TEA1085 datasheet(11/32 Pages) PHILIPS

No. de pieza	TEA1085

Descripción Electrónicos	Listening-in circuit for line-powered telephone sets

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Fabricante Electrónico	PHILIPS [NXP Semiconductors]

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11 / 32 page

March 1992

11

Philips Semiconductors

Preliminary speciﬁcation

Listening-in circuit for line-powered

telephone sets

TEA1085; TEA1085A

Larsen limiter detector (DTI and DCA) pins 15 and 16

The QLA output signal is AC coupled to the detector input

DTI. DTI is biased by potential divider R30 and R31. The

voltage applied to DTI of the Larsen level limiter is

converted into a current for further processing in this

circuit. Current adjustment is achieved using the network

connected between DCA and VBB (see Fig.8).

The equation for DC current is:

The equation for AC current is:

In the basic application:

R30 = 100 k

Ω, R31 = 220 kΩ, R33 = 500 Ω, R32 = 100 kΩ

and C25 = 330 nF

This results in IDCA = 11 µA and the equation:

High-pass ﬁlter

A third order high-pass filter is created between the

microphone input voltage and the current flowing into

DCA. The cut-off frequencies (see Fig.9) of the three

sections are:

Where: R25 = R26 and C22 = C23

The filter reduces the sensitivity of the system to own

speech.

Normal speech is in the frequency range 300 Hz to

3400 Hz, however, the Larsen signal normally occurs at a

frequency

> 3 kHz.

With the component values as used in the basic

application (see Fig.16); f1 = 500 Hz, f2 = 1 kHz and

f3 = 3 kHz

I

DCA

R30

R31

+

-----------------------------

V

BB

1

R32

R33

+

-----------------------------

×

=

i

DCA

V

DTI

R33

------------

forf>

1

2

---

π R33 C25

=

i

DCA

V

DTI

------------

2 (mA/V)

=

f1

1

2

πR

eg C24

-----------------------------

where R

eq

R30

R31

×

R30

R31

+

-----------------------------

==

f2

1

2

πR33C24

------------------------------

=

f3

1

2

πR26C23

------------------------------

1/(2

πR25C22 )

==

Where:

Larsen limiter capacitor (LLC) pin 12

A 1

µF capacitor (C26) is connected externally between

VSS and LLC to determine the attack and release timing of

the Larsen level limiter in the listen-in and Larsen mode.

The timing is also dependent on the value of the resistor

connected between SIC and VSS.

Larsen level limiter threshold (THL1 and THL2) pins

13 and 14

When the signal at DTI exceeds the first threshold level the

capacitor connected to LLC will start to discharge. The first

threshold level is determined by the value of the resistor,

R35, connected to THL1 and VSS. The amount of

discharge of C26 depends on how much the level of the

signal at DTI exceeds the first threshold level (for normal

speech the discharge is small).

The Larsen effect is generally defined as a signal level of

≥ 100 mV(RMS), on line, for a period of more than 100 ms.

The Larsen signal must be reduced to a low level within

200 ms. For Larsen signal levels (f

> f3 in Fig.9) of

≥ 100 mV(RMS) at DTI and, with the component values of

Fig.16, the system will switch from the listen-in mode to the

Larsen mode in a time period of 100 ms to 200 ms;

consequently, the initial Larsen effect will last only for a

short period of time.

Fig.9 Third-order high-pass filter.

handbook, halfpage

MGR038

20 log

(dB)

g

go

20 log f

f3

f2

f1

0

12 dB per octave

6 dB per octave

18 dB per octave

speech

Larsen

g

i

DCA

V

m

-----------

=

g

o

A

pre

R33

-----------

=

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