MC33215 Motorola, MC33215 Datasheet - Page 11

• Manufacturer Part Number: MC33215
• Description: Telephone Line Interface and Speakerphone Circuit
• Manufacturer: Motorola

RXI. A second high pass filtering is introduced by the
combination of C
by C
RXO is not used for setting a high pass filter but merely for dc
decoupling.
amplifier can become unstable due to the highly inductive
characteristic of some of the capsules. To regain stability, a
100 nF capacitor can be connected from RXS to Gnd in
those cases. An additional 10 nF at the RXI input, as shown
in the typical application, improves the noise figure of the
receiver stage.
Sidetone Cancellation
are tied up in a bridge configuration as depicted in Figure 7.
This bridge configuration performs the so–called hybrid
function which, in the ideal case, prevents transmitted signals
from entering the receive channel.
will not pick up any transmit signal when the bridge is in
balance, that is to say when:
acoustic way. The signal at the earpiece when applying a
signal on the microphone is compared with the signal at the
earpiece when applying a signal on the line. The suppression
takes into account the transmit and receive gains set. In fact
the sidetone suppression can be given as a purely electrical
parameter given by the properties of the sidetone bridge
itself. For the MC33215, this so–called electrical sidetone
suppression A
reached in this way.
MOTOROLA ANALOG IC DEVICE DATA
Gnd
Z
In combination with dynamic ear capsules, the EAR
The line driver and the receiver amplifier of the MC33215
As can be seen from Figure 7 by inspection, the receiver
The sidetone suppression is normally measured in an
Values of –12 dB or better, thus A
SPS
R
line
0
0
0
SLP
RXO
//Z
/11
set
Logic Inputs
and R
A
MUT
STE
1
1
0
STE
Figure 7. Sidetone Bridge
RXO
Transmit
GRX
+
can be given as:
R
Z
. The coupling capacitor at the output
1 –
SLB
bal
and R
PRS
X
1
0
R
VLN
SLP
+
Z
SLB
bal
GRX
Z
R
Handset Normal
Handset Privacy
Handset DTMF
line
V
SLP
. A low pass filter is created
R
x
HM
SLP
Z
STE
R
Z set
line
x 15
11
SLP
11
< 0.25, can easily be
Z set
Table 1. Logic Table for Handset Mode
11
M d
Mode
Z
R
bal
SLB
RXI
Receive
Gnd
MC33215
Automatic Gain Control
and receive regardless of the telephone line length, both the
transmit and receive gain can be varied as a function of line
current when the AGC feature is used. The gain reduction as
a function of line current, and thus line length, is depicted in
Figure 8.
reduction is applied and thus the transmit and receive gains
are at their maximum. For line currents higher than I
gain is gradually reduced until a line current I
This should be the equivalent of a very short line, and the
gain reduction equals 6.0 dB. For higher line currents the
gain is not reduced further. For the start and stop currents the
following relations are valid:
will start to be reduced at I
of gain reduction at I
V
reduction for any line current. This is also sometimes called
PABX mode.
amplifiers as well as in the BMIC amplifier. In this way the
AGC is also active in speakerphone mode, see the handsfree
operation paragraph.
Privacy and DTMF Mode
be entered according the logic Table 1.
DD
To obtain more or less constant signal levels for transmit
For small line currents, and thus long lines, no gain
For the typical application, where R
The automatic gain control takes effect in the HMIC and R
During handset operation a privacy and a DTMF mode can
, the AGC function is disabled leading to no gain
–1.0
–2.0
–3.0
–4.0
–5.0
–6.0
HMIC
On
Off
Off
0
0
I start
BMIC
10
Off
Off
Off
Figure 8. Automatic Gain Control
+
stop
R
I stop
R
20
SLP
AGC
DTMF
= 50 mA. When AGC is connected to
1
Off
On
On
= 30 k
start
+
Amplifiers
11
30
R
= 20 mA while reaching 6.0 dB
I
line
SLP
–
(mA)
1
20
R
On
On
Off
R
AGC
40
R
x
11
SLP
AGC
= 20 k
x R
= 30 k , the gain
AGC
11
RX
50
Off
Off
Off
stop
att
is reached.
60
start
EAR
On
On
On
11
, the
70
x