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An Oscillator Problem! (hlp)

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dopradar

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Hi all,

About the overtone oscillator, what's the difference between the output from collector and the one from emitter? I am tuning such oscillator, it works when output from the emitter but not from the collector. Who can tell the reason? Thanks!

Best Regards!
Dopradar
 

Borber

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According to your schematics you have no output from collector. It is connected to +9V.
 

servicemann

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Since the collector is directly power supplied, there is NO signal.
The emitter output has lower impedance anyway! - better for you.
 

E-design

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You need a resistor in the collector line to +9V to get a voltage drop to exrtact any signal on the collector.
 

Davood Amerion

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hi dopradar,
1st reasen: collector and emitter are 180 degree phase shift.
2nd resean: in your schematic collector connect to +9V and can not be used as output.
 

dopradar

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Sorry, it's my mistake. this is my schematic output from emitter.

when i set the output from the collector, remove C4 and use an inductor connecting the collector and VCC then AC coupling output from TR1's collector.
 

BigBoss

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Output loading affects the oscillation conditions. So when an oscillator is designed, output loading and its real and imaginary parts must be taken into account.When a load is connected hardly to the oscillator, the circuit can diverge from barkhausen conditions...

If you should use low load impedance, try to put a buffer between oscillator core and load to prevent loading effect, phase noise, pulling effect and so on...

I recommend you don't use an oscialltor standalone without buffer even the frequency low...

It's a general recommendation in practice...
 

dopradar

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Borber said:
What is the value of collector inductor?
Borber, the inductor is 470nH.

Added after 10 minutes:

BigBoss said:
Output loading affects the oscillation conditions. So when an oscillator is designed, output loading and its real and imaginary parts must be taken into account.When a load is connected hardly to the oscillator, the circuit can diverge from barkhausen conditions...

If you should use low load impedance, try to put a buffer between oscillator core and load to prevent loading effect, phase noise, pulling effect and so on...

I recommend you don't use an oscialltor standalone without buffer even the frequency low...

It's a general recommendation in practice...
Hi BigBoss,

From your experiences if i want to loading the oscillator with a resistor, what are the approximate values of the resistors needed output from collector and from emitter? Thanks!
 

VSWR

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It seems from the schematics, that the oscillator is designed for a load of 1 kohm. In that case there will be a 500 mV signal across the 1 kohm load.

Is this sufficient for your need? If not, what output level do you want? Across what load impedance? 50 ohms?
 

dopradar

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VSWR said:
It seems from the schematics, that the oscillator is designed for a load of 1 kohm. In that case there will be a 500 mV signal across the 1 kohm load.

Is this sufficient for your need? If not, what output level do you want? Across what load impedance? 50 ohms?
Yes, 50ohms!
 

Borber

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You must use buffer stage between oscillator and 50ohm load. Such a load has too low impedance to be connected directly to oscillator.
In case of collector output try to increase load inductance from 470nH to 2uH and use emitter folover.
Buffer stage can be as simple as emitter folower in both cases.
 

pthoppay

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I am not sure whether you can see the oscillation at the collector terminal. The reason is, as per the cicruit there are 3 stages of operation, 1)in linear 2)cutoff 3)active.

So during the cutoff there are no oscillation in the collector terminal while at the emitter the oscillation is due to the discharge of the capacitaor through the emitter resistor.

Please refer to Crystal oscillator circuits by R.Matthys for more information.

Prakash,
 

dopradar

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Borber said:
You must use buffer stage between oscillator and 50ohm load. Such a load has too low impedance to be connected directly to oscillator.
In case of collector output try to increase load inductance from 470nH to 2uH and use emitter folover.
Buffer stage can be as simple as emitter folower in both cases.
Borber,

what's the emitter folover/folower? could you explain it more for me. Thanks!

Added after 5 minutes:

pthoppay said:
I am not sure whether you can see the oscillation at the collector terminal. The reason is, as per the cicruit there are 3 stages of operation, 1)in linear 2)cutoff 3)active.

So during the cutoff there are no oscillation in the collector terminal while at the emitter the oscillation is due to the discharge of the capacitaor through the emitter resistor.

Please refer to Crystal oscillator circuits by R.Matthys for more information.

Prakash,
Pthoppay,

1) There is oscillation at the collector terminal.

2)
pthoppay said:
So during the cutoff there are no oscillation in the collector terminal while at the emitter the oscillation is due to the discharge of the capacitaor through the emitter resistor.
What's you mean? could you explain it more because I am a beginner. Thanks!

3) Do you have its ebook? could you mail a copy to me? "Crystal Oscillator Circuits by R.Matthys?
 

varunkrish

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i also think you can use oscillator as collector . what's wrong?
 

artem

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To say that we have to figure out oscillation condition in math then add a miller effect (will be there as soon as collectors load is nonzero) which works against it .

Better to do as others said - buffer with amplification , but dont forget to control buffer's input impedance not to exceed recommended for that kind of Colpitts oscillators. Read a book that mentioned above .
 

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