IL34118
recommended input signal is independent of the
volume control setting. The diode clamp on the
inputs limits the input swing, and therefore the
maximum negative output swing. This is the reason
for V
RXOL
and V
TXOL
specification being defined as
they are in the Electrical Characteristics. The output
impedance is <10
Ω
until the output current limit
(typically 2.5 mA) is reached.
Figure 1. Attenuator Input Stage
at -240 millivolts with respect to V
B
, the circuit is in
the transmit mode (transmit attenuator is at +6.0 dB).
The circuit is in an idle mode when the C
T
voltage is
equal to V
B
, causing the attenuators’ gains to be
halfway between their fully on and fully off positions
(-20 dB each). Monitoring the C
T
voltage (with
respect to V
B
) is the most direct method of
monitoring the circuit’s mode.
The inputs to the Control Block are seven: 2 from
the comparators operated by the level detectors, 2
from the background noise monitors, the volume
control, the dial-tone detector, and the AGC circuit.
These seven inputs are described below.
LEVEL DETECTORS
The attenuators are controlled by the single output
of the Control Block, which is measurable at the C
T
pin (Pin 14). When the C
T
pin is at +240 millivolts
with respect to V
B
, the circuit is in the receive mode
(receive attenuator is at +6.0 dB). When the C
T
pin is
There are four level detectors - two on the receive
side and two on the transmit side. Refer to Figure 2 -
the terms in parentheses form one system, and the
other
terms
form
the
second
system
Figure 2. Level Detectors
Each level detector is a high gain amplifier with
back-to-back diodes in the feedback path, resulting in
non-linear gain, which permits operation over a wide
dynamic range of speech levels. The sensitivity of
each level detector is determined by the external
resistor and capacitor at each input (TLI1, TLI2,
RLI1, and RLI2). Each output charges an external
capacitor through a diode and limiting resistor, thus
providing a dc representation of the input ac signal
level. The outputs have a guick rise time (determined
by the capacitor and an internal 350
Ω
resistor), and a
slow decay time set by an internal current source and
the capacitor. The capacitors on the four outputs
should have the same value (±10%) to prevent timing
problems.
Referring to Figure 8, on the receive side, one
level detector (RLI1) is at the receive input receiving
the same signal as at Tip and Ring, and
3
INTEGRAL [ INTEGRAL CORP. ]
