colpitts oscillator question .. ??

[wiwi]

I've designed a colpitts oscillator as shown in the link :

**broken link removed**

I used the values 470 pf for caps and 165 uH inductor which gives about 800 KHz signal . I got a 800 KHz sinusoidal signal but it's slightly distorted from the bottom , my question is is it ok for that signal to be used as carrier signal for RF transmitter or it should be a pure sinewave ?

thnx

 

[kak111]

First you should check transistor bias point.
Ie. calculate or test or simulate right values for Rb1 and Rb2,
so that you find symmetrical oscillation.

KAK

 

[wiwi]

everything is fine with biasing , i checked it as a CE amplifier and it worked fine with high gain , but this occurs when it's used as colpitts , the signal is being clipped from the bottom. but anyway is it ok as carrier signal ? or it is necessary be a pure sinusoid ?

 

[kak111]

Hard to say without knowing your circuit.
But generally non-pure carrier creates some additional frequencies. (Harmonics)

 

[FvM]

You should consider, that the feedback factor of a colpitts oscillator, set by the C1 to C2 ratio and other circuit parameters, should be choosen in a reasonable way. A distorted waveform suggests to reduce the feedback factor.

 

[wiwi]

generally non-pure carrier creates some additional frequencies. (Harmonics)

yes a few additional harmonics are created but they are too low (0.1v or less) , can this be ignored ?

 

[tony_lth]

What is your RF TX applications?
There should be some standards for TX from your goverment.

 

[kak111]

Depends on your needs and how harmful are those harmonics
to other radio communication.
Try reduce feedback or amplification as FvM suggest.

There should be some standards for TX from your goverment.

 

[throwaway18]

Distortion that is visible on an oscilloscope means your are generating energy at multiples of the fundamental frequency.

If that matters depends on how much energy you are radiating to some extent.
If you are applying a couple of volts to a couple of meters of wire as an antenna then the radiated ERP will be very low because your antenna is very small compared to the wavelength.
The chance of someone within a few hundred meters of you noticing that your toy medium wave transmitter is putting out weak carriers on shortwave frequencies is low.

If you are planning to use a large antenna and significant power then yes you should have a clean output spectrum. Emissions on 1.6MHz, 2.4, 3.2,4.0MHz etc could go a long way.
Of course if you are doing that you should also worry about the stability of your oscillator, crystal control is strongly preferred for transmitters so the frequency does not drift.

 

[wiwi]

Distortion that is visible on an oscilloscope means your are generating energy at multiples of the fundamental frequency.

If that matters depends on how much energy you are radiating to some extent.
If you are applying a couple of volts to a couple of meters of wire as an antenna then the radiated ERP will be very low because your antenna is very small compared to the wavelength.
The chance of someone within a few hundred meters of you noticing that your toy medium wave transmitter is putting out weak carriers on shortwave frequencies is low.

U mean that if i am working in short range (say about 20 m) it doesn't matter having a little distortion with the carrier ?

(I am trying to build an AM modulator and i am using this as the carrier signal)

 

[RCinFLA]

For an AM modulator you will have subsequent stages. The tank circuits in these stages will reduce harmonic level.

Any real oscillator must start out with a gain greater then unity or there will be startup problems. The amplifier will compress to bring steady state with a total net loop gain of one. This compression causes the distortiion. You can reduce the feedback level to reduce the needed compression but get too close to unity and startup will be slow and may have issues when loading is applied to oscillator.

There are circuits that have AGC for loop gain to run the oscillator in purely linear mode to achieve low distortion sinewave output.

 

[vfone]

Just use about 100pF for Cb and you will get no distortion in your case.