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CDRIVE

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I think, it is not appropriate for this circuit to think in "ON" and "OFF" terms. The circuit produces a sinusoidal output!
This is why I'm loving this thread. It's a pecking frenzy! Whether the OP is comprehending any of it is debatable but I think it's highly unlikely.

The following are my personal Murphy rules regarding nubes and RF circuitry:

(1) If a nubie is building an oscillator it probably won't.
(2) Spice simulators hate sinusoidal oscillators.
(3) If a nubie is building an amplifier he will unwittingly end up with an oscillator! :p

If I had the talent of my nephew Marty I'd put a clever graphic here, along with a pun that only he understands. :confused:

Chris
 

dorke

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LvW

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(2) Spice simulators hate sinusoidal oscillators.
CDRIVE - I can agree to all of your remarks with one tiny exception:
If properly arranged SPICE simulators will have no problems with linear (sinusoida) oscillators.
However, one must know how to command the simulator.
My experience: In nearly ALL cases, it is the user who makes errors - not the engine.
 

Martaine2005

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Don't wont to join the "Oscillo-wars" ;)

Being productive instead:

Here is an exallent video on design of a crystal colpittz osc.
It is different than the CB Colpittz but still...not by much.

Another video,how to actually build a Colpittz oscilator

and a very good video on RF Circuit Construction Techniques

I think all the above may be very helpful to all,and more so for the newbies to the black-magic RF world... enjoy:)
I see your sense of humour is slowly but surely coming down to mine and uncle Chris's!
Good on ya dorke!
 

Martaine2005

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If I had the talent of my nephew Marty I'd put a clever graphic here, along with a pun that only he understands. :confused:
rjo0619l.jpg

Marty...:p
 

ericstanleysamar

Dec 13, 2015
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Besides what dork stated I'd like to add that one does not make their first oscillator 98MHz. One starts their RF journey far more conservatively .. like 1MHz or lower.

Chris
It is a school project sir, it was never my decision to make it oscillate at 98Mhz, it was my professor's. and he has not taught us much on the theory of this, i am taking a 3 year vocational course in electronics. so i'm very certainly sorry for asking such a silly question. i just want to know how to calculate this, i have seen my professor demonstrate his version of the circuit, and we picked it up in the fm receivers of our phones.. it is supposed to be acting as short range transmitter.,, i am really lost and don't know where to start,, how do i calculate the theoretical values for the biasing, inductor, capacitors, and how do i know if my design does not dampen, sorry for very stupid question
 

CDRIVE

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CDRIVE - I can agree to all of your remarks with one tiny exception:
If properly arranged SPICE simulators will have no problems with linear (sinusoida) oscillators.
However, one must know how to command the simulator.
My experience: In nearly ALL cases, it is the user who makes errors - not the engine.

One thing for certain.. If the simulator in Transit Analysis provides options of
(1) Calculate Operating Point,
(2) Use Initial Conditions.
(3) Zero Initial Values.

I now always choose 2 or 3. Never 1.

Chris
 

ericstanleysamar

Dec 13, 2015
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I MADE a cirrcuit in multisim, and it oscillates,, not sure what frequency though.. just another question,.. how do i modulate the frequency i produced with respect to the ac source i placed there?? is it right if i put this varactor diodes,,, and use the ac source connecting it in both anodes of the varactor w.r.t ground? i am lost... and if so,,what are common varactor diodes i can use,, easy to find in market?
 

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LvW

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One thing for certain.. If the simulator in Transit Analysis provides options of
(1) Calculate Operating Point,
(2) Use Initial Conditions.
(3) Zero Initial Values.

I now always choose 2 or 3. Never 1.

Chris

Yes - me too.
More than that, I always switch on power supply at t=0.
 

dorke

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Eric,
If this is only a "theoretical school drill" to be "tested" in a simulator program but not to be actually physically build you should be able to do it.
On the other end, building it and making it work is something you shouldn't even try with your current knowledge and experience.

1.It should have been posted in the "Homework Help" not "Circuit Help" .

2.Your last circuit is a CE one and not a CB one as the original post.

3.As for calculating the oscillation frequency.
fosc=1/[2*pi* (CT*L)^0.5]
L=L1 ; CT =C1*C4/[C1+C4]

4.Maybe you missed the video in my post #22 above
watch it and ignore the fact it is talking about a crystal - think of it as an inductor instead of the crystal and it is a CC circuit.
you may learn a lot from it.here it is again
design of a crystal colpittz osc.

5.Your last circuit needs some modifications

V1 is too low,make it 9V.
C2 is not necessary-make it a short wire.
C3,C5 too low make them 0.1uF (or higher).
R1,R4 too low should be in a 10x magnitude higher.
 

Colin Mitchell

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I have added this to my website under SPOT THE MISTAKE Page 26:


98MHz OSCILLATOR
This discussion has been taken from an electronics forum where a student has asked for component values for the following 98MHz FM oscillator.

FM-Transmitter.gif


My first comment was this:
You really need a cap across the coil to make a reliable osc.

The answer I got from a technician was:
The effective capacitor across the coil is the one from collector to emitter.

And another answer:
Of course, there is a cap in parallel to the inductor.
As a consequence, we have a tuned circuit, which determines the oscillator frequency.


And a third reply:
The two capacitors are across the inductor with the battery in series.

I made some comments about turning the transistor ON and OFF and get this reply:
I think, it is not appropriate for this circuit to think in "ON" and "OFF" terms. The circuit produces a sinusoidal output!


Then we have another comment:
The tuned circuit will have a Q value of maybe 100, in other words, the circulating current will be much higher than the input current, this will be built up over several cycles.


Let's look at these comments and see how incorrect they are.

Firstly we need to look at the circuit and see if it oscillates.

FM-Transmitter-2.gif


The circuit above oscillates. It has a coil and capacitor connected in parallel to make a circuit called a TUNED CIRCUIT.
This type of circuit produces a sinewave when connected to a power supply and then instantly removed.
It does not need any other components and it does not need a transistor to produce this effect.
So, what do all the surrounding components do?
They connect the TUNED CIRCUIT to the supply and quickly remove it.
It must be quickly removed, otherwise the voltage produced by the TANK CIRCUIT will be reduced. In other words the surrounding circuitry will put a load on the TANK CIRCUIT.
That means the transistor must be turned on and then turned OFF very quickly.
This clears up the faulty thinking of one the replies above.
Now we come to the amplitude of the waveform produced by the TANK CIRCUIT.
We are going to remove all the surrounding components and talk about the TANK CIRCUIT.
For a correctly designed tank circuit, the energy stored in the coil is equal to the energy stored in the capacitor.
This is necessary because the TANK CIRCUIT produces a waveform consisting of half a cycle that is below the power rail and half a cycle that is higher than the power rail.
For both these half-cycles to be identical, the component values must match.
The full amplitude (called the peak-to-peak value) will be the addition of these two values.
Q-values have nothing to do with current. They are a voltage-determined value.
You can now see the first circuit does not have a capacitor across the coil and even though some of the cycle may be produced by the surrounding components, the equal parts of the waveform cannot be produced.
We have not described how the TANK CIRCUIT produces a waveform above AND below the power rail.
The easiest way is to remove all the surrounding components and tap the TANK CIRCUIT across the battery.
This will put energy into the uncharged capacitor and it will be charged to 3v. During this time the voltage will appear across the coil and it will produce a small amount of flux that will oppose the voltage and thus very little current will flow into the coil.
We are delivering energy to the circuit for a very short period of time and this is too short for the coil.
Now the supply is removed and the energy from the capacitor is slowly fed to the coil and it produces magnetic flux. The coil does not accept energy any faster than a certain rate because the "applied voltage" - the voltage on the capacitor, produces magnetic flux called EXPANDING FLUX and this cuts the turns of the coil to produce a voltage in the opposite direction to OPPOSE the incoming voltage and that's why the capacitor discharges slowly.
So far we are producing the lower part of the waveform because the bottom plate of the capacitor is at 0v and the capacitor is gradually discharging.
The capacitor continues to deliver current until a point is reached where the coil is producing a "back voltage" equal to the capacitor and suddenly the capacitor cannot deliver any current.
The magnetic flux in the air surrounding the coil cannot be maintained and it collapses. This produces a voltage in the turns of the coil that is in the OPPOSITE DIRECTION.
We not have a situation where the voltage produced by the coil is OPPOSITE to the previous voltage and as the magnetic flux collapses it delivers a voltage to charge the capacitor in the opposite direction.
Since the two components are equally matched in "energy storage" capability, the capacitor is charged to a voltage EQUAL to the original voltage (but in the opposite direction).
Now you can see how the waveform rises to the voltage of the supply rail when the capacitor has no charge and then rises ABOVE rail voltage as the capacitor charges in the opposite direction.
This exchange could continue FOREVER but there are some losses with the magnetic flux and each cycle becomes smaller and smaller.
If we have a transistor that delivers a small amount of energy at exactly the right time during each cycle, the full waveform will be maintained. That is what the transistor and surrounding components do.
For a 3v supply, the peak-to-peak value of the waveform will be 6v.
The "Q" of the circuit is 2. Not 100.
 

Martaine2005

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Colin, that is the best post from you that I have seen!
Well done sir!

Do you have a new prescription? I already like him!.:p
Seriously..

Martin
 

CDRIVE

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Colin, I've been busting your chops for quite some time now but I must admit that my harassment had nothing to do with breaking you out of your 10 word maximum post rule.

Though I didn't find anything in your post that gave me pause I don't think it would matter if there was. The fact that something (not my continual pecking) prompted you to punch back convincingly is a milestone!

Even if someone finds a single misstatement in your post it wouldn't matter to me. The fact that you were bugged enough to really debate the particulars is a game changer. Your post is a complete reversal of what I've become accustomed to. I loved your post!

Chris
 

CDRIVE

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I have added this to my website under SPOT THE MISTAKE Page 26:


98MHz OSCILLATOR
This discussion has been taken from an electronics forum where a student has asked for component values for the following 98MHz FM oscillator.

FM-Transmitter.gif


My first comment was this:
You really need a cap across the coil to make a reliable osc.
Colin, I'm surprised that no one has bit the hook, so I guess I'll be the first to give it a go.
As I see it the effective capacitance across the coil is approximately the series equivalent of C2 & C3. This is because you totally missed the fact that the Vcc rail is bypassed and therefore at signal ground potential! Yes, C3 is way too large for proper feedback to the Emitter but that fact doesn't negate this.
Edit: Just to reiterate and be as clear as possible, I'm saying that C2, C3 are effectively across the coil because the top end of the coil is connected to the Vcc rail,... which is ideally at RF ground potential.
Hey, I still liked your post though!

Chris
https://www.electronicspoint.com/attachments/gedc0063-jpg.23754/
 
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Colin Mitchell

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Chris

You have got to understand how the oscillator works.
I have already covered your point and you still keep bringing it up.
That's why I don't add long comments to the forum because I am basically wasting my time.
However I get 7,000 visitors a day to my website and thousands have seen the 26 pages of circuit mistakes and many have commented on how much they have learnt.
I have emails to say they have learnt more from my site than the whole 4 years of University.
I have already said the circuit relies on only two components. The coil and capacitor in parallel to produce the waveform and the frequency.
All the transistor does is to deliver a little bit of energy at exactly the right time to keep the circuit oscillating at its maximum.
If you think the circuit works in some other way, you are totally incorrect. In fact I have been designing and producing these circuits for the past 40 years and made millions out of selling kits.
You will find my circuits and PCB's copied by all sorts of other sellers and never has anyone given me credit for the design.
Not only do they use exactly the same component values, the same layout but the board is exactly the same shape.
What a coincidence !!
You desperately need to ready my website and get an understanding of how the circuit works.
I used to lecture the teachers on electronics and found how futile the situation was.
Out of 70 teachers each year for 5 years, they were supposed to buy a set of kits to give to their students.
Out of 350 teachers, only one teacher bought any kits and he taught at a GIRLS SCHOOL !!!
In the following year I contacted 2,300 teachers by mail with an electronics book.
In the following 18 months only 19 teachers responded.
That's an accurate record of the enthusiasm, capability and dedication of Australian teachers.
And there's absolutely no point in encouraging technology.
The Chinese have taken over.
The Chinese understand technology - especially electronics.
Their mechanical understand is abysmal - buy they are learning rapidly.
If you see their cars and motorcycles now, you will be impressed.
They have copied western technology to the nth degree and the results are impressive.
They are now at a point of unstopability.
 

dorke

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Collin,
A lot of brag and a lot of miss-information coming from you,
and not for the first time ...:mad:

In short,
to create an oscillator we have to satisfy the barkhausen criteria.
It can be done in numerous ways.

For the sake of learning something:

Here is another (this time C.E) Colpittz oscillator video

And here is an old ,and very good, US army Video about LC oscillators.
 

Colin Mitchell

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Dorke:

To start with, the first video is a common-emitter stage, it is operating at 14MHz and uses a series tuned circuit, so it is nothing like the 94MHz oscillator we are describing.
On top of this, the video does not explain how the oscillator works AT ALL and is of no importance in our discussion.

In the second video he describes current incorrectly. Current does not flow from negative to positive, otherwise the description of how the full cycle is produced, is correct.
 
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CDRIVE

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Chris

You have got to understand how the oscillator works.
I have already covered your point and you still keep bringing it up.

If you think the circuit works in some other way, you are totally incorrect.
Colin, my post did not address how this circuit or sinusoidal oscillators in general work. You seem to have missed or chose to ignore the point I was making. You don't have exclusive rights to disliking repeating one's self, so I will not do so either.

I'm more than willing to debate any issue and also admit being wrong if proven so. We have to agree what were debating about first!

As I see it this is never going to happen because you're too wrapped up in the world according to Colin Michell and if Colin Mitchell says it then there is no debate! You seem to have the aloof attitude that everyone here at EP are just superfluous and annoying (know nothing) Harpies that just get in the way of Colin Mitchell's undisputed, paramount answer (Click Here) to all things.

EDIT: When you first came aboard EP I was convinced that the only reason you're here is to funnel members to your website. Now that time has passed my opinion hasn't changed a wit!

Chris
 

LvW

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However I get 7,000 visitors a day to my website and thousands have seen the 26 pages of circuit mistakes and many have commented on how much they have learnt.
I have emails to say they have learnt more from my site than the whole 4 years of University.
................................
You desperately need to ready my website and get an understanding of how the circuit works.
I used to lecture the teachers on electronics and found how futile the situation was.
Out of 70 teachers each year for 5 years, they were supposed to buy a set of kits to give to their students.
Out of 350 teachers, only one teacher bought any kits and he taught at a GIRLS SCHOOL !!!

.

Colin, I must admit that - up to now - I did not read all of your contributions in"talking electronics".
However, I only can hope that none of the thousands of vistors of your webside was trying to follow your "explanations" about BJT basics (in particular base biasing and function of the emitter resistor RE). It seems that you still are following the wrong approach that the BJT and its collector current would be physically controlled by the base current IB.
As a consequence, we can read some "surprising novel explanations" how current stabilization works.
Something similar applies to all parts dealing with "linear" oscillators. Surprisingly, I didn`t find any mentioning of the terms "loop gain" or "Barkhausen criterion"; instead, you follow the ON/OFF terminology which is not applicable to harmonic oscillators.
 

dorke

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In the second video he describes current incorrectly.
Current does not flow from negative to positive.

Here is another example of your miss-information and actually very basic misunderstanding !o_O
And you brag about teaching teachers...:eek:

Current(of electrons) does infect flow from negative to positive.
it is only by convention that we prefer to describe it flowing the other way around from positive to negative.

In a circuit one can freely decided the direction of current (or voltage),if done consistently it has no effect on the circuit analysis /behavior .
 
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