MH88620IN
Tip/Ring driver circuitry such that impedance in the
feedback path gets reflected to the two wire port.
The MH88620IN’s Tip-ring impedance (Z
in
) is
designed to be 600
Ω
, when used with 25Ω PTC’s as
protection circuitry. For this requirement, Z1 and Z2
should be connected together on the PCB. To
accommodate the use of lower value PTC’s a series
resistance can be connected between Z1 and Z2.
For example, if two 8Ω PTCs are used, connect
340Ω between Z1 and Z2.
The design uses a 0.1 times impedance amplifier so
the 340Ω actually adds 34Ω of additional impedance
to the 550Ω (16 + 34 +550 = 600). For complex
impedance setting, a capacitor and/or resistor can
be connected between Z1 and Z2. For example, if
Return Loss is to be maximized for a Z
in
of 600
+2.2µF, a 0.22µF cap can be connected between Z1
and Z2.
Preliminary Information
Longitudinal Balance
The longitudinal balance specifies the degree of
common mode rejection in the 2 to 4 wire direction.
Precision laser trimming of internal resistors in the
hybrid ensures good overall longitudinal balance.
The interface circuitry can operate in the presence of
induced longitudinal currents of up to 40 mA RMS at
60 Hz.
DTMF
The DTMF tones are transmitted and received at the
4-wire port.
High voltage Capability
Inherent in the thick-film process is the ability of the
substrate to handle high voltage. The standard Mitel
thick-film process provides dielectric strengths of
greater than 1000 VAC or 1500 VDC. The thick-film
process allows easy integration of surface mount
components such as the high voltage bipolar power
transistor line drivers. This allows for simpler, less
elaborate and less expensive protection circuitry
required to handle high voltage transients and fault
conditions caused by lightning, induced voltages and
power line crossings.
Network Balance
Transhybrid loss is maximised when the line
termination impedance and SLIC network balance
are matched. The MH88620IN’s network balance
impedance can be set to Zin, or to a user selectable
value. Thus, the network balance impedance can be
set to any Indian or other international requirement.
An external Network Balance impedance is selected
which 0.1 times the impedance between N1 and
AGND. N2 to GND balances to 600 ohms.
On-Hook Transmission
The MH88620IN provides for on-hook transmission
which supports features such as Automatic Number
Identification (ANI). The ANI information is a FSK or
DTMF signal originating from and sent by the C.O.
during the off period of the ringing voltage being sent
to the subscriber’s set. The subscriber’s set decodes
the identification signal and displays the calling
party’s number.
Off-Hook and Dial Pulse Detection
The SHK pin goes low when the DC-loop current
exceeds a specified level. The threshold level is
internally set by the bias voltage of the switch-hook
detect circuitry. Dial pulses can be detected by
monitoring the interruption rate at the SHK pin.
These dial pulses would be debounced by the
system software.
Ring Trip Detection
The interface permits detection of an Off-Hook
condition during ringing. If the subscriber set goes
Off-Hook when the ringing signal has been applied,
the DC loop current flow will be detected within
approximately 100msecs and the SHK output will go
low. The Ring relay is automatically disabled by the
internal hardware.
Applications
As shown in the application diagram, Figure 7a, the
ringing voltage, typically 80 V
RMS
25Hz biased at -48
VDC, is applied to the subscriber line through an
external relay, K1, Enabling of the relay is performed
by applying a logic high level to the relay driver
control input, RRC.
Figure 7b, shows how balanced ringing can be
accommodated if required.
2-166
MITEL [ MITEL NETWORKS CORPORATION ]
