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The 27 MHz Tx

samy555

May 11, 2010
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I have some simple questions about this circuit and I hope to get answers
Fig1.gif

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.
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.
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.
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?
 

davenn

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samy

its a Colpitts oscillator -- you can do some google searching on that type of osc and do some learning :)

How will I know that the frequency is adjusted? What is the simple way to do that?

You will need a frequency counter so you can monitor the change in freq as you adjust

Quote:
The transistor is configured as a common emitter amplifier.

The crystal grounded the base at 27MHz, so the transistor is configured as a common base amplifier?
(4)

Quote:
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.

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?

once again read up on the osc as said above..

cheers
Dave
 

samy555

May 11, 2010
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samy

its a Colpitts oscillator -- you can do some google searching on that type of osc and do some learning :)



You will need a frequency counter so you can monitor the change in freq as you adjust



once again read up on the osc as said above..

cheers
Dave

Thank you very much for this good advice
 

davenn

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The field strength meter circuit that has been shown for you to build is a good little project on its own

attachment.php


This wont tell you what frequency you are on but it will tell you when maximum power is being produced by the transmitter

even better than the 8cm lead would be a telescopic whip antenna off an old radio receiver so it can be pulled out to full length or pushed in for storage. and at 27MHz a 30cm or more length for the whip will work so much better

Dave
 

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samy555

May 11, 2010
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Dave

How do you know that it is its a Colpitts oscillator?
thanks
 

davenn

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There's 2 oscillators that are very common for crystal oscillators
The Colpitts and the Hartley, if anything it is a little closer to the Hartley
but their operation and layout is very similar. And there are often little variations
that are played around with.

the circuit takes very little changes to make it a Pierce or a Millar osc

Dave
 

davenn

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There's some good info in that link
there's a few reasonable tutorials online that you will find using google

gosh when I was learning all this stuff, the internet didnt exist
very basic computers were just starting to appear. It all came out of mainly amateur radio books like the ARRL Handbook, for me

Dave
 

duke37

Jan 9, 2011
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The feeback for oscillation comes from the capacitance in the transistor, transistor type may therefore be critical.

The crystal does not ground the base, it will have a parallel and series resonance and the circuit will operate somewhere between these.

If I remember correctly, the collector circuit has to be tuned to be inductive to oscillate. Just tune to maximum output.
 

Harald Kapp

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Dave: It now works for me, too. It didn't in the first place. But you explained everything.

Harald
 
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