the components at C
參數(shù)資料
型號(hào)： MC34118DW
廠商： Freescale Semiconductor
文件頁(yè)數(shù)： 6/22頁(yè)
文件大?。?/td> 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
14
DESIGN GUIDELINES
SWITCHING TIME
The switching time of the MC34118 circuit is dominated by
the components at CT (Pin 14, refer to Figure 6), and sec-
ondarily by the capacitors at the level detector outputs
(RLO1, RLO2, TLO1, TLO2).
termined by the capacitor at CT, together with the internal
current sources(refertoFigure6). Theswitchingtimeis:
T =
Vx CT
I
For the typical case where V = 240 mV, I = 60 A, and CT
is 5.0 F, T = 20 ms. If the circuit switches directly from re-
ceive to transmit (or vice--versa), the total switching time
wouldbe40ms.
The switching time from either receive or transmit to idle
depends on which type of idle mode is in effect. If the circuit
is going to “fast idle,” the time constant is determined by the
CT capacitor, and the internal 2.0 k resistor (Figure 6). With
CT =5.0 F, the time constant is ≈10 ms, giving a switching
time to idle of ≈30 ms (for 95% change). Fast idle is an infre-
quent occurrence, however, occurring when both speakers
are talking and competing for control of the circuit. The
switching time from idle back to either transmit or receive is
described above.
If the circuit is switching to “slow idle,” the time constant is
determined by the CT capacitor and RT, the external resistor
(see Figure 6). With CT =5.0 F, and RT = 120 k,the
time constant is ≈600 ms, giving a switching time of
≈1.8 seconds (for a 95% change). The switching period to
slow idle begins when both speakers have stopped talking.
The switching time back to the original mode will depend on
how soon that speaker begins speaking again. The sooner
the speaking starts during the 1.8 second period, the quicker
the switching time since a smaller voltage excursion is re-
quired. That switching time is determined by the internal cur-
rent sources as described above.
The above switching times occur, however, after the level
detectors have detected the appropriate signal levels, since
their outputs operate the Attenuator Control Block. Referring
to Figure 4, the rise time of the level detectors’ outputs to
new speech is quick by comparison (≈1.0 ms), determined
by the internal 350 resistor and the external capacitor (typi-
cally 2.0 F). The output’s decay time is determined by the
external capacitor, and an internal 4.0 A current source giv-
ing a decay rate of ≈60 ms for a 120 mV excursion at RLO
or TLO. However, the overall response time of the circuit is
not a constant since it depends on the relative strength of the
signals at the different level detectors, as well as the timing of
the signals with respect to each other. The capacitors at the
four outputs (RLO1, RLO2, TLO1, TLO2) must be equal
value (± 10%) to prevent problems in timing and level re-
sponse.
The rise time of the level detector’s outputs is not signifi-
cant since it is so short. The decay time, however, provides a
significant part of the “hold time” necessary to hold the circuit
during the normal pauses in speech.
The components at the inputs of the level detectors (RLI1,
RLI2, TLI1, TLI2) do not affect the switching time, but rather
affect the relative signal levels required to switch the circuit,
as well as the frequency response of the detectors.
DESIGN EQUATIONS
Referring to Figure 24 (the coupling capacitors have been
omitted for simplicity), and the circuit of Figure 23, the follow-
ing definitions will be used (all measurements are at
1.0 kHz):
—GMA is the gain of the microphone amplifier measured
from the microphone output to TXI (typically 35 V/V, or
31 dB);
—GTX is the gain of the transmit attenuator, measured from
TXI to TXO;
—GHA is the gain of hybrid amplifiers, measured from TXO
to the HTO--/HTO+ differential output (typically 10.2 V/V,
or 20.1 dB);
—GHT is the gain from HTO--/HTO+ to Tip/Ring for transmit
signals, and includes the balance network (measured at
0.4 V/V, or -- 8.0 dB);
—GST is the sidetone gain, measured from HTO--/HTO+ to
the filter input (measured at 0.18 V/V, or --15 dB);
—GHR is the gain from Tip/Ring to the filter input for receive
signals (measured at 0.833 V/V or -- 1.6 dB);
—GFO is the gain of the filter stage, measured from the in-
put of the filter to RXI, typically 0 dB at 1.0 kHz;
—GRX is the gain of the receive attenuator measured from
RXI to RXO;
—GSA is the gain of the speaker amplifier, measured from
RXO to the differential output of the MC34119 (typically
22 V/V or 26.8 dB);
—GAC is the acoustic coupling, measured from the speaker
differential voltage to the microphone output voltage.
I) Transmit Gain
The transmit gain, from the microphone output (VM)to Tip
and Ring, is determined by the output characteristics of the
microphone, and the desired transmit level. For example, a
typical electret microphone will produce ≈0.35 mVrms un-
der normal speech conditions. To achieve 100 mVrms at Tip/
Ring, an overall gain of 285 V/V is necessary. The gain of the
transmit attenuator is fixed at 2.0 (+ 6.0 dB), and the gain
through the hybrid of Figure 23 (GHT) is nominally 0.4
(-- 8.0 dB). Therefore a gain of 357 V/V is required of the
microphone and hybrid amplifiers. It is desirable to have the
majority of that gain in the microphone amplifier for three rea-
sons: 1) the low level signals from the microphone should be
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