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SL34118N

Voice Switched Speakerphone Circuit

System Logic Semiconductor 

SL34118

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 VRXOL and VTXOL 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

the CT pin is at -240 millivolts with respect to VB, the

circuit is in the transmit mode (transmit attenuator is

at +6.0 dB). The circuit is in an idle mode when the CT

voltage is equal to VB, causing the attenuators’ gains

to be halfway between their fully on and fully off

positions (-20 dB each). Monitoring the CT voltage

(with respect to VB) is the most direct method of

monitoring the circuit’s mode.

The inputs to the Control Block are seven: 2 from

the comp arators 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 CT pin (Pin 14). When the CT pin is at

+240 millivolts with respect to VB, the circuit is in the

receive mode (receive attenuator is at +6.0 dB). When

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

SLS

System Logic

Semiconductor

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