Abstract: A filter (45) is passive and has a complex impedance which gives good impedance matching to the complex impedance of a transmission line. Because the filter is passive, it does not need to be powered and can thus be placed in locations that lack a power supply. The filter can also function in the event of a power failure. Because the filter has an impedance which can be well matched to the complex impedance of a transmission line, problems relating to echo and side tones can be minimized.

Abstract: The present invention relates to a filter (45) for filtering signals in a telecommunications system, a method of designing said filter, and a corresponding splitter filter. The inventive filter (45) is characterised in that it is passive and has a complex impedance which gives good impedance matching to the complex impedance of a transmission line. Because the filter is passive, it does not need to be powered and can thus be placed in locations that lack a power supply. The filter will also function in the event of a power failure. Because the filter has an impedance which can be well matched to the complex impedance of a transmission line, problems relating to echo and side tones can be minimised.

Abstract: An apparatus for detection of a tone signal having a predetermined frequency among other tone signals, includes a combined band reject-band pass filter, amplitude limiting means and a product modulator. The signals appearing across the two outputs of the filter have different phases if an input signal has a frequency equal to the tone signal frequency. The value of the phase difference is converted to a DC-component by the limiting means and the modulator, and the magnitude of the DC-component forms an indication of the desired tone signal.

Abstract: A pilot receiver for level control in carrier frequency systems works with an intermediate frequency which is optimal from the viewpoint of the filter which is an integral part of the receiver and comprises in series connection, a phase-shift demodulator (quadrature demodulator), an active RC-filter and a rectifier.

Abstract: The present invention relates to an apparatus for measuring the level of a signal voltage supplied to the control input of a controllable constant current source whose output is connected to a phase inverting circuit conducting in one direction and to a rectifier conducting in the opposite direction so that a full-wave rectified current is obtained which is adapted to flow through a measurement device in order to obtain a measure of the voltage level.