The 27 MHz transmitter

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samy555

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I have some simple questions about this circuit and I hope to get answers

from: http://www.talkingelectronics.com/projects/27MHz Transmitters/27MHzLinks-1.html
(1) Where is the positive feedback needed to sustain oscillation?
(2)
The frequency is adjusted by a ferrite slug in the centre of the coil until it is exactly the same as the crystal.
Click to expand...
How will I know that the frequency is adjusted? What is the simple way to do that?
(3)
The transistor is configured as a common emitter amplifier.
Click to expand...
The crystal grounded the base at 27MHz, so the transistor is configured as a common base amplifier?

(4)
It has a resistor on the emitter for biasing purposes but the 82p across the 390R effectively takes the emitter to the negative rail as far as the signal is concerned.
Click to expand...
I have calculated the reactance of that 82p cap and it was almost equal to 72 ohm. Is not a large value? Why not choose largest value of the capacitor so that it becomes less reactive?
 

Dear samy555
Hi
The crystal , grounded base at 27 MHZ , and feed back path provided with CE capacitor ( internally capacitor ) and tuned network at collector , provided another selector ( frequency selector) . with this consideration it will be alike with HF coolpitts oscillator .
BTW : hasn't this circuit , any input signal ( my mean is modulating signal ?)
Best Wishes
Goldsmith
 


When you sensed that i can help you , feel free to ask me
Click to expand...
That encouraged me to ask, thanks
The crystal , grounded base at 27 MHZ , and feed back path provided with CE capacitor ( internally capacitor ) and tuned network at collector
Click to expand...
What do you mean by "CE" ? did you mean Emitter cap or the internal cap between collector and emitter?
provided another selector ( frequency selector)
Click to expand...
I did not understand that sentence.
Best Wishes
Samy
 

Dear Samy
Hi
At first :
Crystals has series resonant mode and parallel resonant mode .
Yes , my mean was Collector / emitter capacitor ( internally ) . and about your latest question my mean was that the tank circuit that is at collector will resonant at it's resonance frequency .
Best Luck
Goldsmith
 

Dear goldsmith, thank you for fast response
(1) I forgot to ask you how do you know that it is a colpitts osci.? Why not Buttler?
(2)From the datasheets of the transistor I found that the CE internal capacitance is typically 3.5pF, is this enough for positive FB?
(3) You know that the signal at collector is 180 out of phase wrt base, and we need another 180 degrees to make the FB signal inphase, but the internal CE cap provide only 90, remain 90 degrees. How we obtain them?

Now I'll go to read more about Crystals series resonant mode and parallel resonant mode .
Best Wishes
Samy
 

Circuit will amplify the thermal noise filtered by xtal with fundamental and harmonics and amplify signal to collector tuned fundamental with high gain where phase shift is 180deg with feedback from internal miller capacitance. (CE) THis appears to be series mode osc. Parallel mode might use Xtal in CE feedback path. eg >http://talkingelectronics.com/projects/27MHz%20Transmitters/imagesP2/TX-2Bcct.gif
 

Dear Samy
At first , about series and parallel resonant , see below , please :
https://en.wikipedia.org/wiki/Crystal_oscillator
And about feed back capacitor , yes , it seems ok for providing positive feed back ( don't forget , at CB amplifier the in put is emitter and out put is collector and we don't have any phase shift between input and out put )
And about name of this oscillator : as i said , it is alike coolpitts oscillator ( if you replace a capacitor with crystal , but i don't know the real meaning of this circuit when we have a crystal at base )
Best Wishes
Goldsmith
 


Ok,, thanks SunnySkyguy

Of the 82p and 390R connected to the Emitter I read the following:
1- CE & RE form a hpf and when I calculate the cutoff frequency i got 5MHz, is it good?
2- the reactance of CE must = 1/10 of RE. Xc for the 82p at 27MHz = 72 ohm which only 1/5.5 of RE? conmment please?
3- RE alone provide a -ve FB and adding CE in parallel remove that -ve FB? comment

thanks alot

---------- Post added at 18:54 ---------- Previous post was at 18:43 ----------

 Dear goldsmith, Nqw I'll go to read about crystal from
http://en.wikipedia.org/wiki/Crystal_oscillator
and
I'll back to put my questions
Thank you for your good efforts
Best Wishes
Samy

---------- Post added at 20:40 ---------- Previous post was at 18:54 ----------

Dear goldsmith,,,, it was very long article, I stopped at the middle
I felt a severe headache (don't worry)
I think it's unnecessary to know all these details
I'll deal with crystal very simply
I have two questions remained:
1 - Is the crystal vibrates mechanically normally?
2 - The number written on the body of crystal is it the fundamental freq or the third tone?
thank you very much
 

Dear samy
Hi again
for question number one : yes , crystals are mechanical elements , usually , what made from quartz .
2- Yes , that number is fundamental component .
BTW: why headache ? electronics is sweeter than things that you can find in confectionery !!! :lol:
Best Wishes
Goldsmith
 

goldsmith said:
Dear samy
why headache ? electronics is sweeter than things that you can find in confectionery !!!
Click to expand...
Dear Goldsmith
I love science electronics
But books and long articles always talk a lot about the easy and understandable points and do not mention anything about the important points.
I read all the time for 4 days did not go out of great benefit.
Information, some of them contradictory and some of them incomprehensible and confusing
Crystal, for example, talking about the history of her life and how it is manufactured and so, do not walk much in the details that interest me.
I hope that someone explain the circuit very well, give us the mathematical expressions used in the calculation of each element
I will not despair and I will try my best to understand the story
Greetings
 

Dear Samy
Hi again
We can analyze that circuit with mathematical equations , at first , for oscillations , we should create situation of oscillation and we should create :
1- a good negative resistance .
2- unstable poles
3- to keep oscillations we need Av* beta =1 ( beta is feed back network gain .)
but with these considerations ,design steps will be a bit complicated .
How ever , i can give you some simple rules to design this oscillator ( to select the values of elements with your self ! )
If you want , i can say , simple rules of design .
And about articles and books : you won't find , all of your desired things ,in a book and an article ! you should read some books and articles .
Good luck
Goldsmith
 
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    samy555

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All you tuned circuit fans who remember slide rules have used this one. For the rest of you. print this out. I used it daily.
**broken link removed**
the 47 pf @27MHz intersects with 150 ohms with a quick eye check, my spreadsheet says 125Ω.

Since gain is a ratio of collector to emitter impedance. Look at the effective capacitance ratio (C/E ) including Pi filter and you need is >1 loop gain @360deg to oscillate, one would have to do worst case tolerance analysis to see is variation in hFE will support this solution, but there may be a rule of thumb.

A bigger emitter cap will drive more gain in osc. Too much and it becomes nonlinear, hence need for Pi Filter. I have also used CMOS Xtal oscillator input as a filtered square wave with sine high impedance and then buffer it to create a stable generator, less sensitive to supply variation. Of course xtal is very high Q (10K) Pi filter in parallel mode. Also ceramic resonator is good.
 

Reader's Digest version of XTAL Book https://en.wikipedia.org/wiki/Crystal_oscillator

Note that the cool thing about AT cut XTALs is the inductance of L1 in Henry's like a huge power transformer but the motional capacitance, C1 is < 0.1pF so it cannot transfer much energy, and can burn out but provides extremely high Q of 10K, which translates into an inexpensive 50 ppm crystal. ( 15 cents or less in volume)

Whereas inductors are much less accurate and more expensive and lower Q.
I have also used 5'X cut for VLF filters and SC cut for OCXO ultra-stable designs <1e-11
 

I look forward to discuss this with you, but before that, I have a good book in Electronics and I want to read more about oscillators, and then come back to you.

How ever , i can give you some simple rules to design this oscillator ( to select the values of elements with your self ! )
If you want , i can say , simple rules of design .
Click to expand...
I hope that you give me the rules.

And about articles and books : you won't find , all of your desired things ,in a book and an article ! you should read some books and articles .
Click to expand...
Yes, I agree with you completely.
thank you very much

---------- Post added at 03:57 ---------- Previous post was at 03:31 ----------
1) What do you mean by: effective capacitance ratio (C/E )?
2) What is the relationship of the filter with the oscillator itself?
3) It s a common Emitter config>. so, hFE Will be reasonably fixed

A bigger emitter cap will drive more gain in osc. Too much and it becomes nonlinear, hence need for Pi Filter.
Click to expand...
Did you mean that a 82p cap is largeso the circuit designer need to pi filter?


I'll ask you a simple question and I hope that I receive an easy and clear answer
The question is: if I have a Crystal, what I need to make it to start oscillations?
thank you
 

1) the C/E = collector/emitter impedance ratio in this case at resonance the collector Cap and Ind. have equal magnitudes of impedance but opposite phase. The ratio of these values controls common emitter gain for small signals
2) the filter has a loading effect and thus lowers the resonant frequency.
3) Hfe is never fixed in fact is highly variable, but common emitter design can reduce effects of hFE variation on overall Voltage gain. But oscillator current gain drops sharply when it starts to saturate which tends to improve regulation of voltage gain.
Simple answer is any circuit will oscillate if it has positive feedback >1. Or in other words o or 360 deg phase shift and loop gain >1. That means loss thru crystal * Gain of transistor >1 at phase shift = 0 or 360 or 180deg thru inverter.
The Pi config for crystal with CMOS inverter is most reliable due to ideal impedance properties, voltage swing and gain for this freq range.
THis design needs work,and emitter R possibly needs to be reduced to value of impedance of emitter cap to keep emitter current linear and signal symmetrical but filter will help that.
I think xtal parallel mode design is most reliable.
 
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