5

4743D–CORD–03/06

U4082B

3.2

Transmit and Receive Attenuators TI, TO and RI, RECO

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 Figure 7-1 on page 17). Their purpose is to control the transmit and

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

Ω (see Figure

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

Ω until the output current limit (typically 2.5 mA) is reached.

Figure 3-1.

Attenuator Input Stage

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

T

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.

3.3

Level Detectors

There are four level detectors, two on the receive side and two on the transmit side. As shown in

Figure 3-2 on page 6, the terms in parentheses form one system, and the other terms form the

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, 7-3 and 7-4 on page 18 for their DC and AC

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

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.

As can be seen in Figure 1-2 on page 2, on the receive side, one level detector (RLI1) is located

at the receive input, receiving the same signal as at Tip and Ring, and the other (RLI2) is at the

11 k

Ω

V

B

RI 21

TI 9

to Attenuator

Input

5 k

Ω

95 k

Ω