Preliminary
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
BAL
/Z
LINE
= R5/R1
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
AS2520/21/20B/21B
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.
Z
LINE
Z
BAL
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
Rev. 5.1
Page 6
May 1999
AMSCO [ AUSTRIAMICROSYSTEMS AG ]
