice switched
systems; or a filter/codec, such as the Mitel MT896X,
for use in digital voice switched systems.
The 2-wire side can be interfaces to a crosspoint
switch such as the MT8816 or a junctor SLIC such
as the MH88510 for use in analog voice switched
systems.
Transhybrid Loss
THL = log (VRX/VTX)
Transhybrid loss is maximised when the JUNi
termination impedance is 754W. In addition, good
transhybrid loss is indicated in Figure 4 and AC
Electrical Characteristics.
Fixed Transmit and Receive Gain
Transmit Gain (JUNi to TXi, TXi/JUNi) and receive
Gain (RXi to JUNi, JUNi/RXi) are both fixed at 0dBV
providing the MH88524 JUNi impedance is 754Ω.
Return Loss at Junctor
The MH88524’s Junctor impedance (Zin) is fixed at
604Ω nominal when RXi and TXi in a feedback loop
as shown in Figure 6, the JUNi impedance will
change, see Return Loss with Interface Circuit.
Application with MT8840, MH88500 and
MH88524
Figure 11 illustrates an application for the
MH88524’s wide bandwidth. The MT8840 requires a
2-4 Wire converter which has good transhybrid loss
at 32kHz. Since the MH88524 operates to 50kHz, it
is ideal for this application. In addition, if a SLIC
(Subscriber Line Interface Circuit) is required, the
MH88500 can also be used since it also has a 604Ω
Junctor and a wide bandwidth.
Mechanical Data See Figure 12.
Return Loss with Interface Circuit
To maximise return loss at Tip-Ring of the Interface
Circuit, the termination impedance at Tip-Ring of the
Interface Circuit (COIC or SLIC) should match the
Interface Circuit’s input impedance (600Ω, 900Ω or
complex). However, with the inclusion of the
MH88524, the interface circuit’s input impedance is
dependent on the JUNi termination resistance. For
optimum return loss the JUNi should be terminated
with 754Ω.
2-77
MITEL [ MITEL NETWORKS CORPORATION ]
