MC33215 Просмотр технического описания (PDF) - Motorola => Freescale

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MC33215

Telephone Line Interface and Speakerphone Circuit

Motorola => Freescale 

Freescale SMeCm3i3c2o15nductor, Inc.

ASWD = Switching depth as performed in the

attenuators

To avoid howling, the maximum possible loop gain should

be below 0 dB and preferably below –10 dB for comfort. In a

practical telephone design, both the ABMRX(max) and the

ARXBM(max) will be less than 20 dB thus a switching depth of

50 dB will give a loop gain of less than –10 dB. An optimized

sidetone network is of high importance for handsfree

operation. The better the network matches with the

telephone line the less local feedback and the smaller the

switching range can be.

+ ǒ Ǔ The amount of gain reduction ASWD obtained by the

duplex controller is set via resistor RSWD according:

ASWD

2

20 log

3.6 x RSWD

RREF

(dB)

In the typical application the gain reduction will be 50 dB.

To compare the transmit and receive signals with each

other, they have to be monitored. This is done by making a

signal envelope and a background noise envelope via the

CTSE, CTBN capacitors for the transmit channel and via the

CRSE, CRBN capacitors for the receive channel. In Figure 14,

a schematic behavior of the envelopes is depicted which is

equal for both transmit and receive.

The voltage signal at the input is first transferred to a

current via the sensitivity adjust network. Then this current is

led through a diode which gives a logarithmic compression in

voltage. It is this voltage from which the signal envelope is

created by means of asymmetric charge and discharge of the

signal envelope capacitor. The noise envelope voltage then

follows in a similar way. Based on the envelope levels, the

MC33215 will switch to transmit, receive or idle mode

following Table 3. The fact that in receive mode the signal on

the base microphone is greater than it is in case of transmit

mode, due to the coupling of the high loudspeaker signal, is

automatically taken into account.

In the table, two particulars can be found. At first, the set

will go to idle mode if the signals are not at least 4.5 dB

greater then the noise floor, which calculates as a 13 mV

voltage difference in envelopes. This avoids continuous

switching over between the modes under slight variations of

the background noise due to, for instance, typing on a

keyboard. Second, a dial tone detector threshold is

implemented to avoid that the set goes to idle mode in

presence of a continuous strong receive signal like a dial

tone. The dial tone detector threshold is proportional to the

RRSA resistor. In the typical application with RRSA = 3.3 kΩ,

the threshold is at 1.26 mVrms at the input RXI or 20 mVrms

at the line. Line length AGC is of influence on the dial tone

detector threshold, increasing the level depending on the line

current with a maximum of 6.0 dB.

In order to perform a correct comparison between the

signal strengths, the sensitivity of the envelope detectors can

be adjusted via the resistors connected to TSA and RSA.

Based on the above, and on the fact that there is an effective

gain of 20 dB in the transmit monitor, it can be derived that for

stable operation the following two relations are valid:

ǒ Ǔ t ǒ Ǔ ) 20 log RTSA 20 log RRSA – ABMRX(max) 20 (dB)

ǒ Ǔ u ǒ Ǔ 20 log RTSA 20 log RRSA – ARXBM(max)

) – ASW 20 (dB)

By measuring the gains and choosing the RRSA, the limits

for RTSA follow. The choice for the sensitivity resistors is not

completely free. The logarithmic compressors and the

amplifier stages have a certain range of operation and, on the

receive side, the choice for RRSA is given by the desired dial

tone detector threshold. Figure 15 indicates the available

dynamic range for the selected value of the sensitivity

resistors.

Microphone

Input Signal

Figure 14. Signal and Noise Envelopes

1.8 V

Internal

VHF

CTSE

TSE

VHF

CTBN

TBN

TSA

RTSA

CTSA

MOTOROLA ANALOG IC DEVICEFDoArTAMore Information On This Product,

15

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