RC generator

54 RC generator This tone generator uses two RC networks (all-pass filters) connected in series to achieve the necessary phase shifting. The frequency range is from 20 Hz to 20 kHz and distor- tion is kept to a minimum by the extensive use of amplitude stabil- isation. Op-amps Al and A2 are the bases for two phase shifting networks in the circuit diagram of figure 1. A stereo potentiometer Pl sets the frequency so that each network shifts the phase of the signal by 90°. Switch Si selects the required range; 20 Hz . . .. 200 Hz, 200 Hz ... 2 kHz or 2 kHz. . . 20 kHz. Op-amp A3 gives a further 180° phase shift and amplifies the signal so that the system will keep oscillating. Capaci- tor C5 is included in the feedback loop of A3 to suppress HF oscil- lation above 100 kHz. The output of both A2 and A3 are rectified by diodes Dl and D2 before being passed, via P3, to the inverting input of A4. This op-amp compares the feedback signal to a reference voltage set by zener diode D4. The...