MC34118DW Freescale Semiconductor, MC34118DW Datasheet - Page 7

• Manufacturer Part Number: MC34118DW
• Description: IC VOICE SWITCH SPKPHONE 28-SOIC
• Manufacturer: Freescale Semiconductor

Specifications of MC34118DW

• Function: Voice-Switched Speakerphone
• Number Of Circuits: 1
• Voltage - Supply: 3.5 V ~ 6.5 V
• Current - Supply: 5.5mA
• Operating Temperature: -20°C ~ 60°C
• Mounting Type: Surface Mount
• Package / Case: *
• Includes: Background Noise Monitor, Chip Disable, Dial Tone Detector, Driver Amplifiers, Level Detectors, Microphone Amplifier, MUTE
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power (watts): -
• Interface: -

INTRODUCTION
speakerphone and a handset is that of half--duplex versus
full--duplex. The handset is full duplex since conversation
can occur in both directions (transmit and receive) simulta-
neously. A speakerphone has higher gain levels in both
paths, and attempting to converse full duplex results in oscil-
latory problems due to the loop that exists within the system.
The loop is formed by the receive and transmit paths, the hy-
brid, and the acoustic coupling (speaker to microphone). The
only practical and economical solution used to date is to de-
sign the speakerphone to function in a half duplex mode —
i.e., only one person speaks at a time, while the other listens.
To achieve this requires a circuit which can detect who is
talking, switch on the appropriate path (transmit or receive),
and switch off (attenuate) the other path. In this way, the loop
gain is maintained less than unity. When the talkers ex-
change function, the circuit must quickly detect this, and
switch the circuit appropriately. By providing speech level de-
tectors, the circuit operates in a “hands--free” mode, eliminat-
ing the need for a “push--to--talk” switch.
erphone. But since the gains are considerably lower, and
since the acoustic coupling from the earpiece to the mouth
piece is almost non--existent (the receiver is normally held
against a person’s ear), oscillations don’t occur.
tenuators, and switching control for a properly operating
speakerphone. The detection sensitivity and timing are ex-
ternally controllable. Additionally, the MC34118 provides
background noise monitors which make the circuit insensi-
tive to room and line noise, hybrid amplifiers for interfacing to
Tip and Ring, the microphone amplifier, and other associated
functions. Please refer to the Block Diagram (Figure 2) when
reading the following sections.
ATTENUATORS
in function, i.e., when one is at maximum gain (+ 6.0 dB), the
other is at maximum attenuation (-- 46 dB), and vice versa.
They are never both fully on or both fully off. The sum of their
gains remains constant (within a nominal error band of
± 0.1 dB) at a typical value of -- 40 dB (see Figure 10). Their
purpose is to control the transmit and receive paths to pro-
vide the half--duplex operation required in a speakerphone.
(from max gain) frequency of ≈100 kHz. The input imped-
ance of each attenuator (TXI and RXI) is nominally 10 kΩ
(see Figure 3), and the input signal should be limited to
350 mVrms (990 mVp--p) to prevent distortion. That maxi-
mum recommended input signal is independent of the vol-
ume 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
MOTOROLA
The fundamental difference between the operation of a
The handset, by the way, has the same loop as the speak-
The MC34118 provides the necessary level detectors, at-
The transmit and receive attenuators are complementary
The attenuators are non--inverting, and have a -- 3.0 dB
RXOL
and V
TXOL
FUNCTIONAL DESCRIPTION
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.
Control Block, which is measurable at the C
When the C
circuit is in the receive mode (receive attenuator is at
+ 6.0 dB). When the C
to V
is at + 6.0 dB). The circuit is in an idle mode when the C
voltage is equal to V
halfway between their fully on and fully off positions (-- 20 dB
each). Monitoring the C
most direct method of monitoring the circuit’s mode.
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
and two on the transmit side. Refer to Figure 4 — the terms
in parentheses form one system, and the other terms form
the second system. Each level detector is a high gain ampli-
fier 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. Refer to the graphs of Fig-
ures 11, 12 and 13 for their dc and ac transfer characteristics.
The sensitivity of each level detector is determined by the ex-
ternal 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 rep-
resentation 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 cur-
rent source and the capacitor. The capacitors on the four out-
puts should have the same value (± 10%) to prevent timing
problems.
tor (RLI1) is at the receive input receiving the same signal as
at Tip and Ring, and the other (RLI2) is at the output of the
The attenuators are controlled by the single output of the
The inputs to the Control Block are seven: 2 from the
There are four level detectors — two on the receive side
Referring to Figure 2, on the receive side, one level detec-
(RXI)
TXI
B
, the circuit is in the transmit mode (transmit attenuator
10 k
T
Figure 3. Attenuator Input Stage
pin is at + 240 millivolts with respect to V
4.0 k
B
, causing the attenuators’ gains to be
T
T
pin is at -- 240 millivolts with respect
voltage (with respect to V
96 k
V
B
TO ATTENUATOR
INPUT
T
pin (Pin 14).
MC34118
B
) is the
B
, the
7
T