MC34216A Datasheet, PDF (12/20 Pages) Motorola, Inc – PROGRAMMABLE TELEPHONE LINE INTERFACE CIRCUIT WITH LOUDSPEAKER AMPLIFIER

MC34216A
Figure 9. Receive Part of the MC34216A
Z1
Line +
R20
C17
HYN
+
Ge
2.4 V
IMP ₊
Line –
+
2VBE + VO1
SAI
R1
Line AGC
Gnd
MC34216A
–1 X
RXO2
Vref ₊
RXO1
Cear
Rload
Cload
RXI
Handset
Earpiece
The noise which is transmitted to the line can be reduced
when the microphone threshold feature is enabled. This is
also called squelch. When there is no microphone signal
present, the transmit gain is reduced by 7.0 dB with respect
to nominal. In between, the gain is varied in steps of 1.0 dB
dependent on the signal level. The detection of the
microphone signal level is done via an integrator which uses
the external capacitors C28 and C29 for timing and filtering.
This feature is also very useful to reduce the howling effect
during listening–in.
The MC34216A offers the possibility to mute the
microphone, also called privacy mode. In this case, the far
end user no longer hears the local user. It is possible to send
during this mode an idle tone to the line to make the far end
user aware of this.
During pulse–dialing, the microphone biasing can be
switched off by programming. Pin Mic will then be made high
impedance, meaning that the microphone dc current no
longer flows into this pin. This reduces the current
consumption of the application during pulse–dialing. It can
also be used to disconnect the handset microphone signal
and enable another transmit signal, for instance from a
handsfree microphone.
Receive
The receive portion of the MC34216A is shown in Figure 9.
The receive signal is picked up by the amplifier at the HYN
input. This is the same amplifier as present in the dc loop of
Figure 4. The signal is first converted to current by the
transconductance amplifier with transfer Ge. The multiplier
placed after performs line length dependent gain or line AGC.
Afterwards, the current is converted back to voltage via the
external feedback network Zload. The resulting voltage is
available at output RXO1, and at RXO2 which is the inverted
earpiece output.
+ + ǒ ) Ǔ ^{From the diagram of Figure 9 the receive gain (ARX) can}
be derived as:
ARX
VRXO
Vline
Ge x R1 x Zload x
1
Zh
1
Zin
+ With : Zh
R1 x Z1
R20
,
also see the sidetone characteristics below.
12
The earpiece can either be connected as a single–ended
or as a bridge–tied load. The above calculated gain is valid
for the single–ended case. When connecting as a bridge–tied
load, the gain is increased by 6.0 dB.
The MC34216A offers the possibility to increase the
receive gain to the earpiece with an extra 6.0 dB via the
microcontroller on command of the user of the telephone set.
This is useful for people with hearing problems or when the
far end user speaks very softly.
The MC34216A offers the possibility to mute the signal
coming from the line to the earpiece. This is useful during
pulse–dialing or to mute the far end user.
Sidetone
When a transmit signal is transmitted to the line, a part of
the signal is returned to the receive channel due to the nature
of the 2 to 4 wire conversion of the hybrid. During transmit,
the signal on the line will be –Iline x Zline. During receive, the
signal on the line will be Iline x Zin. When replacing Zin in the
formula for the receive gain, it follows that the signal on the
+ ǒ Ǔ ^{earpiece output due to a sending signal on the line will be:}
Vear
Vline–transmit
Ge x R1 x Zload x
1
Zh
–
1
Zline
The sidetone can be minimized by making Zh equal to
Zline. A complex sidetone network can therefore be made via
a complex Z1. The coupling capacitor C17 in series with R20
is only meant to block dc.
Line Length AGC
The MC34216A offers the possibility to vary the transmit
and receive gains over line length in order to compensate for
the loss in gain at longer line lengths. In the block diagrams
of the transmit and receive channels the attenuator
performing the line AGC is already drawn. The line AGC can
be switched on/off via the microcontroller. In case the line
AGC is switched off, the transmit and receive gain are
lowered 2.0 dB with respect to the value calculated via the
formulas above.
The line AGC characteristics for both transmit and receive
channels have the general shape depicted in Figure 10.
MOTOROLA ANALOG IC DEVICE DATA