U4080B Datasheet, PDF (6/15 Pages) TEMIC Semiconductors – Voice-Switched Circuit for Handsfree Operation

U4080B
Transmit and Receive Attenuators
The transmit and receive attenuators are supplementary
in function. This means that when one is at maximum
gain, the other is at maximum attenuation, and vice versa.
That is, both are never on or off. They are controlled by
the voltage of ACF (attenuator control filter) at Pin 25
being supplied by attenuator control as shown in figure 1
The ACF voltage is provided by the attenuator control
block, which receives the three inputs given below:
D R-T comparator
D Transmit detector comparator
D Volume control
The response of the attenuators is based on the difference
between the ACF voltage and V_CC. If the difference
[ (DV_ACF) is 6 mV, the transmit attenuator is fully on and
[ the receive attenuator is fully off (T mode). If DV_ACF
[ 150 mV, the circuit is in the R mode. If DV_ACF 75 mV,
the circuit is in the idle mode, and the two attenuators are
at gain settings approximately half way (in dB) between
their fully on and fully off positions.
Three resistors R₁, RTG and RD, determine maximum
gain and attenuation values. R₁ effects both attenuators
according to its value relative to RTG and RG. Figure 4
shows the variations versus the ratio of other resistors to
R1. RTG affects the gain and attenuation of only the
transmit attenuator according to the curves of figure 2,
while RG affects only the receive attenuator according to
figure 3 Gain difference from on to off, according to the
figures, is a reasonably constant 45 dB until the upper
gain limit is approached. A value of R1 = 30 kΩ is recom-
mended as a starting point, and then the values of RTG
and RG selected to suit the particular design goals.
The input impedance of the attenuators (at TI and RECI)
is typically 5.0 kΩ, and the maximum input signal which
will not cause output distortion is 250 mV_rms (707 mV_pp).
The 4 kΩ resistor and 0.01 µF capacitor at RECO (in
figure 1) filters out high-frequency components in the
receive path. This helps to minimize high-frequency
acoustic feedback problems which may occur if the filter
were not present.
The filter’s insertion loss is 1.5 dB at 1.0 kHz. The outputs
of the attenuators are inverted from their inputs. Referring
to the attenuator control block, the DV_ACF voltage at its
output is determined by three inputs. The relationship of
the inputs and outputs is summarized in the following
truth table:
T-R
Comp
Transmit
Transmit
Receive
Receive
20
Transmit
Det
Comp
Transmit
Idle
Transmit
Idle
DVR_ACF = 150 mV
0
Volume
Control
No Effect
No Effect
Affects DV_ACF
Affects DV_ACF
DV_ACF
6.0 mV
75.0 mV
50 – 150 mV
50 – 150 mV
Mode
Transmit
Idle
Receive
Receive
20
DVT_ACF = 8 mV
0
–20
20
–40
0
94 7864 e
DVR_ACF = 8 mV
100
RG ( kW )
1000
Figure 2.
40
DVT_ACF = 150 mV
0
94 7863 e
100
RTG ( kW )
Figure 3.
1000
6 (15)
TELEFUNKEN Semiconductors
Preliminary Information
Rev. A1, 20-May-96