Hi all
And thank you so much for reading my blogs .
After a long time since i've promised to write something about crystal oscillators i'm here again . ;-)
All right , here we can see a simple common base amplifier which it's base has been bypassed through the ground as well :
https://s4.picofile.com/file/7736383973/CB_amplifier.jpg
So if we replace RC with a tuned network it will behaves as a narrow band amplifier .
See here please :
https://s4.picofile.com/file/7736384080/tuned_CB_amplifier.jpg
So , as we know out put of a common base amplifier is in phase with it's input . so we can easily make it unstable with adding a feedback path from collector through emitter .
( now we have two poles in out put which will select frequency of operation . )
Take a look here please :
https://s4.picofile.com/file/7736384515/regenerative_oscillator.jpg
As it is obvious , the capacitor C4 will provide a positive feedback through the input of the amplifier , now let's look into it from another vision !
Let's consider it is a perfectly unstable circuit , so we can call it an oscillator . so it has an out put and no input .
There are some problems with such a simple circuit :
Value of feedback capacitor is very important and we should select it as proper as enough to fulfill the best oscillating condition .
and of course frequency of such a circuit won't be so precise and certainly not very pure .
Now let's look into crystals :
About what are crystals and how is their behavior and how are the arrangements for crystals in a circuit , please take a look into my last blog before this .
As we know a crystal has very high quality factor ( Q ) so we can guarantee it's behavior will be so precise .
BW=Fr/Q====> Q= Fr/BW ===== > BW is very narrow and small so Q will be high . and it is all we need !
Ok now let's go through other steps :
what will happen if we replace C4 with a crystal ? take a look here please :
https://s4.picofile.com/file/7736385806/Crystal_added_circuit.jpg
What kind of mode our crystal is attempted to operate in ?
For better clarification see here , an equivalent circuit !
https://s4.picofile.com/file/7736387197/eq_circuit.jpg
According to what we've learned before ( in my latest blog before this ) it is used in series mode of resonant !
So it's phase shifting will be around zero . so still positive feedback is fulfilled .
So thus we can guarantee when unwanted harmonics are coming through the feedback path crystal won't allow them to be transferred into the input of amplifier so we won't have them in out put .
Now let's look into a practical circuit what i've designed :
( out put shape and FFT form can easily demonstrate all of the things that i've told above )
https://s4.picofile.com/file/7736388709/practical_circuit.jpg
https://s4.picofile.com/file/7736388816/out_put_shape.jpg
https://s4.picofile.com/file/7736389030/spectrum.jpg
If there is any question please feel free to ask me .
Best Wishes
Goldsmith :wink:
And thank you so much for reading my blogs .
After a long time since i've promised to write something about crystal oscillators i'm here again . ;-)
All right , here we can see a simple common base amplifier which it's base has been bypassed through the ground as well :
https://s4.picofile.com/file/7736383973/CB_amplifier.jpg
So if we replace RC with a tuned network it will behaves as a narrow band amplifier .
See here please :
https://s4.picofile.com/file/7736384080/tuned_CB_amplifier.jpg
So , as we know out put of a common base amplifier is in phase with it's input . so we can easily make it unstable with adding a feedback path from collector through emitter .
( now we have two poles in out put which will select frequency of operation . )
Take a look here please :
https://s4.picofile.com/file/7736384515/regenerative_oscillator.jpg
As it is obvious , the capacitor C4 will provide a positive feedback through the input of the amplifier , now let's look into it from another vision !
Let's consider it is a perfectly unstable circuit , so we can call it an oscillator . so it has an out put and no input .
There are some problems with such a simple circuit :
Value of feedback capacitor is very important and we should select it as proper as enough to fulfill the best oscillating condition .
and of course frequency of such a circuit won't be so precise and certainly not very pure .
Now let's look into crystals :
About what are crystals and how is their behavior and how are the arrangements for crystals in a circuit , please take a look into my last blog before this .
As we know a crystal has very high quality factor ( Q ) so we can guarantee it's behavior will be so precise .
BW=Fr/Q====> Q= Fr/BW ===== > BW is very narrow and small so Q will be high . and it is all we need !
Ok now let's go through other steps :
what will happen if we replace C4 with a crystal ? take a look here please :
https://s4.picofile.com/file/7736385806/Crystal_added_circuit.jpg
What kind of mode our crystal is attempted to operate in ?
For better clarification see here , an equivalent circuit !
https://s4.picofile.com/file/7736387197/eq_circuit.jpg
According to what we've learned before ( in my latest blog before this ) it is used in series mode of resonant !
So it's phase shifting will be around zero . so still positive feedback is fulfilled .
So thus we can guarantee when unwanted harmonics are coming through the feedback path crystal won't allow them to be transferred into the input of amplifier so we won't have them in out put .
Now let's look into a practical circuit what i've designed :
( out put shape and FFT form can easily demonstrate all of the things that i've told above )
https://s4.picofile.com/file/7736388709/practical_circuit.jpg
https://s4.picofile.com/file/7736388816/out_put_shape.jpg
https://s4.picofile.com/file/7736389030/spectrum.jpg
If there is any question please feel free to ask me .
Best Wishes
Goldsmith :wink:
I want to share you some simple...