U4082B-DEV-BOARD Atmel, U4082B-DEV-BOARD Datasheet - Page 5

• Manufacturer Part Number: U4082B-DEV-BOARD
• Description: WiFi / 802.11 Modules & Development Tools Voice Switched Circuit
• Manufacturer: Atmel

3.2
3.3
4743D–CORD–03/06
Transmit and Receive Attenuators TI, TO and RI, RECO
Level Detectors
The attenuators are functionally complementary; that is, when one is at maximum gain
(+6.0 dB), the other is at maximum attenuation (–46 dB), and vice versa. Neither is ever fully on
or off. The sum of their gains remains constant (within a nominal error band of ±0.1 dB) at a typ-
ical value of –40 dB (see
receive paths to provide the half-duplex operation required in a speakerphone.
The non-inverting attenuators have a –3.0 dB (from maximum gain) frequency of approximately
100 kHz. The input impedance of each attenuator (TI and RI) is nominally 10 k (see
3-1). To prevent distortion, the input signal should be limited to 350 mVrms. The maximum
recommended input signal is independent of the volume control setting. The diode clamp on the
inputs limits the input swing, and thus the maximum negative output swing. This results in a
specific V
The output impedance is less than 10 until the output current limit (typically 2.5 mA) is reached.
Figure 3-1.
The attenuators are controlled by the single output of the control block, which is measurable at
pin C
(receive attenuator is at +6.0 dB). When pin C
transmit mode (transmit attenuator is at +6.0 dB). The circuit is in an idle mode when the C
voltage is equal to V
off positions (–20 dB each). Monitoring the C
method of monitoring the circuit's mode.
The attenuator control has seven inputs: two from the comparators operated by the level detec-
tors, two from the background noise monitors, volume control, dial-tone detector, and AGC.
They are described in the sections that follow.
There are four level detectors, two on the receive side and two on the transmit side. As shown in
Figure 3-2 on page
second system. Each level detector is a high-gain amplifier with back-to-back diodes in the feed-
back path, resulting in nonlinear gain which permits operation over a wide dynamic range of
speech levels. Refer to the graphs of Figures 7-2,
transfer characteristics. The sensitivity of each level detector is determined by the external resis-
tor 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 quick rise time (determined by the capacitor and an internal
350 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.
As can be seen in
at the receive input, receiving the same signal as at Tip and Ring, and the other (RLI2) is at the
T
(pin 14). When pin C
RECO
Attenuator Input Stage
and V
Figure 1-2 on page
6, the terms in parentheses form one system, and the other terms form the
B
TOL
causing the attenuators' gain to be halfway between their fully on and fully
RI 21
TI 9
definition as given in the table
Figure 7-1 on page
T
is at +240 mV with respect to V
5 k
11 k
2, on the receive side, one level detector (RLI1) is located
T
17). Their purpose is to control the transmit and
T
is at –240 mV with respect to V
voltage (with respect to V
95 k
7-3
and
“Electrical Characteristics” on page
7-4
to Attenuator
B
on page
Input
, the circuit is in receive mode
V
B
18
B
for their DC and AC
) is the most direct
B
, the circuit is in
U4082B
Figure
14.
5
T