The RC network also acts as the frequency-determining device. For the circuit to produce regenerative feedback, the RC network must provide a 180-degree phase shift of the collector signal. Using the same size resistor and capacitor as before will cause another 60-degree phase shift to take place. Now the signal at point P has been shifted about 120 degrees and its amplitude is reduced still further. This signal experiences another 60-degree phase shift and has further amplitude reduction.
R is the input to the base of the common-emitter amplifier.
This meets the regenerative feedback requirement. An exact 60-degree phase shift per stage is not required, but the sum of the three phase shifts must equal 180 degrees. For a given RC network, only one frequency of the initial noise signal will be shifted exactly 180 degrees. In other words, the network is frequency selective. Therefore, the RC network is the frequency-determining device since the lengths of the vectors and their phase relationships depend on frequency.