When the value of the dc signal exceeds {HEAD ~ a threshold value}, the comparator provides a first output signal which causes the tone switching logic circuit to change the reference frequency to the frequency f2, and to control the demultiplexer and multiplexer so that the mixed signal is thereafter alternately coupled, at the multiplex frequency, to the inputs of {TAIL ~ the third and fourth low pass filters}, and so that the outputs thereof are alternately coupled, at the multiplex frequency, to the squarer 
When the value of the dc signal exceeds { HEAD ~ a threshold value } , the comparator provides a first output signal which causes the tone switching logic circuit to change the reference frequency to the frequency f2 , and to control the demultiplexer and multiplexer so that the mixed signal is thereafter alternately coupled , at the multiplex frequency , to the inputs of { TAIL ~ the third and fourth low pass filters } , and so that the outputs thereof are alternately coupled , at the multiplex frequency , to the squarer
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH