Meter pulse filtering circuit for analog modems
View Patent ↗A filtering circuit is described for filtering out meter pulses of analog telephone communication, for example in analog modems. The circuit is mainly composed of resistors and capacitors and is configured as a double T element or a Wien bridge circuit. The filtering circuit is needed for attenuating interferences to such an extent that integrated circuits operating with low voltages are not overloaded.
1. A filtering circuit for filtering out meter pulses of analog telephone connections, consisting essentially of:
a plurality of resistors;
a plurality of capacitors connected to said resistors such that an RC filter is formed for filtering out the meter pulses;
said resistors and said capacitors connected in a form of a twin-T section;
two of said resistors being connected in series forming a first series circuit having a center connecting node between said two of said resistors;
two of said capacitors being connected in series forming a second series circuit having a center connecting node between said two of said capacitors;
a common ground line;
an input line branching and connected to both said first series circuit and said second series circuit;
an output line branching from and connected to both of said first series circuit and said second series circuit on a side opposite to said input line;
a further one of said capacitors having a first terminal connected to said common ground line and a second terminal connected to said center connecting node of said first series circuit;
a further one of said resistors having a first terminal connected to said common ground line and a second terminal connected to said center connecting node of said second series circuit; and
said further one of said capacitors exhibits twice a capacitance of each of said capacitors in said second series circuit and said further one of said resistors exhibits half a resistance of the resistance of each of said resistors in said first series circuit.
2. The filtering circuit according to claim 1 , wherein the capacitance of each of said capacitors in said second series circuit is 32.935 nF and the resistance of each of said resistors in said first series circuit is 302 Ω, and including a terminating resistance connected to said output line and having a resistance of 417.63 Ω.
3. A filtering circuit for filtering out meter pulses of analog telephone connections, consisting essentially of:
a plurality of resistors;
a plurality of capacitors connected to said resistors such that an RC filter is formed for filtering out the meter pulses;
said resistors and said capacitors are connected in a form of a Wien-Robinson bridge circuit;
said resistors include a first resistor, a second resistor, a third resistor, and a fourth resistor each having a first terminal and a second terminal;
said capacitors include a first capacitor and a second capacitor each having a first terminal and a second terminal;
a first input terminal connected to said first terminal of said first resistor and to said first terminal of said second resistor;
said first capacitor is connected between said second resistor and said third resistor;
a first output line connected to said second terminal of said first resistor;
a second output line connected to said first capacitor;
said fourth resistor is connected to said second terminal of said first resistor;
said second capacitor is connected in parallel with said third resistor and together define a parallel circuit, said parallel circuit connected to said first capacitor;
a second input terminal connected to said parallel circuit and to said second terminal of said fourth resistor; and
all of said capacitors exhibit an equivalent capacitance, said second resistor and said third resistor have an equivalent first resistance, said first resistor has a second resistance different from said first resistance, said fourth resistor has a third resistance being half of the second resistance.
4. The filtering circuit according to claim 3 , wherein the capacitance of said capacitors is in each case 74.61 nF, the first resistance is 133.3 Ω, the second resistance is 700 Ω, and including a terminating resistor having a resistance of 6 kΩ and connected to said first output line.