MC33215
RXI. A second high pass filtering is introduced by the
Automatic Gain Control
combination of CGRX and RGRX. A low pass filter is created
by CRXO and RRXO. The coupling capacitor at the output
RXO is not used for setting a high pass filter but merely for dc
decoupling.
In combination with dynamic ear capsules, the EAR
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.
To obtain more or less constant signal levels for transmit
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.
Figure 8. Automatic Gain Control
0
–1.0
R
= 20 k
AGC
–2.0
Sidetone Cancellation
R
= 30 k
AGC
The line driver and the receiver amplifier of the MC33215
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.
–3.0
–4.0
–5.0
–6.0
Figure 7. Sidetone Bridge
0
10
20
30
40
(mA)
50
60
70
VLN
I
line
For small line currents, and thus long lines, no gain
reduction is applied and thus the transmit and receive gains
are at their maximum. For line currents higher than Istart, the
gain is gradually reduced until a line current Istop is reached.
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:
Z
//Z
Z
bal
line set
V
x 15
11
HM
RXI
Receive
Transmit
R
Gnd
SLP
Gnd
R
/11
R
SLB
SLP
SLP
1
I
stop
As can be seen from Figure 7 by inspection, the receiver
will not pick up any transmit signal when the bridge is in
balance, that is to say when:
R
11
SLP
20 µ x R
AGC
11
1
Z
Z
Z
I
–
set
11
start
bal
line
R
11
R
SLP
SLP
R
R
SLB
SLP
For the typical application, where RAGC = 30 kΩ, the gain
will start to be reduced at Istart = 20 mA while reaching 6.0 dB
of gain reduction at Istop = 50 mA. When AGC is connected to
The sidetone suppression is normally measured in an
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 ASTE can be given as:
V
DD, the AGC function is disabled leading to no gain
reduction for any line current. This is also sometimes called
PABX mode.
The automatic gain control takes effect in the HMIC and Rx
amplifiers as well as in the BMIC amplifier. In this way the
AGC is also active in speakerphone mode, see the handsfree
operation paragraph.
Z
R
11
bal
SLP
Privacy and DTMF Mode
During handset operation a privacy and a DTMF mode can
be entered according the logic Table 1.
A
1 –
x
STE
R
Z
Z
set
SLB
line
Values of –12 dB or better, thus ASTE < 0.25, can easily be
reached in this way.
Table 1. Logic Table for Handset Mode
Logic Inputs
Amplifiers
DTMF
SPS
MUT
PRS
HMIC
On
BMIC
Off
R
RX
EAR
On
Mode
x
att
0
0
0
1
1
0
1
0
Handset Normal
Handset Privacy
Handset DTMF
Off
On
On
On
On
Off
Off
Off
Off
Off
Off
On
X
Off
Off
On
11
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
