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UTCMC34018

VOICE SWITCHED SPEAKER-PHONE CIRCUIT

Unisonic Technologies 

UTCMC34018 LINEAR INTEGRATED CIRCUIT

LOG AMPLIFIERS(Transmit and Receive level Detectors)

The logs amps monitor the levels of tharsmit and receive signal, so as to tell the Tx-Rx comparator which mode

should be in effect. The input signals are applied to the amplifiers(at TLI and RLI) through AC coupling capacitors

and current limiting resistors. The value of these components determines the sensitivity of the respective

amplifiers ,and has an effect on the switching times between transmit and receive modes. The feedback elements for

the amplifier are back-to-back diodes which provide a logarithmic gain curve, thus allowing operation over a wide

range of signal levels. The outputs of the amplifiers are rectified, having a quick rise time and a slow decay time. The

rise time is determined by the external resistor and capacitor, and is on the order of a fraction of a second. The

switching time is not fixed, but depends on relative values of the transmit and receive signals, as well as these

external components. Fig 7 indicates the DC transfer characteristics of the log amps, and Fig 8 indicates the transfer

characteristics with respect to an AC input signal the DC level at TLI,RLI,TLO, and RLO

is approximately VB.

The Tx-Rx comparator responds to the voltages at TLO and RLO, which in turn are functions of the currents

sourced out of TLI and RLI, respectively. If an offset at the comparator input is desired, e.g. to prevent noise from

switching the system, or to give preference to either the transmit or receive channel, this may be achieved by biasing

the appropriate input(TLI or RLI).A resistor to ground will cause a DC current to flow out of that input, thus forcing the

output of that amplifier to be biased slightly higher than normal. This amplifier then becomes the preferred one in the

system operation. Resistor values from 500k to 10M ohms are recommended for this purpose.

SPEAKER AMPLIFIER

The speaker amplifier has a fixed gain of 34dB(50V/V),and is noninverting. The input impedance is nominally

22 k ohms as long as the output signal is below that required to activate the peak limiter. The Fig 9 indicates the

typical output swing available at SKO(pin 15).Since the output current capability is 100mA,the lower curve is limited

to 1 5.0Volt swing. The output impedance depends on the output signal level and is a relatively low as long as the

signal level is not near maximum limits. At 3 Vp-p the output impedance is less than 0.5 ohms, and at 4.5Vp-p it is

less than 3 ohms. The output is short circuit protected at approximately 300 mA.

When the amplifier is over driven, the peak limiter causes a portion of the input signal to be shunted to ground,

in order to maintain a constant output level. The effect is that of a gain reduction caused by a reduction of the input

impedance (at SKI) to a value not less than 2.0k ohms.

The capacitor at pin 17(AGF) determines the response time of the peak limiter circuit. When a large input

signal is applied to SKI, the voltage at AGC(pin 17) will drop quickly as a current source is applied to the external

capacitor. When the large input signal is reduced, the current source is turned off, and an internal 110k ohms

resistor discharges the capacitor so that the voltage at AGC can return toots normal value(1.9Vdc).The capacitor

additionally stabilizes the peak limiting feedback loop.

If there is a need to mute the speaker amplifier without disabling the rest of the circuit, this may be

accomplished by connecting a resistor from the AGC pin to GND.A 100k ohms resistor will reduce the gain by 34dB

(0dB from SKI To SKO), and a 10k ohms resistor will reduce the gain by almost 50dB.

TRANSMIT DETECTOR CIRCUIT

The transmit detector circuit, also known as the background noise monitor, distinguishes speech(which

consists of bursts) from the background noise( a relatively constant signal).It does this by storing a voltage level,

representative of the average background noise, in the capacitor at CP1(pin 11).The resistor and capacitor at this

pin have a time constant approximately 5 second(in fig 1).The voltage at pin 1 is applied to the inverting input of the

transmit detector comparator. In the absence of speech signal, the noninverting input receiving the same as voltage

level minus an offset of 36mV.In this condition, the output transistor turned off, and the voltage at XDC(pin 23) will be

at GND. If the Tx-Rx comparator is in the transmit position, the attenuators will be in the idle mode(∆Vacf=75mV).

When speech is presented to the microphone, the signal burst appearing at XDI reaches the noninverting input of

the transmit detector comparator before the voltage at the inverting input can change, causing the output to switch

high, driving the voltage ad XDC up to approximately 4Volts.This high level causes the attenuator control block to

switch the attenuators from the idle mode to the transmit mode(assuming the Tx-Rx comparator is in the transmit

mode).As long as the speech continues to arrive, and is maintained at a level above the background, the voltage at

XDC will be maintained at a high level, and the circuit will remain in transmit mode. The time constant of the

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UNISONIC TECHNOLOGIES CO., LTD. 7

QW-R108-006,A

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