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參數(shù)資料
| 型號： | MC34118DW |
| 廠商： | Freescale Semiconductor |
| 文件頁數(shù)： | 20/22頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC VOICE SWITCH SPKPHONE 28-SOIC |
| 標準包裝： | 26 |
| 功能： | 語音切換式揚聲電話 |
| 電路數(shù)： | 1 |
| 電源電壓： | 3.5 V ~ 6.5 V |
| 電流 - 電源： | 5.5mA |
| 工作溫度： | -20°C ~ 60°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 28-SOIC（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 28-SOIC W |
| 包裝： | 管件 |
| 包括： | 背景噪聲監(jiān)視器，芯片禁用，撥號音調(diào)檢測器，驅(qū)動器放大器，電平檢測器，麥克風放大器，靜音 |
MC34118
MOTOROLA
7
FUNCTIONAL DESCRIPTION
INTRODUCTION
The fundamental difference between the operation of a
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.
The handset, by the way, has the same loop as the speak-
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.
The MC34118 provides the necessary level detectors, at-
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
The transmit and receive attenuators are complementary
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.
The attenuators are non--inverting, and have a -- 3.0 dB
(from max gain) frequency of ≈100 kHz. The input imped-
ance of each attenuator (TXI and RXI) is nominally 10 k
(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 VRXOL and VTXOL specification
being defined as they are in the Electrical Characteristics.
The output impedance is < 10 until the output current limit
(typically 2.5 mA) is reached.
Figure 3. Attenuator Input Stage
10 k
VB
TO ATTENUATOR
INPUT
4.0 k
96 k
TXI
(RXI)
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 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
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
There are four level detectors — two on the receive side
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 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.
Referring to Figure 2, on the receive side, one level detec-
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
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| MC34118-E28-T | 制造商:UTC-IC 制造商全稱:UTC-IC 功能描述:VOICE SWITCHED SPEAKER-PHONE CIRCUIT |
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