Preliminary

AS2520/21/20B/21B

Rev. 5.1

Page 6

May 1999

The acoustic stability is provided as follows:

When the volume is increased, e.g. by 10 dB, the

receive gain maintains the same as long as no

receive signal is applied. Applying a receive signal

will cause a 10 dB increase of the receive gain and a

corresponding decrease of the transmit gain. This

squelch function improves the signal-to-noise ratio.

In other words, a certain increase of the volume

introduces a similar amount of dynamic voice

switching, controlled by the receive signal, also in the

handset mode.

Sidetone

A good sidetone cancellation is achieved by using

the following equation:

Z

The sidetone cancellation signal is applied to the

STB input.

By using two separate Wheatstone Bridges for return

loss and sidetone cancellation it is very easy to

calculate the sidetone balance network (see figure 4).

This

unique

configuration

provides

a

sidetone

cancellation less sensitive to tolerances on the

external balance network and totally independent of

the ac impedance and its tolerances.

A good and stable sidetone cancellation improves the

handsfree function considerably and ensures a safe

margin

against

acoustic

instability

under

all

circumstances.

R1

30 ohm

R5

300 ohm

Figure 4: Sidetone Bridge

Furthermore, the dual Wheatstone bridge makes it

very simple to adapt the circuit to different PTT

requirements as these two parameters (return loss

and sidetone balance) are independent of each

other.

Line Loss Compensation

The line loss compensation (Rx and Tx AGC

controlled by the line current) is a pin option. When it

is activated, the transmit and receive gains are

changed by -6 dB in 1 dB steps at line currents from

20 to 50 mA or 45 to 75 mA depending on selected

range.

Loudhearing

The loudhearing mode is enabled when HS and LE

are high. In order to prevent acoustic coupling

between

the

handset

microphone

and

the

loudspeaker, the AS252x incorporate an anti-Larsen

circuit.

The anti-Larsen circuit decreases the gain of the

loudspeaker amplifier when a microphone signal is

applied. If no signal is applied from the microphone,

the loudspeaker amplifier is at its full gain.

Anti-Clipping (not AS2520B/21B)

The anti-clipping circuit is activated in loudhearing

and handsfree mode. The circuit prevents harsh

distortion at very high signal levels.

Furthermore, the circuit assures that the integrity of

the whole telephone circuit is maintained under

extreme load conditions, since it prevents that the

supply voltage drops below a certain minimum level.

The attack time is fast (120 µs/6 dB) for preventing

harsh

distortion

when

the

amplitude

rapidly

increases. For avoiding chopper effects and to

assure low distortion, the decay time is longer,

approx. 128 ms/6 dB.

When the anti-clipping circuit has been activated by a

large receive signal, the channel control will increase

the Tx gain corresponding to the reduction in Rx gain

caused by the anti-clipping.

Handsfree

The handsfree function allows voice communication

without using the handset (full 2-way speaker phone).

Two voice controlled attenuators prevent acoustic

coupling

between

the

loudspeaker

and

the

microphone.

A conventional voice switching circuit has a channel

control with three states, namely idle, transmit or

receive. In idle state, when no signal is applied, both

the transmit and the receive channels are attenuated

by approx. 20 dB to keep the total loop gain below 0

dB.

When a signal is applied to the microphone, the

circuit switches to transmit state, i.e. the gain in the

transmit channel is increased and the gain in the

receive channel is decreased accordingly. And vice

versa when a receive signal is applied.