MH88437AS-PI datasheet(5/16 Pages) MITEL

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Advance Information

MH88437-P

Ringing Voltage Detection

The sensitivity of the ringing voltage detection

circuitry can be adjusted by applying an external

resistor (R7, Figure 5) between the RS and LOOP

pins. With a short circuit, the threshold sensitivity is

~10Vrms, therefore R7 = 30k

Ω x (Desired threshold

voltage - 10Vrms).

Example: 300k

Ω gives ~20Vrms and 600kΩ gives

~30Vrms.

An AC ringing voltage across Tip and Ring will cause

RV to output TTL pulses at the ringing frequency,

with an envelope determined by the ringing cadence.

Parallel Phone and Dummy Ringer

An external parallel phone or dummy ringer circuit

can be connected across Tip and Ring as shown in

Figure 5. A Dummy Ringer is an AC load which

represents a telephone’s mechanical ringer.

In normal circumstances when a telephone is On-

Hook and connected to the PSTN, its AC (Ringer)

load is permanently presented to the network. This

condition is used by many PTT’s to test line

continuity, by placing a small AC current onto the line

and measuring the voltage across tip (A) and ring

(B).

Today’s telecom equipment may not have an AC load

present across tip and ring (e.g. modems), therefore

any testing carried out by the PTT will see an open

circuit across tip and ring. In this instance the PTT

assumes that the line continuity has been damaged.

To overcome this problem many PTT’s specify that a

"Dummy Ringer" is presented to the network at all

times. Ideally its impedance should be low in the

audio band and high at the ringing frequencies (e.g.

25Hz). Note that the requirement for the "Dummy

Ringer" is country speciﬁc.

Parallel phone detection is used mostly in set-top

box applications. This is when a modem call will

need to be disconnected from the central ofﬁce by

the equipment when the parallel phone is in the off-

hook state. This is to allow the subscriber to make

emergency calls.

To detect this state, additional circuitry will be

required. Refer to Application Note MSAN-154.

2-4 Wire Conversion

The device converts the balanced 2-Wire input,

presented by the line at Tip and Ring, to a ground

referenced signal at VX, biased at 2.0V. This

simpliﬁes the interface to a modem chip set.

Conversely, the device converts the differential signal

input at VR+ and VR- to a balanced 2-Wire signal at

Tip and Ring. The device can also be used in a

single ended mode at the receive input, by leaving

VR+ open circuit and connecting the input signal to

VR- only. Both inputs are biased at 2.0V.

During full duplex transmission, the signal at Tip and

Ring consists of both the signal from the device to

the line and the signal from the line to the device.

The signal input at VR+ and VR- being sent to the

line, must not appear at the output VX. In order to

prevent this, the device has an internal cancellation

circuit, the measure of this attenuation is Transhybrid

Loss (THL).

The MH88437 has the ability to transmit analog

signals from Tip and Ring through to VX when on-

hook. This can be used when receiving caller line

identiﬁcation information.

Transmit Gain

The Transmit Gain of the MH88437 is the gain from

the differential signal across Tip and Ring to the

ground

referenced

signal

VX.

The

internal

Transmit Gain of the device is ﬁxed as shown in the

AC Electrical Characteristics table. For the correct

gain, the Input Impedance of the MH88437, must

match the speciﬁed line impedance.

By adding an external potential divider to VX, it is

possible to reduce the overall gain in the application.

The output impedance of VX is approximately 10

Ω

and the minimum resistance from VX to ground

should be 2k

Ω.

Example: If R3 = R4 = 2k

Ω, in Figure 5, the overall

gain would reduce by 6.0dB.

Receive Gain

The Receive Gain of the MH88437 is the gain from

the differential signal at VR+ and VR- to the

differential signal across Tip and Ring. The internal

Receive Gain of the device is ﬁxed as shown in the

AC Electrical Characteristics table. For the correct

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