U4080B Datasheet, PDF (7/15 Pages) TEMIC Semiconductors – Voice-Switched Circuit for Handsfree Operation

20
G_T
G_R
0
20
G_T
0
U4080B
–20
–20
DVR_ACF = 150 mV
DVR_ACF = 8 mV
–40
G_RT
–60
0
94 7865 e
G_TR
R₁ = 30 kW
100
RTG / R₁ or RG / R₁
1000
Figure 4.
–40
G_R
–60
0
94 7866 e
0.2
0.4
0.6
VCI
Figure 5.
0.8 1.0
As can be seen from the truth table, the T-R comparator
dominates. The transmit detector comparator is effective
only in the transmit mode, and the volume control is
effective only in the receive mode. The T-R comparator
is in the transmit position when the transmit signal present
is greater than the receive signal. The transmit detector
comparator then determines whether the transmit signal
is a result of background noise (a relatively constant
signal), or speech, which consists of bursts. If the signal
is due to background noise, the attenuators will be put into
the idle mode (DV_ACF = 75 mV). If the signal consists of
speech, the attenuators will be switched to the transmit
mode (DV_ACF = 6.0 mV). For further explanation, please
refer to the transmit detector circuit (Log. amplifiers).
The T-R comparator is in the receive position when there
is sufficient receive signal to overcome the background
noise AND any speech signals. The DV_ACF voltage will
x ^{be 150 mV if the volume control is at the maximum}
position, i.e. VCI (Pin 24) = V_B. If VCI V_B’, RG and
TG will vary in a complementary manner as shown in
figure 5 It can be seen that at the minimum recommended
operating level (VCI = 0.55 V_B) the gain of the transmit
attenuator is actually greater than that of the receive atten-
uator. The effect of varying VCI is to vary DV_ACF, with
a resulting variation in the gains of the attenuators.
Figure 6 shows the gain variations with DV_ACF.
20
R
0
–20
–40
T
–60
0
94 7867 e
40
80
120
160
DV_ACF ( mV )
Figure 6.
A 4.7 µF capacitor at Pin 25 smooths the transition
between operating modes. This keeps down any “clicks”
in the speaker or transmit signal when the ACF voltage
switches. The gain separation of the two attenuators can
be reduced from the typical 45 dB by connecting a resistor
between Pins 20 and 25. The effect is a reduction of
voltage in the receive mode, but no effect on the transmit
mode voltage.
For example, adding a 12 kΩ resistor will reduce DV_ACF
by approximately 15 mV (to 135 mV), decrease the gain
of the receive attenuator by approximately 5.0 dB, and
increase the gain of the transmit attenuator by a similar
amount. If the circuit requires the receive attenuator gain
to be +6.0 dB in the receive mode, RG must be adjusted
(to 27 kΩ) to re-establish this value. This change will also
increase the receive attenuator’s gain in the transmit
mode by a similar amount. The resistor at TLI may also
require changing to reset the sensitivity of the transmit
level detector.
TELEFUNKEN Semiconductors
7 (15)
Rev. A1, 20-May-96
Preliminary Information