All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum

Notices

The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 08-11-2011, 11:28 AM
Purnendu Kumar rath Purnendu Kumar rath is offline
New Member
 
Join Date: Jun 2011
Location: Bhubaneswar,India
Posts: 7
Default RC phase shift oscillator.

1.It's said that the source for oscillations is a noise signal,generated in resistors.Can anyone please explain me ,how it occurs?
2.In a oscillator what actually do we do,i mean can anyone please explain me the working of an oscillator(say RC phase shift oscillator)?
Heyy frndz please help me out.
Reply With Quote
  #2  
Old 08-11-2011, 07:18 PM
Veracohr Veracohr is offline
Senior Member
 
Join Date: Jan 2011
Posts: 192
Default

1. "Johnson–Nyquist noise... is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor..."

So basically it's just the sound of electrons moving around, and it's a physical property of electrical conductors. In an oscillator the amplifier amplifies the naturally-occuring noise of the resistors.

2. A sine wave oscillator requires an amplifier with positive feedback at a particular frequency. Noise is broadband, so the RC network is used to isolate a frequency for feeback.



Notice the feedback path goes to the inverting input, which is normally negative feedback. We want positive feedback. The RC network introduces frequency-dependent phase shift. Since the feedback path is going to the inverting input, the frequency of interest needs to be shifted by 180 degrees in order to result in positive feedback. The phase shift of the RC network varies by frequency, so there will be only one frequency where the exact amount of required phase shift occurs.

So even though you've got a broadband noise signal, positive feedback occurs at only one frequency, and you get oscillation.

I find Wien bridge oscillators better. Instead of phase shift they use a bandpass filter to isolate a frequency for feedback.
Reply With Quote
The Following User Says Thank You to Veracohr For This Useful Post:
Purnendu Kumar rath (08-13-2011)
  #3  
Old 08-13-2011, 04:05 PM
Purnendu Kumar rath Purnendu Kumar rath is offline
New Member
 
Join Date: Jun 2011
Location: Bhubaneswar,India
Posts: 7
Default oscillators

Thank you so much,that was a wonderful reply.
But it again brings some doubts to mind that of course for a particular frequency oscillations do occur because at that frequency only the phase shift is 180 degrees in the feedback network for RC phase shift oscillator ,for which the total phase shift becomes 360 degrees and hence satisfying barkhausen's criteria.
My question to you is that what happens to the frequencies other than the frequency of oscillation....?
and does the noise signal have a fixed frequency?
AND why is noise broadband in nature????

Last edited by Purnendu Kumar rath; 08-13-2011 at 04:09 PM. Reason: one more question.......
Reply With Quote
  #4  
Old 08-13-2011, 04:55 PM
Veracohr Veracohr is offline
Senior Member
 
Join Date: Jan 2011
Posts: 192
Default

Quote:
Originally Posted by Purnendu Kumar rath View Post
My question to you is that what happens to the frequencies other than the frequency of oscillation....?
and does the noise signal have a fixed frequency?
AND why is noise broadband in nature????
The other frequencies are also shifted by the RC network, just at different amounts, such that they are not at the proper phase to result in positive feedback.

Noise doesn't have a fixed frequency, it has all frequencies (theoretically).

Why is it broadband? I have no idea, that's beyond me. I'm sure there's a nice scientific explanation of the physics behind it, I just don't know what it is.
Reply With Quote
The Following User Says Thank You to Veracohr For This Useful Post:
Purnendu Kumar rath (08-15-2011)
  #5  
Old 08-15-2011, 07:26 AM
Purnendu Kumar rath Purnendu Kumar rath is offline
New Member
 
Join Date: Jun 2011
Location: Bhubaneswar,India
Posts: 7
Default

thanks again...
noise is continuously generated in conductors so when that amount of noise is again fed back positively then the amplitude of output must rise every cycle,how cum it stays constant??
Reply With Quote
  #6  
Old 08-15-2011, 07:35 AM
Purnendu Kumar rath Purnendu Kumar rath is offline
New Member
 
Join Date: Jun 2011
Location: Bhubaneswar,India
Posts: 7
Default

http://www.electronics-tutorials.ws/...r/hartley.html

please help me through this page......

it talks about tank circuits ..how are these tank circuits different from an RC circuit as in case of wein bridge?
initially it sayz that we need continuous feedback to meet the losses in the tank circuit which produce the oscillations,but den it sayz that the source of oscillations are noise signals and that these tank circuits are just frequency selecting networks...
can anyone clarify......please
Reply With Quote
  #7  
Old 08-15-2011, 06:57 PM
Veracohr Veracohr is offline
Senior Member
 
Join Date: Jan 2011
Posts: 192
Default

The LC 'tank' circuit accomplishes the same thing as the RC circuit in the Wien Bridge - frequency selecting - just in a different way.

LC tank circuits use the property of inductor-capacitor resonance to isolate a frequency.

RC bandpass filters use cascaded low-pass and high-pass RC filters to isolate a frequency.
Reply With Quote
The Following User Says Thank You to Veracohr For This Useful Post:
Purnendu Kumar rath (08-16-2011)
  #8  
Old 08-15-2011, 07:08 PM
Veracohr Veracohr is offline
Senior Member
 
Join Date: Jan 2011
Posts: 192
Default

Quote:
Originally Posted by Purnendu Kumar rath View Post
initially it sayz that we need continuous feedback to meet the losses in the tank circuit which produce the oscillations,but den it sayz that the source of oscillations are noise signals and that these tank circuits are just frequency selecting networks...
can anyone clarify......please
I think that means that if you give an LC resonant circuit a momentary noise signal, it will oscillate briefly but decay. The amplifier provides the "continuous feedback" needed to sustain the oscillations. The source of the oscillation is noise, from which the tank isolates one frequency for oscillation.
Reply With Quote
The Following User Says Thank You to Veracohr For This Useful Post:
Purnendu Kumar rath (08-16-2011)
  #9  
Old 08-16-2011, 06:52 AM
Purnendu Kumar rath Purnendu Kumar rath is offline
New Member
 
Join Date: Jun 2011
Location: Bhubaneswar,India
Posts: 7
Default

incase of an hartley oscillator 180 degree phase shift is provided by the amplifier and further 180 by the LC circuit,please explain how the Lc ircuit provides 180 degree phase shift....
Reply With Quote
  #10  
Old 08-16-2011, 11:27 PM
Veracohr Veracohr is offline
Senior Member
 
Join Date: Jan 2011
Posts: 192
Default

No, the Hartley oscillator doesn't work based on phase shift, it works on inductor-capacitor resonance which is reinforced and sustained by an amp. I provided a link that explains it. Also, the link you posted has a page just before it that explains resonance.
Reply With Quote
Reply   Post New Thread

Tags
, ,


Related Site Pages
Section Title
Worksheet Opamp oscillator circuits
Worksheet Oscillator circuits
Worksheet Passive filter circuits
Worksheet Mixed-frequency signals
Worksheet Resonance
Textbook Frequency and phase measurement : Ac Metering Circuits
Textbook Special transformers and applications : Transformers
Textbook Oscillator circuits -- INCOMPLETE : Practical Analog Semiconductor Circuits
Textbook Resonance in series-parallel circuits : Resonance
Textbook Input and output coupling : Bipolar Junction Transistors


Similar Threads
Thread Thread Starter Forum Replies Last Post
Oscillation criteria for a phase shift oscillator? Envergure Math 4 04-08-2010 02:43 AM
phase shift oscillator (Formula) Ghasemzadeh Homework Help 3 11-26-2009 04:35 AM
RC Phase Shift Oscillator Please help hasie Homework Help 13 10-03-2009 09:11 PM
Phase shift oscillator elektro1 The Projects Forum 4 08-27-2009 06:21 AM
Power Engineering Help (Star-Delta) mksa Homework Help 2 05-09-2006 05:41 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 04:59 AM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.