An apparatus for detecting a tone signal whose frequency is within a particular tonefrekvensbaand

摘要

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.