MC34216A Datasheet, PDF (13/20 Pages) Motorola, Inc – PROGRAMMABLE TELEPHONE LINE INTERFACE CIRCUIT WITH LOUDSPEAKER AMPLIFIER

MC34216A
Figure 10. General Line AGC Characteristics
Gain
Nominal
Gain
+ ₎ Gain
Nominal Gain
1
IL – ILstart
ILrange
The first part of the loudspeaker amplifier equals the
receive part of the earphone amplifier. It therefore also
includes the anti sidetone circuit and the line current AGC.
+ ǒ ) Ǔ ǒ ) Ǔ ^{The overall gain from line to loudspeaker output LPO is}
therefore:
ALSP
1
R30
R28
x RT x Ge x R1 x
1
Zh
1
Zin
Reduced
Gain
Iline
ILstart
ILrange
For small line currents, and thus long lines, the gains are
nominal. When the line current has increased above ILstart
with a current ILrange, the gain is reduced by 6.0 dB. Due to
the general characteristics of the line AGC curve, the gain will
be decreased further for higher currents.
For France and the U.K., the line AGC will be active in
Region 3 of the dc characteristics. The ILstart is approximately
equal to the ILK. The range is determined by:
+ ILrange
Z1 x (I2R – I2CD)
RE3
For Low Voltage mask, the line AGC is active in Region 2.
Loudspeaking Facility
The MC34216A incorporates a loudspeaking facility which
facilitates group listening–in with incorporated anti–howling
circuitry, handsfree conversation with external duplex
controller and ringing via loudspeaker with external switched
mode power supply.
Loudspeaker Amplifier
The block diagram of the loudspeaker amplifier which is
incorporated in the MC34216A is depicted in Figure 11.
The gain of the loudspeaker amplifier can be adjusted by
the external components as indicated in the formula above.
However, the gain is also influenced by line length AGC,
volume control and the signal limiter AGC.
The influence of line length AGC on the loudspeaker
channel is equal to its influence on the receive amplifier of the
earpiece because it is in their common path.
The volume of the loudspeaker is controlled via the
microcontroller. In total, 8 levels can be programmed: 7 levels
with equal steps of 4.5 dB and one level which completely
mutes the loudspeaker amplifier.
The output signal is limited by an AGC circuit when the
peaks of the signal are larger than a certain threshold (peak
AGC) and when the line current available is not sufficient
(current AGC). This prevents distortion of the output signal
and a drop of the supply voltage VCC.
Listening–In
During group listening–in, both the handset and the
loudspeaker are active. Via the acoustic coupling from
loudspeaker to handset microphone, howling can occur
when the handset is held close to the base. In order to reduce
howling, an anti–howling correlator and a microphone
threshold are incorporated in the MC34216A. Both functions
can be switched on/off via the microcontroller.
The anti–howling correlator monitors the microphone and
loudspeaker signal via capacitors C26 through C29. When
both signals show a high correlation, then both contain a
common signal. In practice, this means that howling is
present. In this case, the anti–howling correlator will reduce
the loudspeaker gain in steps of 1.25 dB, until the correlation
is below a certain threshold.
Figure 11. Block Diagram of the Loudspeaker Amplifier
Z1
Line +
R20
C17
HYN
+
2.4 V
IMP
MC34216A
Microphone Volume
Threshold Control
IRX
RT
Line –
+
2VBE + VO1
Line AGC
Anti–
Howl
SAI
Gnd
R1
+
LPO
LPI
Signal ^ACG
Limiter
C24
C23 R30
R28
C25 C21
Loudspeaker
MOTOROLA ANALOG IC DEVICE DATA
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