Colpitts Oscillator Problem: Frequency won't stay put

Thread Starter

64C113M Abe.

Joined Sep 13, 2016
14
Hello all,

I have successfully made a shortwave transmitter using a crystal oscillator, now I want to try making a transmitter using a colpitts oscillator.

I made a video of the problem, but screwed up editing the video. You can still hear the carrier wave moving across the frequency.


The circuit I used was from this website:
http://www.electronics-tutorials.ws/oscillator/colpitts.html

What I first thought was the problem was that supply voltage was fluctuating, so I ran it through a 7805 voltage regulator, problem was still there.

I suspecting that the RF generated by the circuit is feeding back into the circuit, but I have no way of knowing this.
 

Thread Starter

64C113M Abe.

Joined Sep 13, 2016
14
Well as an update I have noticed a change in stability when I use different transistors but it still is suffering from frequency "hunting"

I'm thinking about putting in a constant current source at the emitter, but I feel this might not have any effect.
 

SLK001

Joined Nov 29, 2011
1,548
Maybe the frequency is being pulled by the transmitter. You need to isolate the outputs of oscillators from everything, as everything can and will detune your tuned circuits. I don't see any schematics, but maybe you need a buffer amp b/t the osc and xmitter.
 

BR-549

Joined Sep 22, 2013
4,938
Colpitts frequency drift.


"
  • 1. Isolate the oscillator from succeeding stages with a well designed buffer stage followed by a stage of amplification. Large signals can often then be reduced by a 3 or 6 dB attenuator which also has the benefit of presenting a well defined load impedance to the amplifier. If the stage is feeding a mixer, as is most often the case, then another benefit is the mixer (you are using double balanced mixers?), also see a source impedance of 50 ohms.

  • 2. Ensure the mechanical stability of your oscillator is such that mechanical vibration can have no effect on components, especially those frequency determining components.

  • 3. Supply the oscillator with a clean well regulated supply. If using varactor tuning, doubly ensure the tuning DC voltage is as clean as possible, a few hundred micro volts of noise can be imposed on the oscillator signal. Use back to back diodes for the variable element. Air variables are hard to come by although they offer far superior Q figures. DC tuning tends to be more versatile.

  • 4. Minimize circuit changes from ambient variations by using NPO capacitors, polystyrene are dearer but excellent, silvered mica in my opinion are not what many people believe and are highly over rated.

  • 5. The inductor should be air wound on a coil form with a configuration to maximize Qu. If you must use a toroid, where possible try to use the 6 type as it offers the best Q. Sometimes, for other reasons you might have to use a slug tuned form.

  • 6. Parallel a number of smaller value NPO capacitors rather than using one large one in frequency determining components. For trimmers try and use an air variable. Keep an eye out for small value N750, N1500 capacitors, < 15 pF, when available and are found to be dirt cheap. These are sometimes useful in taming drift in an oscillator.

  • 7. Bipolar or FETS for active device seems to be a matter of personal preference and I've seen some ferocious arguments over that one. Consensus seems to come down in favour of FETS. Me, I'm a bipolar man because FETS hate me pure and simple.
"
I would recommend an Hartley with a fet.
 

RichardO

Joined May 4, 2013
2,271
I would add an item 0 to the @BR-549 list:
0. Bypass, bypass, bypass. Use both large and small value ceramic capacitors for power supply bypassing. Something like 0.1 uf, 1000 pf and 100 pf all in parallel. The smallest cap should be closest to the oscillator circuit.
 

ian field

Joined Oct 27, 2012
6,539
I would add an item 0 to the @BR-549 list:
0. Bypass, bypass, bypass. Use both large and small value ceramic capacitors for power supply bypassing. Something like 0.1 uf, 1000 pf and 100 pf all in parallel. The smallest cap should be closest to the oscillator circuit.
Quite a large proportion of radio hams have websites, and you can usually find those by googling their call letters.

Its a very valuable resource that's well worth tapping into.

You can collect callsigns to try from various ham radio books and magazines - there's a pretty good stack on americanradiohistory.com

There shouldn't be much difficulty finding articles on VFO design.
 

radiohead

Joined May 28, 2009
508
The Colpitts Oscillator is a poor choice for frequency stability in my opinion. Can you post a schematic of the video transmitter you intend to use? It will make circuit analytics easier for those of us trying to help you.
 

Thread Starter

64C113M Abe.

Joined Sep 13, 2016
14
Thank you all for your replies, I have done some research on my own and it lead me to a video series by Stefan0719


It was made clear stated that you use a buffer to get frequency stability.

As it stands for now I've dropped all work for on the colpitts oscillator in favor of crystal oscillators for my radio transmitter projects.

Though in the future I would like to have something that has a variable frequency, the end goal is be able to build a homebrew VHF/UHF transmitter.
 

upand_at_them

Joined May 15, 2010
766
I'd like to point out that just because the Colpitts is less stable than a crystal doesn't mean you shouldn't use or experiment with it. There's much to be learned from all kinds of circuits.
 

ian field

Joined Oct 27, 2012
6,539
I'd like to point out that just because the Colpitts is less stable than a crystal doesn't mean you shouldn't use or experiment with it. There's much to be learned from all kinds of circuits.
Indeed - you can learn a lot about "flogging a dead horse" by choosing the wrong oscillator for an application.
 
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