Help me debug a Clapp oscillator that I've built

[obrien135]

Re: Clapp Oscillator

This is the circuit



Added after 8 minutes:

I am having trouble using the edaboard tools to post an image, but the above image is a Clapp oscillator which I built. Can anybody see whats wrong with it? It shows no sign of oscillation.

Added after 3 hours 25 minutes:

Here is a better picture of it.



Added after 36 minutes:

Do I need the 120Kohm resister?

 

[keith1200rs]

Re: Clapp Oscillator

Those are crazy component values! I cannot read what frequency you are after - it looks like 10MHz.

Anyway, I have attached some suggested component values for 10MHz. You should take the output from the transistor source.

Keith.

 

[obrien135]

Re: Clapp Oscillator

wOULDN'T MOST OF THOSE CAPACITANCE VALUES BE TOO CLOSE TO THE STRAY CAPACITANCE? i HAVE BEEN TRYING TO WORK WITH an air variable tuning cap which runs between 14 and 380pF, which I bought for an earlier project, in the interest of saving money (by reusing it). I find it difficult to find them with lower ranges. The one I'm using is more or less dictating the other capacitance values to get the range of frequencies between 10 and 10.3 MHz (LO for a 40M SSB rig). Maybe I would do better with a wider range.

 

[keith1200rs]

Re: Clapp Oscillator

Your values do not get anywhere near 10MHz. 27uH and 10nF = 306kHz. If you want to re-use the capacitor you will have to use it in series with something quite small, particularly if you don't reduce the inductor.

The 1nF capacitors I suggested are quite a lot larger than any stray capacitance I would expect. Stray should only be a few pF.

Keith.

 

[obrien135]

Re: Clapp Oscillator

Thank you Keith, but if I used a capacitor small enough that the 14-380pF in series would only affect the frequency by 5 or 10% it would have to be down around the stray capacitance wouldn't it?

 

[keith1200rs]

Re: Clapp Oscillator

The trimmer is in parallel with the 1nF capacitors so stray isn't a big issue, I would have thought. If you want to use 10nF as you showed you would need to reduce the inductance to around 33nH which is not a normal value for a 10MHz oscillator. It can probably be made to work if that is what you want. However with 33nH of inductance you need to be careful about the construction due to stray inductances.

Keith.

 

[obrien135]

Re: Clapp Oscillator

According to the attached document the caps are all in series. That didn't seem correct to me either, but I was referred to this PDF by another member.

 

[keith1200rs]

Re: Clapp Oscillator

From a resonance point of view that is correct, but your values have 100nF in series with 2x100nF (=200nF) in series with 10nF with your trimmer in parallel with the 10nF. So, your capacitance is not much less than 10nF (around 8.7nF I think).

Keith.

 

[obrien135]

Re: Clapp Oscillator

I don't quite follow what you are saying. I know that caps in parrallel add, and caps in series are the inverse of the sum of the inverses. I'm not sure how you came up with those figures. Can you explain further?

 

[keith1200rs]

Re: Clapp Oscillator

If you follow what is round the inductor, you firs come across C3. Then in series you have C2, then in series you have C1, then you end up at the other end of the inductor. All those capacitors are in series as far as the inductor is concerned. Just delete all the other components and it may be easier to see.

Keith.

 

[obrien135]

Re: Clapp Oscillator

Yes, Keith, but you said in the previous message that the trimmer was in parrallel with the 1nF caps, but it is only in parrallel with the 15nF cap. So although it's true that the total capacitance would be more that 14pf (the min value of the trimmer), which is above the stray capacitance, if you don't make the capacitance it's in parrallel with significantly higher that 14pF you will get too much delta C to stay in the 40M band. Wouldn't you say that is correct?

 

[keith1200rs]

Re: Clapp Oscillator

Yes, Keith, but you said in the previous message that the trimmer was in parrallel with the 1nF caps, but it is only in parrallel with the 15nF cap.

No, I said the trimmer was in parallel with the 10nF capacitor which it is in your circuit.

You need to work out your capacitance to get the tuning range you want with the trimmer range you have.

Keith

 

[obrien135]

Re: Clapp Oscillator

I see what you are saying now Keith. I'm a little slow but eventually I get it. I will try your circuit. I think I misread those 1nF as 1pF.

 

[obrien135]

Re: Clapp Oscillator

I have to buy some 1 nF caps on Monday. But in the meantime, does it look like the right circuit configuration? The pdf file I went by didn't show a practical circuit. I assumed I would need the two resisters in the gate circuit. I am going to try the cap and inductor values you suggested, but other than that, does it look like it will oscillate? I have made a lot of attempts to get a VFO working but no luck so far.

 

[keith1200rs]

Re: Clapp Oscillator

The pdf I posted earlier simulated ok, so I would suggest that as a starting point. I didn't check the resistor values, but those values seem to work.

Keith

 

[slwthr]

Re: Clapp Oscillator

If you follow what is round the inductor, you firs come across C3. Then in series you have C2, then in series you have C1, then you end up at the other end of the inductor. All those capacitors are in series as far as the inductor is concerned. Just delete all the other components and it may be easier to see.

Keith.

Dear Keith,
I wish a big help from you.

I have some trouble with answering with below question.Please help me out with this question.I tried a lot but I failed.Please reply me soon I have to submit it tomorrow.

here is the question :

Question 1.

You are required to simulate an oscillator circuit and an amplifier circuit. The output of the oscillator is to be connected to the input of the
amplifier.

The circuit supply voltage will be 12V and both parts of the circuit should be designed using a single BJT. A non-distorted sine wave should be present at the output.(One BJT for Oscillator and other BJT for common collector amplifier)

Oscillator type - Clapp
Amplifier type - common collector

Oscillator frequency 3kHz and amplifier output voltage 4 V


Here is some example links:

Transistor Tutorial, Part 7: Oscillators ( see Fig8)

Clapp Oscillator example : Clapp oscillator - Wikipedia, the free encyclopedia

Basic common collector amplifier : Common collector - Wikipedia, the free encyclopedia


Please try to give me a simple diagram for clapp oscillator connecting with basic amplifier circuit(which when is clapp oscillator 3Khz out put signal when using as a input of the the basic amplifier and then want to get 4V Out put from the amplifier)

Please send me a simple circuit for this : slwthr@yahoo.com

Here is my new post also :https://www.edaboard.com/threads/228670/

Regards
Harith

 

[mvs sarma]

Re: Clapp Oscillator

@keith,
C2&C3 (c2 at 100n and C3 at 200n). is the ratio correct for a proper feed back
i doubt it at 10MHz

 

[slwthr]

Re: Clapp Oscillator

@keith,
C2&C3 (c2 at 100n and C3 at 200n). is the ratio correct for a proper feed back
i doubt it at 10MHz

Hello Sarma,

Can you help me with this please

I have some trouble with answering with below question.Please help me out with this question.I tried a lot but I failed.Please reply me soon I have to submit it tomorrow.

here is the question :

Question 1.

You are required to simulate an oscillator circuit and an amplifier circuit. The output of the oscillator is to be connected to the input of the
amplifier.

The circuit supply voltage will be 12V and both parts of the circuit should be designed using a single BJT. A non-distorted sine wave should be present at the output.(One BJT for Oscillator and other BJT for common collector amplifier)

Oscillator type - Clapp
Amplifier type - common collector

Oscillator frequency 3kHz and amplifier output voltage 4 V


Here is some example links:

Transistor Tutorial, Part 7: Oscillators ( see Fig8)

Clapp Oscillator example : Clapp oscillator - Wikipedia, the free encyclopedia

Basic common collector amplifier : Common collector - Wikipedia, the free encyclopedia


Please try to give me a simple diagram for clapp oscillator connecting with basic amplifier circuit(which when is clapp oscillator 3Khz out put signal when using as a input of the the basic amplifier and then want to get 4V Out put from the amplifier)

Please send me a simple circuit for this : slwthr@yahoo.com

Here is my new post also :clapp oscillator amplifier help

Regards
Harith

 

[keith1200rs]

I have locked this thread - don't run two threads on the same subject.

Keith.