KL34018 IK Semicon Co., Ltd, KL34018 Datasheet - Page 6

• Manufacturer Part Number: KL34018
• Description: Linear Integrated Circuit Voice Switched Speaker Phone Circuit
• Manufacturer: IK Semicon Co., Ltd

ATTENUATORS
complementary in function, i.e., when one is at
maximum gain the other is at maximum attenuation, and
vice versa. They are never both on or both off. Their
main purpose is to control the transmit and receive paths
to provide the half-duplex operation required of a
speakerphone. The attenuators are controlled solely by
the voltage at the ACF pin (Pin 25). The ACF voltage is
provided by the Attenuator Control block, which
receives 3 inputs: a) the Rx-Tx Comparator, b) the
Transmit Detector Comparator, and c) the Volume
Control. The response of the attenuators is based on the
difference of the ACF voltage from VCC, and therefore
a simple method for monitoring the circuit operation is
to monitor this voltage difference (referred to as
AVacf). If AVacf is approximately 6 millivolts the
transmit attenuator is fully on and the receive attenuator
is fully off (transmit mode). If AVacf is approximately
150 millivolts the circuit is in the re- ceive mode. If
AVacf is approximately 75 millivolts, 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.
determined by the three resistors RR, RTX, and RRX
(Refer to Figures 2, 3 and 4). RR affects both
attenuators ac- cording to its value RELATIVE to RTX
and RRX, which is why Figure 4 indicates the variations
versus the ratio of the other resistors to RR. (GRX and
GTX are the maximum gains, and ARX and ATX are
the maximum attenuations). RTX affects the gain and
attenuation of only the transmit attenuator according to
the curves of Figure 2, while RRX affects only the
receive attenuator according to Figure 3. As can be seen
from the figures, the gain difference (from on to off) is a
reasonably constant 45 dB until the upper gain limit is
approached. A value of 30 k is recommended for RR as
a starting point, and then RTX and RRX selected to suit
the particular design goals.
The input impedance of the attenuators (at TXI and
RXI) is typically 5.0 kR, and the maximum input signal
which will not cause output distortion is 250 mVrms
(707 mVp-p). The 4300 ohm resistor and 0.01 pF
capacitor at RXO (in Figure 1) filters out high frequency
components in the receive path. This helps 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 AVacf voltage at its output
is determined by three inputs. The relationship of the
inputs and output is summarized in the following truth
table:
The maximum gain and attenuation values are
The
transmit
and
receive
attenuators
FUNCTION DECRIPTION
are
parator dominates. The Transmit Detector Comparator is
effective only in the transmit mode, and the Volume
Control is effective only in the receive mode.
there is sufficient transmit signal present over and above
any receive signal. The Transmit Detector Com- parator
then determines whether the transmit signal is a result of
background noise (a relatively stable signal), or speech
which consists of bursts. If the signal is due to
background noise, the attenuators will be put into the
idle mode (AVacf = 75 mV). If the signal consists of
speech, the attenuators will be switched to the trans- mit
mode (AVacf = 6.0 mV.) A further explanation of this
function will be found in the section on the Transmit
Detector Circuit.
The Tx-Rx comparator is in the receive position when
there is sufficient receive signal to overcome the back-
ground noise AND any speech signals. The AVacf volt-
age will now be 150 mV IF the volume control is at the
maximum position, i.e. VLC (Pin 24) = VB. IF VLC is
less than VB, the gain of the receive attenuator, and the
attenuation of the transmit attenuator, will vary in a
complementary manner as shown in Figure 5. It can be
seen that at the minimum recommended operating level
(VLC = 0.55 VB) the gain of the transmit attenuator is
actually greater than that of the receive attenuator. The
effect of varying VLC is to vary AVacf, with a resulting
variation in the gains of the attenuators. Figure 6 shows
the gain variations with ΔVacf..
between operating modes. This keeps down any "clicks"
in the speaker or transmit signal when the ACF voltage
switches.
reduced from the typical 45 dB by adding a resistor
between Pins 20 (VCC) and 25 (ACF). The effect is a
reduction of the maximum AVacf voltage in the receive
mode, while not affecting AVacf in the transmit mode.
As an example, adding a 12 kQ resistor will reduce
AVacf 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, RRX
must be adjusted (to - 27 k) to re-establish this value.
This change will also increase the receive attenuator gain
in the transmit mode by a similar amount. The resistor at
TLI may also require changing to reset the sensitivity of
As can be seen from the truth table, the Tx-Rx com-
The Tx-Rx comparator is in the transmit position when
The capacitor at ACF (Pin 25) smooths the transition
The gain separation of the two attenuators can be
Transmit
Transmit
Receive
Receive
Tx-Rx
Comp
Transmit
Transmit
Transmit
Comp
Det
Idle
Idle
Affects ΔVacf
Affects ΔVacf
No Effect
No Effect
Volume
Control
50-150 mV
50-150 mV
Δ Vacf
6.0 mV
75 mV
Rev. 00
KL34018
Transmit
Receive
Receive
Mode
Idle