TCA3388 Datasheet, PDF (10/16 Pages) Motorola, Inc – TELEPHONE LINE INTERFACE

TCA3388
Line +
Line –
Figure 10. Receive Part of the TCA3388
Z1
R20
C17
Z2
R19
HYS
HYL
Ge
IMP
TCA3388
1.6 V
Vref
RXO1
Ge
2V _BE+ V01
Line Mute
AGC
RXI
SAI
Gnd
R1
–1X
RXO2
Cear
RLoad CLoad
Handset
Earpiece
RECEIVE
The receive part of the TCA3388 is shown in Figure 10.
The receive signal is picked up by the amplifiers at the
HYL/HYS inputs. These are the same amplifiers present in
the dc loop of Figure 5. The signal is first converted to current
by the transconductance amplifier with transfer Ge. The
multiplier placed after performs the line length AGC. It
switches over between the 2 signals at HYS and HYL
according to the line current via a modulation factor m.
Afterwards, the current is converted back to voltage via the
external feedback network ZLoad. The resulting voltage is
available at output RXO1, and inverted at RXO2.
From the diagram of Figure 10 the receive gain (ARX) can
+ + ǒ ) Ǔ ^{be derived as:}
ARX
VRXO
VLine
Ge x R1 x ZLoad x
1
ZH
1
Zin
+ _With:
1
ZH
R20
R1 x Z1
in case of 1 sidetone network and
HYS connected to HYL, or
+ ) ¹
ZH
m
x
R20
R1 x Z1
(m
–1)
R19
R1 x Z2
in case of
2 sidetone networks
More information on ZH and the modulation factor m can
be found under the sidetone characteristics.
The earpiece can either be connected as a single ended or
as a differential load. The above calculated gain is valid for the
single ended case. When connecting as a differential load, the
gain is increased by 6.0 dB. The TCA3388 offers the
possibility to mute the signal coming from the line to the
earpiece. This can be useful during pulse– and DTMF–dialing.
10
SIDETONE
When a transmit signal is transmitted to the line, a part of
the signal is returned to the receive channel due to the
architecture 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
+ ǒ Ǔ ^{the signal on the 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
In applications with 1 sidetone network where HYS is
connected to HYL, it follows:
+ + ¹
R20
1
ZH R1 x Z1 ZHL
ZH has to be chosen according the average line
impedance, and the average linelength of the countries
involved in the application. A complex sidetone network can
be made via a complex Z1 which is preferred above making
R20 complex. The coupling capacitor C17 in series with R20
is meant only to block dc.
For applications with 2 sidetone networks it follows:
+ ) + ¹
ZH
m
R20
R1 x Z1
(m
–1)
R19
R1 x Z2
) m
1
ZHL
(m
–
1)
1
ZHS
The ZH thus exists as ZHL for long lines with low line
currents and as ZHS for short lines with high line currents.
This can be useful in applications such as DECT and
handsfree where the sidetone has to be minimized to reduce
the effect of delayed echoing and howling respectively. The
TCA3388 will automatically switch over between the 2 hybrid
networks as a function of line current. This is expressed in the
MOTOROLA ANALOG IC DEVICE DATA