Branch circuit to isolate pilot tone - performs isolation of tone through differential condenser and series circuit of appropriate characteristics

A selective branch circuit with continuously adjustable branch attenuation is for use in isolating a pilot frequency or a narrow frequency band for control purposes which is superimposed on a comonnunications channel. A signal source of very low internal impedance is connected to a static plate of a variable capacitor which has a second static plate which is earthed; the rotor is connected to the branch output through a coil and a resistance; the characteristics of the coil depend on the desired resonant frequency and the total capacity of the variable capacitor and the value of the resistance is approx. equal to the desired characteristic impedance of the branch line.