Building a Colpitts

[ciscorey]

Hey guys, when it comes to EE I am the epitome of a novice. In fact, I picked myself up a experiment lab from Radioshack the other day to learn some basics. Unfortunately the superiors at work need me to develop a colpitts oscillator that oscillates at 13.46MHz. I was wondering if anyone had any pointers. I'm most confused about which components are necessary to acheive this result.

Thanks in advance.

 

[Pud]

Colpitts: do a search for "dead bug construction" - crude but stable.


Cheers

Harry
(G4NTK in another life!)

 

[GonzaloEE]

Try to learn some basics on oscillators from good sources first, then go looking for circuits in Google.

Be aware of those DIY sites with RF bugs and spy microphones, often written by non EEs (tech geeks, amateurs, etc.)

Good Luck!
Gonzalo

 

[ijl]

Check the frequency. I am pretty sure it should be 13.56 MHz, not 13.46 MHz

 

[MacGyverS2000]

It's over unity... maybe he wanted to make some contact with it ;-) a few less kiloHertz should do it...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[richs]

Hiya-

Continuing this thread. Hint. Get the boss to pay for the ARRL handbook. When it comes, put YOUR name on it in big bold letters across the top (where all the pages are). When you leave, due to new job, financial problems, whatever, take the book with you!

Cheers,

Rich S.

 

[MacGyverS2000]

Hmmmm, should we turn this thread into one about ethical dilemmas and stealing from the company? ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[xray]

I have liked the idea of putting together a colpitts oscillator. Tried one from a nice description by valdim Manassewitsch and dead bugged a 10 MHz colpitts with a 2222 BJT and passives I had at the office. A small amount time a bit more effort in the calcultations an order to any of the mirad of elctronic distributors a good osicllator could be completed. Valdim's book is called "Frequency Synthesizers: Theory and Design". Would like to discuss the circuitry behind the colpitts a bit more.

 

[Skogsgurra]

If this dynamic and inventive boss and his willing and cluless hand want to build a Colpitts oscillator. Then let them do so.

But do not tell them to use a crystal. That is not Colpitts any more.

I can not understand why the Colpitts is the only oscillator configuration deemed suitable. There are dozens of other topologies. Could it be that the positive feedback obtained in the Colpitts oscillator reminds some creative souls about over-unity? Which it is, voltage-wise, but not power-wise. Then, why wouldn't a Hartley be just as good? It also has over-unity feedback.

Also, by using a tunable ciecuit (lika a Colpitts) they may be able to find that sweet spot somewhere between 13.46 and 13.56 where the pot of gold resides.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Or the tarpit of extinction.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[VEBill]

"But do not tell them to use a crystal. That is not Colpitts any more."

What is a Colpitts-like xtal oscillator supposed to be called?

 

[Skogsgurra]

A crystal oscillator - I suppose?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GonzaloEE]

You may use 'faster' transistors, like MPF102 (FET) or BF240 (NPN), etc. Those work well even at UHF.

Regards,
Gonzalo

 

[VEBill]

"A crystal oscillator - I suppose?"

Such a name fails to provide much info on the architecture. It would be like calling an oscillator an 'LC Oscillator' - perhaps accurate but unenlightening.

My ARRL handbook (1989, p. 10-1) shows four xtal oscillators in Figure 1: Pierce, Colpitts, Harmonic, and Overtone. The (xtal) Colpitts has a 10k resistor and two capacitors (47 & 100pF) in series across the xtal, and the feedback is tapped into their junction. Given the difficulty of tapping into the middle of a xtal, perhaps this explains the nomenclature discrepancy.

Cheers.

 

[Skogsgurra]

The idea of tapping into a fraction of a series resonance circuit is common to Colpitts and Hartley. Using an artificial midpoint may very well correspond to either the Colpitts or Hartley structure. Not just Colpitts.

So, which one should give name to such a circuit? Not that it matters much.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[richs]

Hiya again-

In my grandfather's old (circa 48) ARRL hand book they mention and build a VFXCO (variable frequency crystal oscillator). Using the old FT243 (????) cases (the ones with the three machine screws in them and the "thick" metal plate. Popular from WWII era stuff), they drilled and tapped a 4-40 screw in it and mounted a knob on the outside of the screw. This variable pressure changed the frequency of the crystal up to 10KHz or so.

One could use that for finding the "sweet spot".

They also had an article on "hand lapping" the crystal to move the frequency of surplus crystals UP. Included, if I remember correctly was to lap a slight angle on one corner of the crystal to get it to "ring" better.

Cheers,

Rich S.