NCN49599
www.onsemi.com
22
Alternatively, automatic level control (ALC) may be used
by clearing the bit R_ALC_CTRL[3].
In this mode, the signal on the analogue input pin ALC_IN
controls the transmitter output level. First, peak detection is
performed. The peak value is then compared to two
threshold levels VTL
ALC_IN
and VTH
ALC_IN
. Depending
on the value of the measured peak level on ALC_IN the
attenuation is updated using
Vp
ALC_IN
< VTL
ALC:
increase the level with one 3 dB step
VTL
ALC
≤ Vp
ALC_IN
≤ VTH
ALC:
do not change the attenuation
Vp
ALC_IN
> VTH
ALC:
decrease the level with one 3 dB step
The gain changes in the next chip clock. Therefore, an
evaluation phase and a level adjustment phase take two
CHIP_CLK periods. ALC operation is enabled only during
the first 16 CHIP_CLK cycles after switching to transmit
mode.
Following reset, the level is set at minimum level
(maximum attenuation). When switching to reception mode
the last level is kept in memory. As a result the next transmit
frame starts with the old level.
Note that the DC level on the ALC_IN pin is fixed
internally to 1.65 V. As a result, a coupling capacitor is
usually required.
If the automatic level control feature is not used, the pin
ALC_IN may be left floating (not recommended) or tied to
ground.
Transmitter Output TX_OUT
The transmitter output is DC coupled to the TX_OUT pin.
Because the entire analogue part of the NCN49599 is
referenced to the analogue reference voltage REF_OUT
(about 1.65 V), a decoupling capacitor (C
1
in Figure 21) is
usually required.
To suppress the second and third order harmonic of the
generated S−FSK signal it is recommended to use a low pass
filter. Figure 21 illustrates an MFB topology of a 2
nd
order
filter.
ALC
control
ALC_IN
Transmitter (S−FSK Modulator )
ARM
Interface
&
Control
TX_OUT
LP
Filter
TX_EN
TO TX POWER
OUTPUT STAGE
FROM LINE
DRIVER
C
1
R
1
V
SSA
C
2
R
2
R
3
C
3
C
4
R
4
Figure 21. TX_OUT Filter
The modem indicates whether it is transmitting or
receiving on the digital output pin TX_ENB. This is driven
low when the transmitter is activated. The signal can be used
to turn on an external line driver.
TX_ENB is a 5 V safe with open drain output; an external
pull−up resistor must be added (Figure 21, R
4
).
When the modem switches from transmit to receive mode,
TX_ENB is kept active (i.e., low) for a short period
t
dTX_ENB
(Figure 13).
