ucb1100 NXP Semiconductors, ucb1100 Datasheet - Page 13

• Manufacturer Part Number: ucb1100
• Description: Advanced Modem/audio Analog Front-end
• Manufacturer: NXP Semiconductors

Philips Semiconductors
1998 May 08
Figure 10. Telecom codec Sidetone Suppression Circuitry
Advanced modem/audio analog front-end
Shown with the Typical Connection between UCB1100 and
A
B
Telephone Line (No Protections Circuits Shown)
Rt
Rt
1:1 transformer
Rt
Rt
Ro
Ro
toutp
toutn
tinp
tinn
Ri
Rs
Rs
Ri
SN00135
Rg
Rg
13
An important built in feature of the telecom codec is the sidetone
suppression circuit. The sidetone suppression circuit is activated
when the sidetone suppression enable bit in the telecom control
register B is set. The telecom input signal contains a large part of
the telecom output signal Tout, when the sidetone suppression
circuit is disabled. The available dynamic range of the telecom input
is occupied largely by the telecom output voltage.
The sidetone suppression circuit subtracts a part of the telecom
output signal from the telecom input signal when activated. The
available dynamic range is in that case used more effectively than
without sidetone suppression.
The built in side tone suppression circuit, shown in Figure 9, has a
fixed subtraction ratio, set be the resistors Rs and Ri, which equals
600 / 456. This ratio is calculated from the following relations:
The impedance seen by the telephone line equals:
In which Rt represents winding resistance of the transformer, divided
by 2. Assuming Ri >> Ro then
A typical transformer has 156 ohm winding impedance, thus Re
should be 144 ohm. The ratio of the telecom input and output
voltage is therefore
Rline
Rline
Tin
Tout *
2 * Rt
Rt
156
Rt
156
Rt
Ro
300
300
Ro
Ro * Ri
600
144
2
Ri
300
Tout * 456
Preliminary specification
600
UCB1100