Analog filter for Teletax pulse attenuation and method for tone detection through decimation and selective bandpass filtering
Original Publication Date: 1999-Apr-01
Included in the Prior Art Database: 2003-Jul-21
The detection of the Teletax (TTX) tones at 12 or 16 kHz is often a requirement for the line interface (e.g. Siemens ANIC-Chipset). But because the maximum TTX amplitude could reach 10.5 Vrms at the tip-ring it needs to be attenuate to the maximum allowed value by the input-stages (about 3 Vrms). Because of the two different frequencies a suitable analog filter must provide this attenuation at both the TTX frequencies. The structure proposed for the attenuation consists of a capacitance in series with a resistor (R 2 ), between Tip and Ring, and two coils in parallel with a resistor (R 1 ) along both Tip and Ring, as shown in Fig.1. The Lowpass action is provided by the capacitor C, in combination with loops over the ANIC-Chipset, while the coils are suitable to let the filter be transparent to DC and to the voice-band signals. At high frequencies the input impedance will converge to the sum 2*R 1 +R 2 , so it is possible to control this value by a suitable choice of R 1 and R 2 .