UTCMC34018 查看數據表（PDF） - Unisonic Technologies

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UTCMC34018

VOICE SWITCHED SPEAKER-PHONE CIRCUIT

Unisonic Technologies 

UTCMC34018 LINEAR INTEGRATED CIRCUIT

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 ∆Vacf).If ∆Vacf is

approximately 150 millivolts the circuit is in the receive mode. If ∆Vacf is approximately 75 millivolts the circuit is in

the idle mode, and two attenuators are at gain setting approximately half way(in dB) between their fully on and fully

off positions.

The maximum gain and attenuation values are determined by the three resistors RR,RTX,RRX(referred to

Fig2,3,4).RR affects both attenuators according to its value RELATIVE to RTX and RRX, which is why Fig 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 maximum attenuations).RTX affects the gain and attentuator according to Fig 3.As can be seen

from the figures, the gain difference(from on to off) is a reasonably constant 45dB until the upper gain limit is

approached. A value of 30K 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 5.0k ohms, and the maximum input signal

which will not cause output distortion is 250mVrms(707mVp-p).The 4300 ohms resistor and 0.01µCapacitor at

RXO(in fig 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 a 1.5dB at

1.0kHz.The outputs of the attenuators are inverted from their inputs.

Referring to the attenuator control block, the ∆Vacf voltage at its output is determined by three inputs. The

relationship of the inputs and outputs is summarized in the following truth table.

TX-RX COMP

TRANSMIT DET

COMP

VOLUME

CONTROL

∆VACF

MODE

Transmit

Transmit

No Effect

6.0mV

Transmit

Transmit

Idle

No Effect

75mV

Idle

Receive

Transmit

Affects ∆Vacf

50-150mV

Receive

Receive

Idle

Affects ∆Vacf

50-50mV

Receive

As can be seen from the truth table, the Tx-Rx comparator dominates. The transmit detector comparator is

effective only in the receive mode.

The Tx-Rx comparator is in the transmit position when there is sufficient transmit signal present over and

above any receive signal. The transmit detector comparator then determined whether the transmit signal is a result

of background noise(1 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(∆Vacf=75mV).If the signal consists of speech, the attenuator will

be switched to the transmit mode(∆Vacf=6.0mV).A further explanation of this function will be found in the section on

the transmit detector circuit.

The Rx-Tx Comparator is in the receive position when there is sufficient receive signal to overcome the

background noise AND any speech signals. The ∆Vacf voltage will now be 150mV 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, will vary in a

complementary manner as shown in fig 5.It can be seen that at the minimum recommended operating

level(VLC=0.55VB) the gain of the transmit atternuator is actually greater than that of the receive attenuator. The

effect of varying VLC is to vary ∆Vacf, with a resulting variation in the gains of the attenuators, Fig 6 shows the

gain variation with ∆Vacf.

The capacitor at ACF(pin 25) smoothes 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 45dB by adding a resistor between

pin 20(VCC) and pin 25(ACF).The effect is a reduction of the maximum ∆Vacf voltage in receive mode, while not

affecting ∆Vacf in the transmit mode. as an example, adding a 12 k ohms resistor will reduce ∆Vacf by approximately

15mV(to 135mV).decrease the gain of the receive attenuator by approximately 5.0dB, and increase the gain of the

transmit attenuator by a similar amount. If the circuit requires the receive attenuator gain to be +6.0dB in the receive

mode, RRX must be adjusted (to 27k ohms) 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 the thansmit level detector.

UTC

UNISONIC TECHNOLOGIES CO., LTD. 6

QW-R108-006,A

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