Analogue echo filter
The invention relates to an analog echo filter for a modem, to which an analog transmit signal is supplied. According to the invention, a passive analog input filter and an active analog filter, whose transfer function is adjustable, are provided. The transfer function of the active analog filter can preferably be switched between a first-order transfer function and a second-order transfer function.
1. An analog echo filter for an echo damping path of a modem, which is located between a transmit path and a receive path, in which case the modem can be supplied with an analog transmit signal, which is transmitted in full-duplex via a transmission line, via the transmit path, with the modem having an analog hybrid circuit for separating and combining the transmit signal and a received signal which can be output via the receive path, and having an active analog filter, whose transfer function is adjustable,
wherein
a passive analog input filter is provided for input balancing of the echo damping path and for adjusting a terminating impedance for the analog hybrid circuit;
the transfer function of the active analog filter can be switched between a first-order transfer function and a second-order transfer function;
the active analog filter has three operational amplifiers, which are connected in series; and the second operational amplifier in the series circuit can be bridged by switches in order to switch the active analog filter from a second-order transfer function to a first-order transfer function.
2. The analog echo filter as claimed in claim 1
wherein
the passive analog input filter comprises resistors and capacitors.
3. The analog echo filter as claimed in claim 1 ,
wherein
the first operational amplifier and the second operational amplifier in the series circuit have capacitors which respectively feed back the outputs to the inputs of an operational amplifier.