14. RC PHASE SHIFT OSCILLATOR

AIM: To calculate the frequency of the RC phase shift oscillator & to measure the phase angles at different RC sections.

  

APPARATUS:

                   1. Transistor BC107                        

                    2. Resistors:  10KΩ              -3Nos

                                           8KΩ or 10KΩ

                                           22KΩ                         

                                           1.2KΩ                        

                                           100KΩ                        

                  3. Capacitors:  0.001µf  – 3 Nos

                                           10µF      – 2Nos

1µf                    

                  4. Regulated power Supply  

                  5. CRO                                  

THEORY:

                 RC-Phase shift Oscillator has a CE amplifier followed by three sections of RC phase shift feed back Networks the out put of the last stage is return to the input of the amplifier. The values of R and C are chosen such that the phase shift of each RC section is 60º.Thus The RC ladder network produces a total phase shift of 180º between its input and output voltage for the given frequencies. Since CE Amplifier produces 180 º phases shift the total phase shift from the base of the transistor around the circuit and back to the base will be exactly 360º or 0º. This satisfies the Barkhausen condition for sustaining oscillations and total loop gain of this circuit is greater than or equal to 1, this condition used to generate the sinusoidal oscillations.

The frequency of oscillations of RC-Phase Shift Oscillator is,                                                               

                                                        

                                                                  1

                                                    f =     -----------

                                                             2pRC* √6

CIRCUIT DIAGRAM:

                                           

PROCEDURE:        

1.    Make the connection as per the circuit diagram as shown above.

2.    Observe the output signal and note down the output amplitude and time period (T_d).

3.    Calculate the frequency of oscillations theoretically and verify it practically (f=1/T_d).

4.     Calculate the phase shift at each RC section by measuring the time shifts (T_p) between the final waveform and the waveform at that section by using the below formula.     

OBSERVATIONS:

THEORITICAL CALCULATIONS:  R = 10KΩ, C = 0.001 μf

                                               1                                     

                              f =     --------------     =     

                                          2pRC* √6       

PRACTICAL CALCULATIONS:          

                                      T_d =

                                              1                                      

                                    f =     -----   

                                                T_d                                   

                                               Tp₁

                              (1). θ ₁=  --------*360⁰   =  

                                               T_d                       

                                                 Tp₂                         

                             (2).  θ ₂ =  ------- * 360^0  =    

                                                  T_d                          

                                                 Tp₃                              

                            (3). θ ₃= ----------- *360⁰  =    

                                              T_d                    

MODEL WAVE FORMS:

OUT PUT WAVE FORM :

OUT PUT WAVE FORM : θ = 60⁰

OUT PUT WAVE FORM : θ =  120⁰

OUT PUT WAVE FORM : θ =  180

RESULT:  The frequency of RC phase shift oscillator is calculated and the phase shift at different RC sections is noted.

VIVA QUESTIONS:

1. What are the conditions of oscillations?
2. Give the formula for frequency of oscillations?
3. What is the total phase shift produce by the RC ladder network?
4. Whether the oscillator is positive feedback or negative feedback?
5. What are the types of oscillators?
6.   What is the gain of RC phase shift oscillator?
7.  What is the difference between damped oscillations undamped oscillations?
8.  What are the applications of RC oscillations?
9. How many resistors and capacitors are used in RC phase shift network
10. How the Barkhausen criterion is satisfied in RC phase shift oscillator