GAC--MAX of -- 37 dB. Experience has shown, however, that an" />
參數(shù)資料
型號(hào)： MC34118DW
廠商： Freescale Semiconductor
文件頁(yè)數(shù)： 10/22頁(yè)
文件大小： 0K
描述： IC VOICE SWITCH SPKPHONE 28-SOIC
標(biāo)準(zhǔn)包裝： 26
功能： 語(yǔ)音切換式揚(yáng)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(hào)音調(diào)檢測(cè)器，驅(qū)動(dòng)器放大器，電平檢測(cè)器，麥克風(fēng)放大器，靜音
MC34118
MOTOROLA
18
GAC--MAX of -- 37 dB. Experience has shown, however, that
an acoustic coupling loss of > 40 dB is desirable.
—The MTX line helps define the maximum sidetone cou-
pling (GST) allowed in the system, which can be found from
the following equation:
(Equation 14)
GST =
R4
2xR2 xGFO
Using the component values of Figure 23 in Equation 14
yields a maximum sidetone of 0 dB. Experience has shown,
however, that a minimum of 6.0 dB loss is preferable.
The above equations can be used to determine the resis-
tor values for the level detector inputs. Equation 6 can be
used to determine the R1/R3 ratio, and Equation 10 can be
used to determine the R4/R2 ratio. In Figure 24, R1 -- R4
each represent the combined impedance of the resistor and
coupling capacitor at each level detector input. The magni-
tude of each RC’s impedance should be kept within the
range of 2.0 k -- 15 k in the voiceband (due to the typical
signal levels present) to obtain the best performance from
the level detectors. The specific R and C at each location will
determine the frequency response of that level detector.
APPLICATION INFORMATION
DIAL TONE DETECTOR
The threshold for the dial tone detector is internally set at
15 mV (10 mVrms) below VB (see Figure 5). That threshold
can be reduced by connecting a resistor from RXI to ground.
The resistor value is calculated from:
R = 10 k
VB
V
–1
where VB is the voltage at Pin 15, and V is the amount of
threshold reduction. By connecting a resistor from VCC to
RXI, the threshold can be increased. The resistor value is
calculated from:
R = 10 k
VCC–VB
V
–1
where V is the amount of the threshold increase.
BACKGROUND NOISE MONITORS
For testing or circuit analysis purposes, the transmit or re-
ceive attenuators can be set to the “on” position, by disabling
the background noise monitors, and applying a signal so as
to activate the level detectors. Grounding the CPR pin will
disable the receive background noise monitor, thereby indi-
cating the “presence of speech” to the attenuator control
block. Grounding CPT does the same for the transmit path.
Additionally, the receive background noise monitor is auto-
matically disabled by the dial tone detector whenever the re-
ceive signal exceeds the detector’s threshold.
TRANSMIT/RECEIVE DETECTION PRIORITY
Although the MC34118 was designed to have an idle mode
such that the attenuators are halfway between their full on
and full off positions, the idle mode can be biased towards
the transmit or the receive side. With this done, gaining con-
trol of the circuit from idle will be easier for that side towards
which it is biased since that path will have less attenuation at
idle.
By connecting a resistor from CT (Pin 14) to ground, the
circuit will be biased towards the transmit side. The resistor
value is calculated from:
R = RT
VB
V
–1
where R is the added resistor, RT is the resistor normally be-
tween Pins 14 and 15 (typically 120 k), and V is the differ-
ence between VB and the voltage at CT at idle (refer to
Figure 10).
By connecting a resistor from CT (Pin 14) to VCC,the cir-
cuit will be biased towards the receive side. The resistor val-
ue is calculated from:
R = RT
VCC–VB
V
–1
R, RT, and V are the same as above. Switching time will be
somewhat affected in each case due to the different voltage
excursions required to get to transmit and receive from idle.
For practical considerations, the V shift should not exceed
100 mV.
VOLUME CONTROL
If a potentiometer with a standard linear taper is used for
the volume control, the graph of Figure 14 indicates that the
receive gain will not vary in a linear manner with respect to
the pot’s position. In situations where this may be objection-
able, a potentiometer with an audio taper (commonly used in
radio volume controls) will provide a more linear relationship
as indicated in Figure 26. The slight non--linearity at each
end of the graph is due to the physical construction of the
potentiometer, and will vary among different manufacturers.
Figure 26. Receive Attenuator Gain versus
Potentiometer Position Using Audio Taper
0
--10
--20
--30
40
80
120
160
DEGREES OF ROTATION
+10
-- 40
200
240
GAIN
(dB)
320
280
APPLICATION CIRCUIT
The circuit of Figure 23 is a basic speakerphone, to be
used in parallel with any other telephone which contains the
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